Thanks to visit codestin.com
Credit goes to doc.rust-lang.org

alloc/vec/
mod.rs

1//! A contiguous growable array type with heap-allocated contents, written
2//! `Vec<T>`.
3//!
4//! Vectors have *O*(1) indexing, amortized *O*(1) push (to the end) and
5//! *O*(1) pop (from the end).
6//!
7//! Vectors ensure they never allocate more than `isize::MAX` bytes.
8//!
9//! # Examples
10//!
11//! You can explicitly create a [`Vec`] with [`Vec::new`]:
12//!
13//! ```
14//! let v: Vec<i32> = Vec::new();
15//! ```
16//!
17//! ...or by using the [`vec!`] macro:
18//!
19//! ```
20//! let v: Vec<i32> = vec![];
21//!
22//! let v = vec![1, 2, 3, 4, 5];
23//!
24//! let v = vec![0; 10]; // ten zeroes
25//! ```
26//!
27//! You can [`push`] values onto the end of a vector (which will grow the vector
28//! as needed):
29//!
30//! ```
31//! let mut v = vec![1, 2];
32//!
33//! v.push(3);
34//! ```
35//!
36//! Popping values works in much the same way:
37//!
38//! ```
39//! let mut v = vec![1, 2];
40//!
41//! let two = v.pop();
42//! ```
43//!
44//! Vectors also support indexing (through the [`Index`] and [`IndexMut`] traits):
45//!
46//! ```
47//! let mut v = vec![1, 2, 3];
48//! let three = v[2];
49//! v[1] = v[1] + 5;
50//! ```
51//!
52//! # Memory layout
53//!
54//! When the type is non-zero-sized and the capacity is nonzero, [`Vec`] uses the [`Global`]
55//! allocator for its allocation. It is valid to convert both ways between such a [`Vec`] and a raw
56//! pointer allocated with the [`Global`] allocator, provided that the [`Layout`] used with the
57//! allocator is correct for a sequence of `capacity` elements of the type, and the first `len`
58//! values pointed to by the raw pointer are valid. More precisely, a `ptr: *mut T` that has been
59//! allocated with the [`Global`] allocator with [`Layout::array::<T>(capacity)`][Layout::array] may
60//! be converted into a vec using
61//! [`Vec::<T>::from_raw_parts(ptr, len, capacity)`](Vec::from_raw_parts). Conversely, the memory
62//! backing a `value: *mut T` obtained from [`Vec::<T>::as_mut_ptr`] may be deallocated using the
63//! [`Global`] allocator with the same layout.
64//!
65//! For zero-sized types (ZSTs), or when the capacity is zero, the `Vec` pointer must be non-null
66//! and sufficiently aligned. The recommended way to build a `Vec` of ZSTs if [`vec!`] cannot be
67//! used is to use [`ptr::NonNull::dangling`].
68//!
69//! [`push`]: Vec::push
70//! [`ptr::NonNull::dangling`]: NonNull::dangling
71//! [`Layout`]: crate::alloc::Layout
72//! [Layout::array]: crate::alloc::Layout::array
73
74#![stable(feature = "rust1", since = "1.0.0")]
75
76#[cfg(not(no_global_oom_handling))]
77use core::cmp;
78use core::cmp::Ordering;
79use core::hash::{Hash, Hasher};
80#[cfg(not(no_global_oom_handling))]
81use core::iter;
82use core::marker::PhantomData;
83use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
84use core::ops::{self, Index, IndexMut, Range, RangeBounds};
85use core::ptr::{self, NonNull};
86use core::slice::{self, SliceIndex};
87use core::{fmt, intrinsics, ub_checks};
88
89#[stable(feature = "extract_if", since = "1.87.0")]
90pub use self::extract_if::ExtractIf;
91use crate::alloc::{Allocator, Global};
92use crate::borrow::{Cow, ToOwned};
93use crate::boxed::Box;
94use crate::collections::TryReserveError;
95use crate::raw_vec::RawVec;
96
97mod extract_if;
98
99#[cfg(not(no_global_oom_handling))]
100#[stable(feature = "vec_splice", since = "1.21.0")]
101pub use self::splice::Splice;
102
103#[cfg(not(no_global_oom_handling))]
104mod splice;
105
106#[stable(feature = "drain", since = "1.6.0")]
107pub use self::drain::Drain;
108
109mod drain;
110
111#[cfg(not(no_global_oom_handling))]
112mod cow;
113
114#[cfg(not(no_global_oom_handling))]
115pub(crate) use self::in_place_collect::AsVecIntoIter;
116#[stable(feature = "rust1", since = "1.0.0")]
117pub use self::into_iter::IntoIter;
118
119mod into_iter;
120
121#[cfg(not(no_global_oom_handling))]
122use self::is_zero::IsZero;
123
124#[cfg(not(no_global_oom_handling))]
125mod is_zero;
126
127#[cfg(not(no_global_oom_handling))]
128mod in_place_collect;
129
130mod partial_eq;
131
132#[unstable(feature = "vec_peek_mut", issue = "122742")]
133pub use self::peek_mut::PeekMut;
134
135mod peek_mut;
136
137#[cfg(not(no_global_oom_handling))]
138use self::spec_from_elem::SpecFromElem;
139
140#[cfg(not(no_global_oom_handling))]
141mod spec_from_elem;
142
143#[cfg(not(no_global_oom_handling))]
144use self::set_len_on_drop::SetLenOnDrop;
145
146#[cfg(not(no_global_oom_handling))]
147mod set_len_on_drop;
148
149#[cfg(not(no_global_oom_handling))]
150use self::in_place_drop::{InPlaceDrop, InPlaceDstDataSrcBufDrop};
151
152#[cfg(not(no_global_oom_handling))]
153mod in_place_drop;
154
155#[cfg(not(no_global_oom_handling))]
156use self::spec_from_iter_nested::SpecFromIterNested;
157
158#[cfg(not(no_global_oom_handling))]
159mod spec_from_iter_nested;
160
161#[cfg(not(no_global_oom_handling))]
162use self::spec_from_iter::SpecFromIter;
163
164#[cfg(not(no_global_oom_handling))]
165mod spec_from_iter;
166
167#[cfg(not(no_global_oom_handling))]
168use self::spec_extend::SpecExtend;
169
170#[cfg(not(no_global_oom_handling))]
171mod spec_extend;
172
173/// A contiguous growable array type, written as `Vec<T>`, short for 'vector'.
174///
175/// # Examples
176///
177/// ```
178/// let mut vec = Vec::new();
179/// vec.push(1);
180/// vec.push(2);
181///
182/// assert_eq!(vec.len(), 2);
183/// assert_eq!(vec[0], 1);
184///
185/// assert_eq!(vec.pop(), Some(2));
186/// assert_eq!(vec.len(), 1);
187///
188/// vec[0] = 7;
189/// assert_eq!(vec[0], 7);
190///
191/// vec.extend([1, 2, 3]);
192///
193/// for x in &vec {
194///     println!("{x}");
195/// }
196/// assert_eq!(vec, [7, 1, 2, 3]);
197/// ```
198///
199/// The [`vec!`] macro is provided for convenient initialization:
200///
201/// ```
202/// let mut vec1 = vec![1, 2, 3];
203/// vec1.push(4);
204/// let vec2 = Vec::from([1, 2, 3, 4]);
205/// assert_eq!(vec1, vec2);
206/// ```
207///
208/// It can also initialize each element of a `Vec<T>` with a given value.
209/// This may be more efficient than performing allocation and initialization
210/// in separate steps, especially when initializing a vector of zeros:
211///
212/// ```
213/// let vec = vec![0; 5];
214/// assert_eq!(vec, [0, 0, 0, 0, 0]);
215///
216/// // The following is equivalent, but potentially slower:
217/// let mut vec = Vec::with_capacity(5);
218/// vec.resize(5, 0);
219/// assert_eq!(vec, [0, 0, 0, 0, 0]);
220/// ```
221///
222/// For more information, see
223/// [Capacity and Reallocation](#capacity-and-reallocation).
224///
225/// Use a `Vec<T>` as an efficient stack:
226///
227/// ```
228/// let mut stack = Vec::new();
229///
230/// stack.push(1);
231/// stack.push(2);
232/// stack.push(3);
233///
234/// while let Some(top) = stack.pop() {
235///     // Prints 3, 2, 1
236///     println!("{top}");
237/// }
238/// ```
239///
240/// # Indexing
241///
242/// The `Vec` type allows access to values by index, because it implements the
243/// [`Index`] trait. An example will be more explicit:
244///
245/// ```
246/// let v = vec![0, 2, 4, 6];
247/// println!("{}", v[1]); // it will display '2'
248/// ```
249///
250/// However be careful: if you try to access an index which isn't in the `Vec`,
251/// your software will panic! You cannot do this:
252///
253/// ```should_panic
254/// let v = vec![0, 2, 4, 6];
255/// println!("{}", v[6]); // it will panic!
256/// ```
257///
258/// Use [`get`] and [`get_mut`] if you want to check whether the index is in
259/// the `Vec`.
260///
261/// # Slicing
262///
263/// A `Vec` can be mutable. On the other hand, slices are read-only objects.
264/// To get a [slice][prim@slice], use [`&`]. Example:
265///
266/// ```
267/// fn read_slice(slice: &[usize]) {
268///     // ...
269/// }
270///
271/// let v = vec![0, 1];
272/// read_slice(&v);
273///
274/// // ... and that's all!
275/// // you can also do it like this:
276/// let u: &[usize] = &v;
277/// // or like this:
278/// let u: &[_] = &v;
279/// ```
280///
281/// In Rust, it's more common to pass slices as arguments rather than vectors
282/// when you just want to provide read access. The same goes for [`String`] and
283/// [`&str`].
284///
285/// # Capacity and reallocation
286///
287/// The capacity of a vector is the amount of space allocated for any future
288/// elements that will be added onto the vector. This is not to be confused with
289/// the *length* of a vector, which specifies the number of actual elements
290/// within the vector. If a vector's length exceeds its capacity, its capacity
291/// will automatically be increased, but its elements will have to be
292/// reallocated.
293///
294/// For example, a vector with capacity 10 and length 0 would be an empty vector
295/// with space for 10 more elements. Pushing 10 or fewer elements onto the
296/// vector will not change its capacity or cause reallocation to occur. However,
297/// if the vector's length is increased to 11, it will have to reallocate, which
298/// can be slow. For this reason, it is recommended to use [`Vec::with_capacity`]
299/// whenever possible to specify how big the vector is expected to get.
300///
301/// # Guarantees
302///
303/// Due to its incredibly fundamental nature, `Vec` makes a lot of guarantees
304/// about its design. This ensures that it's as low-overhead as possible in
305/// the general case, and can be correctly manipulated in primitive ways
306/// by unsafe code. Note that these guarantees refer to an unqualified `Vec<T>`.
307/// If additional type parameters are added (e.g., to support custom allocators),
308/// overriding their defaults may change the behavior.
309///
310/// Most fundamentally, `Vec` is and always will be a (pointer, capacity, length)
311/// triplet. No more, no less. The order of these fields is completely
312/// unspecified, and you should use the appropriate methods to modify these.
313/// The pointer will never be null, so this type is null-pointer-optimized.
314///
315/// However, the pointer might not actually point to allocated memory. In particular,
316/// if you construct a `Vec` with capacity 0 via [`Vec::new`], [`vec![]`][`vec!`],
317/// [`Vec::with_capacity(0)`][`Vec::with_capacity`], or by calling [`shrink_to_fit`]
318/// on an empty Vec, it will not allocate memory. Similarly, if you store zero-sized
319/// types inside a `Vec`, it will not allocate space for them. *Note that in this case
320/// the `Vec` might not report a [`capacity`] of 0*. `Vec` will allocate if and only
321/// if <code>[size_of::\<T>]\() * [capacity]\() > 0</code>. In general, `Vec`'s allocation
322/// details are very subtle --- if you intend to allocate memory using a `Vec`
323/// and use it for something else (either to pass to unsafe code, or to build your
324/// own memory-backed collection), be sure to deallocate this memory by using
325/// `from_raw_parts` to recover the `Vec` and then dropping it.
326///
327/// If a `Vec` *has* allocated memory, then the memory it points to is on the heap
328/// (as defined by the allocator Rust is configured to use by default), and its
329/// pointer points to [`len`] initialized, contiguous elements in order (what
330/// you would see if you coerced it to a slice), followed by <code>[capacity] - [len]</code>
331/// logically uninitialized, contiguous elements.
332///
333/// A vector containing the elements `'a'` and `'b'` with capacity 4 can be
334/// visualized as below. The top part is the `Vec` struct, it contains a
335/// pointer to the head of the allocation in the heap, length and capacity.
336/// The bottom part is the allocation on the heap, a contiguous memory block.
337///
338/// ```text
339///             ptr      len  capacity
340///        +--------+--------+--------+
341///        | 0x0123 |      2 |      4 |
342///        +--------+--------+--------+
343///             |
344///             v
345/// Heap   +--------+--------+--------+--------+
346///        |    'a' |    'b' | uninit | uninit |
347///        +--------+--------+--------+--------+
348/// ```
349///
350/// - **uninit** represents memory that is not initialized, see [`MaybeUninit`].
351/// - Note: the ABI is not stable and `Vec` makes no guarantees about its memory
352///   layout (including the order of fields).
353///
354/// `Vec` will never perform a "small optimization" where elements are actually
355/// stored on the stack for two reasons:
356///
357/// * It would make it more difficult for unsafe code to correctly manipulate
358///   a `Vec`. The contents of a `Vec` wouldn't have a stable address if it were
359///   only moved, and it would be more difficult to determine if a `Vec` had
360///   actually allocated memory.
361///
362/// * It would penalize the general case, incurring an additional branch
363///   on every access.
364///
365/// `Vec` will never automatically shrink itself, even if completely empty. This
366/// ensures no unnecessary allocations or deallocations occur. Emptying a `Vec`
367/// and then filling it back up to the same [`len`] should incur no calls to
368/// the allocator. If you wish to free up unused memory, use
369/// [`shrink_to_fit`] or [`shrink_to`].
370///
371/// [`push`] and [`insert`] will never (re)allocate if the reported capacity is
372/// sufficient. [`push`] and [`insert`] *will* (re)allocate if
373/// <code>[len] == [capacity]</code>. That is, the reported capacity is completely
374/// accurate, and can be relied on. It can even be used to manually free the memory
375/// allocated by a `Vec` if desired. Bulk insertion methods *may* reallocate, even
376/// when not necessary.
377///
378/// `Vec` does not guarantee any particular growth strategy when reallocating
379/// when full, nor when [`reserve`] is called. The current strategy is basic
380/// and it may prove desirable to use a non-constant growth factor. Whatever
381/// strategy is used will of course guarantee *O*(1) amortized [`push`].
382///
383/// It is guaranteed, in order to respect the intentions of the programmer, that
384/// all of `vec![e_1, e_2, ..., e_n]`, `vec![x; n]`, and [`Vec::with_capacity(n)`] produce a `Vec`
385/// that requests an allocation of the exact size needed for precisely `n` elements from the allocator,
386/// and no other size (such as, for example: a size rounded up to the nearest power of 2).
387/// The allocator will return an allocation that is at least as large as requested, but it may be larger.
388///
389/// It is guaranteed that the [`Vec::capacity`] method returns a value that is at least the requested capacity
390/// and not more than the allocated capacity.
391///
392/// The method [`Vec::shrink_to_fit`] will attempt to discard excess capacity an allocator has given to a `Vec`.
393/// If <code>[len] == [capacity]</code>, then a `Vec<T>` can be converted
394/// to and from a [`Box<[T]>`][owned slice] without reallocating or moving the elements.
395/// `Vec` exploits this fact as much as reasonable when implementing common conversions
396/// such as [`into_boxed_slice`].
397///
398/// `Vec` will not specifically overwrite any data that is removed from it,
399/// but also won't specifically preserve it. Its uninitialized memory is
400/// scratch space that it may use however it wants. It will generally just do
401/// whatever is most efficient or otherwise easy to implement. Do not rely on
402/// removed data to be erased for security purposes. Even if you drop a `Vec`, its
403/// buffer may simply be reused by another allocation. Even if you zero a `Vec`'s memory
404/// first, that might not actually happen because the optimizer does not consider
405/// this a side-effect that must be preserved. There is one case which we will
406/// not break, however: using `unsafe` code to write to the excess capacity,
407/// and then increasing the length to match, is always valid.
408///
409/// Currently, `Vec` does not guarantee the order in which elements are dropped.
410/// The order has changed in the past and may change again.
411///
412/// [`get`]: slice::get
413/// [`get_mut`]: slice::get_mut
414/// [`String`]: crate::string::String
415/// [`&str`]: type@str
416/// [`shrink_to_fit`]: Vec::shrink_to_fit
417/// [`shrink_to`]: Vec::shrink_to
418/// [capacity]: Vec::capacity
419/// [`capacity`]: Vec::capacity
420/// [`Vec::capacity`]: Vec::capacity
421/// [size_of::\<T>]: size_of
422/// [len]: Vec::len
423/// [`len`]: Vec::len
424/// [`push`]: Vec::push
425/// [`insert`]: Vec::insert
426/// [`reserve`]: Vec::reserve
427/// [`Vec::with_capacity(n)`]: Vec::with_capacity
428/// [`MaybeUninit`]: core::mem::MaybeUninit
429/// [owned slice]: Box
430/// [`into_boxed_slice`]: Vec::into_boxed_slice
431#[stable(feature = "rust1", since = "1.0.0")]
432#[rustc_diagnostic_item = "Vec"]
433#[rustc_insignificant_dtor]
434pub struct Vec<T, #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global> {
435    buf: RawVec<T, A>,
436    len: usize,
437}
438
439////////////////////////////////////////////////////////////////////////////////
440// Inherent methods
441////////////////////////////////////////////////////////////////////////////////
442
443impl<T> Vec<T> {
444    /// Constructs a new, empty `Vec<T>`.
445    ///
446    /// The vector will not allocate until elements are pushed onto it.
447    ///
448    /// # Examples
449    ///
450    /// ```
451    /// # #![allow(unused_mut)]
452    /// let mut vec: Vec<i32> = Vec::new();
453    /// ```
454    #[inline]
455    #[rustc_const_stable(feature = "const_vec_new", since = "1.39.0")]
456    #[rustc_diagnostic_item = "vec_new"]
457    #[stable(feature = "rust1", since = "1.0.0")]
458    #[must_use]
459    pub const fn new() -> Self {
460        Vec { buf: RawVec::new(), len: 0 }
461    }
462
463    /// Constructs a new, empty `Vec<T>` with at least the specified capacity.
464    ///
465    /// The vector will be able to hold at least `capacity` elements without
466    /// reallocating. This method is allowed to allocate for more elements than
467    /// `capacity`. If `capacity` is zero, the vector will not allocate.
468    ///
469    /// It is important to note that although the returned vector has the
470    /// minimum *capacity* specified, the vector will have a zero *length*. For
471    /// an explanation of the difference between length and capacity, see
472    /// *[Capacity and reallocation]*.
473    ///
474    /// If it is important to know the exact allocated capacity of a `Vec`,
475    /// always use the [`capacity`] method after construction.
476    ///
477    /// For `Vec<T>` where `T` is a zero-sized type, there will be no allocation
478    /// and the capacity will always be `usize::MAX`.
479    ///
480    /// [Capacity and reallocation]: #capacity-and-reallocation
481    /// [`capacity`]: Vec::capacity
482    ///
483    /// # Panics
484    ///
485    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
486    ///
487    /// # Examples
488    ///
489    /// ```
490    /// let mut vec = Vec::with_capacity(10);
491    ///
492    /// // The vector contains no items, even though it has capacity for more
493    /// assert_eq!(vec.len(), 0);
494    /// assert!(vec.capacity() >= 10);
495    ///
496    /// // These are all done without reallocating...
497    /// for i in 0..10 {
498    ///     vec.push(i);
499    /// }
500    /// assert_eq!(vec.len(), 10);
501    /// assert!(vec.capacity() >= 10);
502    ///
503    /// // ...but this may make the vector reallocate
504    /// vec.push(11);
505    /// assert_eq!(vec.len(), 11);
506    /// assert!(vec.capacity() >= 11);
507    ///
508    /// // A vector of a zero-sized type will always over-allocate, since no
509    /// // allocation is necessary
510    /// let vec_units = Vec::<()>::with_capacity(10);
511    /// assert_eq!(vec_units.capacity(), usize::MAX);
512    /// ```
513    #[cfg(not(no_global_oom_handling))]
514    #[inline]
515    #[stable(feature = "rust1", since = "1.0.0")]
516    #[must_use]
517    #[rustc_diagnostic_item = "vec_with_capacity"]
518    #[track_caller]
519    pub fn with_capacity(capacity: usize) -> Self {
520        Self::with_capacity_in(capacity, Global)
521    }
522
523    /// Constructs a new, empty `Vec<T>` with at least the specified capacity.
524    ///
525    /// The vector will be able to hold at least `capacity` elements without
526    /// reallocating. This method is allowed to allocate for more elements than
527    /// `capacity`. If `capacity` is zero, the vector will not allocate.
528    ///
529    /// # Errors
530    ///
531    /// Returns an error if the capacity exceeds `isize::MAX` _bytes_,
532    /// or if the allocator reports allocation failure.
533    #[inline]
534    #[unstable(feature = "try_with_capacity", issue = "91913")]
535    pub fn try_with_capacity(capacity: usize) -> Result<Self, TryReserveError> {
536        Self::try_with_capacity_in(capacity, Global)
537    }
538
539    /// Creates a `Vec<T>` directly from a pointer, a length, and a capacity.
540    ///
541    /// # Safety
542    ///
543    /// This is highly unsafe, due to the number of invariants that aren't
544    /// checked:
545    ///
546    /// * If `T` is not a zero-sized type and the capacity is nonzero, `ptr` must have
547    ///   been allocated using the global allocator, such as via the [`alloc::alloc`]
548    ///   function. If `T` is a zero-sized type or the capacity is zero, `ptr` need
549    ///   only be non-null and aligned.
550    /// * `T` needs to have the same alignment as what `ptr` was allocated with,
551    ///   if the pointer is required to be allocated.
552    ///   (`T` having a less strict alignment is not sufficient, the alignment really
553    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
554    ///   allocated and deallocated with the same layout.)
555    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes), if
556    ///   nonzero, needs to be the same size as the pointer was allocated with.
557    ///   (Because similar to alignment, [`dealloc`] must be called with the same
558    ///   layout `size`.)
559    /// * `length` needs to be less than or equal to `capacity`.
560    /// * The first `length` values must be properly initialized values of type `T`.
561    /// * `capacity` needs to be the capacity that the pointer was allocated with,
562    ///   if the pointer is required to be allocated.
563    /// * The allocated size in bytes must be no larger than `isize::MAX`.
564    ///   See the safety documentation of [`pointer::offset`].
565    ///
566    /// These requirements are always upheld by any `ptr` that has been allocated
567    /// via `Vec<T>`. Other allocation sources are allowed if the invariants are
568    /// upheld.
569    ///
570    /// Violating these may cause problems like corrupting the allocator's
571    /// internal data structures. For example it is normally **not** safe
572    /// to build a `Vec<u8>` from a pointer to a C `char` array with length
573    /// `size_t`, doing so is only safe if the array was initially allocated by
574    /// a `Vec` or `String`.
575    /// It's also not safe to build one from a `Vec<u16>` and its length, because
576    /// the allocator cares about the alignment, and these two types have different
577    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
578    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1. To avoid
579    /// these issues, it is often preferable to do casting/transmuting using
580    /// [`slice::from_raw_parts`] instead.
581    ///
582    /// The ownership of `ptr` is effectively transferred to the
583    /// `Vec<T>` which may then deallocate, reallocate or change the
584    /// contents of memory pointed to by the pointer at will. Ensure
585    /// that nothing else uses the pointer after calling this
586    /// function.
587    ///
588    /// [`String`]: crate::string::String
589    /// [`alloc::alloc`]: crate::alloc::alloc
590    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
591    ///
592    /// # Examples
593    ///
594    // FIXME Update this when vec_into_raw_parts is stabilized
595    /// ```
596    /// use std::ptr;
597    /// use std::mem;
598    ///
599    /// let v = vec![1, 2, 3];
600    ///
601    /// // Prevent running `v`'s destructor so we are in complete control
602    /// // of the allocation.
603    /// let mut v = mem::ManuallyDrop::new(v);
604    ///
605    /// // Pull out the various important pieces of information about `v`
606    /// let p = v.as_mut_ptr();
607    /// let len = v.len();
608    /// let cap = v.capacity();
609    ///
610    /// unsafe {
611    ///     // Overwrite memory with 4, 5, 6
612    ///     for i in 0..len {
613    ///         ptr::write(p.add(i), 4 + i);
614    ///     }
615    ///
616    ///     // Put everything back together into a Vec
617    ///     let rebuilt = Vec::from_raw_parts(p, len, cap);
618    ///     assert_eq!(rebuilt, [4, 5, 6]);
619    /// }
620    /// ```
621    ///
622    /// Using memory that was allocated elsewhere:
623    ///
624    /// ```rust
625    /// use std::alloc::{alloc, Layout};
626    ///
627    /// fn main() {
628    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
629    ///
630    ///     let vec = unsafe {
631    ///         let mem = alloc(layout).cast::<u32>();
632    ///         if mem.is_null() {
633    ///             return;
634    ///         }
635    ///
636    ///         mem.write(1_000_000);
637    ///
638    ///         Vec::from_raw_parts(mem, 1, 16)
639    ///     };
640    ///
641    ///     assert_eq!(vec, &[1_000_000]);
642    ///     assert_eq!(vec.capacity(), 16);
643    /// }
644    /// ```
645    #[inline]
646    #[stable(feature = "rust1", since = "1.0.0")]
647    pub unsafe fn from_raw_parts(ptr: *mut T, length: usize, capacity: usize) -> Self {
648        unsafe { Self::from_raw_parts_in(ptr, length, capacity, Global) }
649    }
650
651    #[doc(alias = "from_non_null_parts")]
652    /// Creates a `Vec<T>` directly from a `NonNull` pointer, a length, and a capacity.
653    ///
654    /// # Safety
655    ///
656    /// This is highly unsafe, due to the number of invariants that aren't
657    /// checked:
658    ///
659    /// * `ptr` must have been allocated using the global allocator, such as via
660    ///   the [`alloc::alloc`] function.
661    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
662    ///   (`T` having a less strict alignment is not sufficient, the alignment really
663    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
664    ///   allocated and deallocated with the same layout.)
665    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
666    ///   to be the same size as the pointer was allocated with. (Because similar to
667    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
668    /// * `length` needs to be less than or equal to `capacity`.
669    /// * The first `length` values must be properly initialized values of type `T`.
670    /// * `capacity` needs to be the capacity that the pointer was allocated with.
671    /// * The allocated size in bytes must be no larger than `isize::MAX`.
672    ///   See the safety documentation of [`pointer::offset`].
673    ///
674    /// These requirements are always upheld by any `ptr` that has been allocated
675    /// via `Vec<T>`. Other allocation sources are allowed if the invariants are
676    /// upheld.
677    ///
678    /// Violating these may cause problems like corrupting the allocator's
679    /// internal data structures. For example it is normally **not** safe
680    /// to build a `Vec<u8>` from a pointer to a C `char` array with length
681    /// `size_t`, doing so is only safe if the array was initially allocated by
682    /// a `Vec` or `String`.
683    /// It's also not safe to build one from a `Vec<u16>` and its length, because
684    /// the allocator cares about the alignment, and these two types have different
685    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
686    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1. To avoid
687    /// these issues, it is often preferable to do casting/transmuting using
688    /// [`NonNull::slice_from_raw_parts`] instead.
689    ///
690    /// The ownership of `ptr` is effectively transferred to the
691    /// `Vec<T>` which may then deallocate, reallocate or change the
692    /// contents of memory pointed to by the pointer at will. Ensure
693    /// that nothing else uses the pointer after calling this
694    /// function.
695    ///
696    /// [`String`]: crate::string::String
697    /// [`alloc::alloc`]: crate::alloc::alloc
698    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
699    ///
700    /// # Examples
701    ///
702    // FIXME Update this when vec_into_raw_parts is stabilized
703    /// ```
704    /// #![feature(box_vec_non_null)]
705    ///
706    /// use std::ptr::NonNull;
707    /// use std::mem;
708    ///
709    /// let v = vec![1, 2, 3];
710    ///
711    /// // Prevent running `v`'s destructor so we are in complete control
712    /// // of the allocation.
713    /// let mut v = mem::ManuallyDrop::new(v);
714    ///
715    /// // Pull out the various important pieces of information about `v`
716    /// let p = unsafe { NonNull::new_unchecked(v.as_mut_ptr()) };
717    /// let len = v.len();
718    /// let cap = v.capacity();
719    ///
720    /// unsafe {
721    ///     // Overwrite memory with 4, 5, 6
722    ///     for i in 0..len {
723    ///         p.add(i).write(4 + i);
724    ///     }
725    ///
726    ///     // Put everything back together into a Vec
727    ///     let rebuilt = Vec::from_parts(p, len, cap);
728    ///     assert_eq!(rebuilt, [4, 5, 6]);
729    /// }
730    /// ```
731    ///
732    /// Using memory that was allocated elsewhere:
733    ///
734    /// ```rust
735    /// #![feature(box_vec_non_null)]
736    ///
737    /// use std::alloc::{alloc, Layout};
738    /// use std::ptr::NonNull;
739    ///
740    /// fn main() {
741    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
742    ///
743    ///     let vec = unsafe {
744    ///         let Some(mem) = NonNull::new(alloc(layout).cast::<u32>()) else {
745    ///             return;
746    ///         };
747    ///
748    ///         mem.write(1_000_000);
749    ///
750    ///         Vec::from_parts(mem, 1, 16)
751    ///     };
752    ///
753    ///     assert_eq!(vec, &[1_000_000]);
754    ///     assert_eq!(vec.capacity(), 16);
755    /// }
756    /// ```
757    #[inline]
758    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
759    pub unsafe fn from_parts(ptr: NonNull<T>, length: usize, capacity: usize) -> Self {
760        unsafe { Self::from_parts_in(ptr, length, capacity, Global) }
761    }
762
763    /// Decomposes a `Vec<T>` into its raw components: `(pointer, length, capacity)`.
764    ///
765    /// Returns the raw pointer to the underlying data, the length of
766    /// the vector (in elements), and the allocated capacity of the
767    /// data (in elements). These are the same arguments in the same
768    /// order as the arguments to [`from_raw_parts`].
769    ///
770    /// After calling this function, the caller is responsible for the
771    /// memory previously managed by the `Vec`. Most often, one does
772    /// this by converting the raw pointer, length, and capacity back
773    /// into a `Vec` with the [`from_raw_parts`] function; more generally,
774    /// if `T` is non-zero-sized and the capacity is nonzero, one may use
775    /// any method that calls [`dealloc`] with a layout of
776    /// `Layout::array::<T>(capacity)`; if `T` is zero-sized or the
777    /// capacity is zero, nothing needs to be done.
778    ///
779    /// [`from_raw_parts`]: Vec::from_raw_parts
780    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
781    ///
782    /// # Examples
783    ///
784    /// ```
785    /// #![feature(vec_into_raw_parts)]
786    /// let v: Vec<i32> = vec![-1, 0, 1];
787    ///
788    /// let (ptr, len, cap) = v.into_raw_parts();
789    ///
790    /// let rebuilt = unsafe {
791    ///     // We can now make changes to the components, such as
792    ///     // transmuting the raw pointer to a compatible type.
793    ///     let ptr = ptr as *mut u32;
794    ///
795    ///     Vec::from_raw_parts(ptr, len, cap)
796    /// };
797    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
798    /// ```
799    #[must_use = "losing the pointer will leak memory"]
800    #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
801    pub fn into_raw_parts(self) -> (*mut T, usize, usize) {
802        let mut me = ManuallyDrop::new(self);
803        (me.as_mut_ptr(), me.len(), me.capacity())
804    }
805
806    #[doc(alias = "into_non_null_parts")]
807    /// Decomposes a `Vec<T>` into its raw components: `(NonNull pointer, length, capacity)`.
808    ///
809    /// Returns the `NonNull` pointer to the underlying data, the length of
810    /// the vector (in elements), and the allocated capacity of the
811    /// data (in elements). These are the same arguments in the same
812    /// order as the arguments to [`from_parts`].
813    ///
814    /// After calling this function, the caller is responsible for the
815    /// memory previously managed by the `Vec`. The only way to do
816    /// this is to convert the `NonNull` pointer, length, and capacity back
817    /// into a `Vec` with the [`from_parts`] function, allowing
818    /// the destructor to perform the cleanup.
819    ///
820    /// [`from_parts`]: Vec::from_parts
821    ///
822    /// # Examples
823    ///
824    /// ```
825    /// #![feature(vec_into_raw_parts, box_vec_non_null)]
826    ///
827    /// let v: Vec<i32> = vec![-1, 0, 1];
828    ///
829    /// let (ptr, len, cap) = v.into_parts();
830    ///
831    /// let rebuilt = unsafe {
832    ///     // We can now make changes to the components, such as
833    ///     // transmuting the raw pointer to a compatible type.
834    ///     let ptr = ptr.cast::<u32>();
835    ///
836    ///     Vec::from_parts(ptr, len, cap)
837    /// };
838    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
839    /// ```
840    #[must_use = "losing the pointer will leak memory"]
841    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
842    // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
843    pub fn into_parts(self) -> (NonNull<T>, usize, usize) {
844        let (ptr, len, capacity) = self.into_raw_parts();
845        // SAFETY: A `Vec` always has a non-null pointer.
846        (unsafe { NonNull::new_unchecked(ptr) }, len, capacity)
847    }
848}
849
850impl<T, A: Allocator> Vec<T, A> {
851    /// Constructs a new, empty `Vec<T, A>`.
852    ///
853    /// The vector will not allocate until elements are pushed onto it.
854    ///
855    /// # Examples
856    ///
857    /// ```
858    /// #![feature(allocator_api)]
859    ///
860    /// use std::alloc::System;
861    ///
862    /// # #[allow(unused_mut)]
863    /// let mut vec: Vec<i32, _> = Vec::new_in(System);
864    /// ```
865    #[inline]
866    #[unstable(feature = "allocator_api", issue = "32838")]
867    pub const fn new_in(alloc: A) -> Self {
868        Vec { buf: RawVec::new_in(alloc), len: 0 }
869    }
870
871    /// Constructs a new, empty `Vec<T, A>` with at least the specified capacity
872    /// with the provided allocator.
873    ///
874    /// The vector will be able to hold at least `capacity` elements without
875    /// reallocating. This method is allowed to allocate for more elements than
876    /// `capacity`. If `capacity` is zero, the vector will not allocate.
877    ///
878    /// It is important to note that although the returned vector has the
879    /// minimum *capacity* specified, the vector will have a zero *length*. For
880    /// an explanation of the difference between length and capacity, see
881    /// *[Capacity and reallocation]*.
882    ///
883    /// If it is important to know the exact allocated capacity of a `Vec`,
884    /// always use the [`capacity`] method after construction.
885    ///
886    /// For `Vec<T, A>` where `T` is a zero-sized type, there will be no allocation
887    /// and the capacity will always be `usize::MAX`.
888    ///
889    /// [Capacity and reallocation]: #capacity-and-reallocation
890    /// [`capacity`]: Vec::capacity
891    ///
892    /// # Panics
893    ///
894    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
895    ///
896    /// # Examples
897    ///
898    /// ```
899    /// #![feature(allocator_api)]
900    ///
901    /// use std::alloc::System;
902    ///
903    /// let mut vec = Vec::with_capacity_in(10, System);
904    ///
905    /// // The vector contains no items, even though it has capacity for more
906    /// assert_eq!(vec.len(), 0);
907    /// assert!(vec.capacity() >= 10);
908    ///
909    /// // These are all done without reallocating...
910    /// for i in 0..10 {
911    ///     vec.push(i);
912    /// }
913    /// assert_eq!(vec.len(), 10);
914    /// assert!(vec.capacity() >= 10);
915    ///
916    /// // ...but this may make the vector reallocate
917    /// vec.push(11);
918    /// assert_eq!(vec.len(), 11);
919    /// assert!(vec.capacity() >= 11);
920    ///
921    /// // A vector of a zero-sized type will always over-allocate, since no
922    /// // allocation is necessary
923    /// let vec_units = Vec::<(), System>::with_capacity_in(10, System);
924    /// assert_eq!(vec_units.capacity(), usize::MAX);
925    /// ```
926    #[cfg(not(no_global_oom_handling))]
927    #[inline]
928    #[unstable(feature = "allocator_api", issue = "32838")]
929    #[track_caller]
930    pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
931        Vec { buf: RawVec::with_capacity_in(capacity, alloc), len: 0 }
932    }
933
934    /// Constructs a new, empty `Vec<T, A>` with at least the specified capacity
935    /// with the provided allocator.
936    ///
937    /// The vector will be able to hold at least `capacity` elements without
938    /// reallocating. This method is allowed to allocate for more elements than
939    /// `capacity`. If `capacity` is zero, the vector will not allocate.
940    ///
941    /// # Errors
942    ///
943    /// Returns an error if the capacity exceeds `isize::MAX` _bytes_,
944    /// or if the allocator reports allocation failure.
945    #[inline]
946    #[unstable(feature = "allocator_api", issue = "32838")]
947    // #[unstable(feature = "try_with_capacity", issue = "91913")]
948    pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
949        Ok(Vec { buf: RawVec::try_with_capacity_in(capacity, alloc)?, len: 0 })
950    }
951
952    /// Creates a `Vec<T, A>` directly from a pointer, a length, a capacity,
953    /// and an allocator.
954    ///
955    /// # Safety
956    ///
957    /// This is highly unsafe, due to the number of invariants that aren't
958    /// checked:
959    ///
960    /// * `ptr` must be [*currently allocated*] via the given allocator `alloc`.
961    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
962    ///   (`T` having a less strict alignment is not sufficient, the alignment really
963    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
964    ///   allocated and deallocated with the same layout.)
965    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
966    ///   to be the same size as the pointer was allocated with. (Because similar to
967    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
968    /// * `length` needs to be less than or equal to `capacity`.
969    /// * The first `length` values must be properly initialized values of type `T`.
970    /// * `capacity` needs to [*fit*] the layout size that the pointer was allocated with.
971    /// * The allocated size in bytes must be no larger than `isize::MAX`.
972    ///   See the safety documentation of [`pointer::offset`].
973    ///
974    /// These requirements are always upheld by any `ptr` that has been allocated
975    /// via `Vec<T, A>`. Other allocation sources are allowed if the invariants are
976    /// upheld.
977    ///
978    /// Violating these may cause problems like corrupting the allocator's
979    /// internal data structures. For example it is **not** safe
980    /// to build a `Vec<u8>` from a pointer to a C `char` array with length `size_t`.
981    /// It's also not safe to build one from a `Vec<u16>` and its length, because
982    /// the allocator cares about the alignment, and these two types have different
983    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
984    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1.
985    ///
986    /// The ownership of `ptr` is effectively transferred to the
987    /// `Vec<T>` which may then deallocate, reallocate or change the
988    /// contents of memory pointed to by the pointer at will. Ensure
989    /// that nothing else uses the pointer after calling this
990    /// function.
991    ///
992    /// [`String`]: crate::string::String
993    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
994    /// [*currently allocated*]: crate::alloc::Allocator#currently-allocated-memory
995    /// [*fit*]: crate::alloc::Allocator#memory-fitting
996    ///
997    /// # Examples
998    ///
999    // FIXME Update this when vec_into_raw_parts is stabilized
1000    /// ```
1001    /// #![feature(allocator_api)]
1002    ///
1003    /// use std::alloc::System;
1004    ///
1005    /// use std::ptr;
1006    /// use std::mem;
1007    ///
1008    /// let mut v = Vec::with_capacity_in(3, System);
1009    /// v.push(1);
1010    /// v.push(2);
1011    /// v.push(3);
1012    ///
1013    /// // Prevent running `v`'s destructor so we are in complete control
1014    /// // of the allocation.
1015    /// let mut v = mem::ManuallyDrop::new(v);
1016    ///
1017    /// // Pull out the various important pieces of information about `v`
1018    /// let p = v.as_mut_ptr();
1019    /// let len = v.len();
1020    /// let cap = v.capacity();
1021    /// let alloc = v.allocator();
1022    ///
1023    /// unsafe {
1024    ///     // Overwrite memory with 4, 5, 6
1025    ///     for i in 0..len {
1026    ///         ptr::write(p.add(i), 4 + i);
1027    ///     }
1028    ///
1029    ///     // Put everything back together into a Vec
1030    ///     let rebuilt = Vec::from_raw_parts_in(p, len, cap, alloc.clone());
1031    ///     assert_eq!(rebuilt, [4, 5, 6]);
1032    /// }
1033    /// ```
1034    ///
1035    /// Using memory that was allocated elsewhere:
1036    ///
1037    /// ```rust
1038    /// #![feature(allocator_api)]
1039    ///
1040    /// use std::alloc::{AllocError, Allocator, Global, Layout};
1041    ///
1042    /// fn main() {
1043    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
1044    ///
1045    ///     let vec = unsafe {
1046    ///         let mem = match Global.allocate(layout) {
1047    ///             Ok(mem) => mem.cast::<u32>().as_ptr(),
1048    ///             Err(AllocError) => return,
1049    ///         };
1050    ///
1051    ///         mem.write(1_000_000);
1052    ///
1053    ///         Vec::from_raw_parts_in(mem, 1, 16, Global)
1054    ///     };
1055    ///
1056    ///     assert_eq!(vec, &[1_000_000]);
1057    ///     assert_eq!(vec.capacity(), 16);
1058    /// }
1059    /// ```
1060    #[inline]
1061    #[unstable(feature = "allocator_api", issue = "32838")]
1062    pub unsafe fn from_raw_parts_in(ptr: *mut T, length: usize, capacity: usize, alloc: A) -> Self {
1063        ub_checks::assert_unsafe_precondition!(
1064            check_library_ub,
1065            "Vec::from_raw_parts_in requires that length <= capacity",
1066            (length: usize = length, capacity: usize = capacity) => length <= capacity
1067        );
1068        unsafe { Vec { buf: RawVec::from_raw_parts_in(ptr, capacity, alloc), len: length } }
1069    }
1070
1071    #[doc(alias = "from_non_null_parts_in")]
1072    /// Creates a `Vec<T, A>` directly from a `NonNull` pointer, a length, a capacity,
1073    /// and an allocator.
1074    ///
1075    /// # Safety
1076    ///
1077    /// This is highly unsafe, due to the number of invariants that aren't
1078    /// checked:
1079    ///
1080    /// * `ptr` must be [*currently allocated*] via the given allocator `alloc`.
1081    /// * `T` needs to have the same alignment as what `ptr` was allocated with.
1082    ///   (`T` having a less strict alignment is not sufficient, the alignment really
1083    ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
1084    ///   allocated and deallocated with the same layout.)
1085    /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
1086    ///   to be the same size as the pointer was allocated with. (Because similar to
1087    ///   alignment, [`dealloc`] must be called with the same layout `size`.)
1088    /// * `length` needs to be less than or equal to `capacity`.
1089    /// * The first `length` values must be properly initialized values of type `T`.
1090    /// * `capacity` needs to [*fit*] the layout size that the pointer was allocated with.
1091    /// * The allocated size in bytes must be no larger than `isize::MAX`.
1092    ///   See the safety documentation of [`pointer::offset`].
1093    ///
1094    /// These requirements are always upheld by any `ptr` that has been allocated
1095    /// via `Vec<T, A>`. Other allocation sources are allowed if the invariants are
1096    /// upheld.
1097    ///
1098    /// Violating these may cause problems like corrupting the allocator's
1099    /// internal data structures. For example it is **not** safe
1100    /// to build a `Vec<u8>` from a pointer to a C `char` array with length `size_t`.
1101    /// It's also not safe to build one from a `Vec<u16>` and its length, because
1102    /// the allocator cares about the alignment, and these two types have different
1103    /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
1104    /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1.
1105    ///
1106    /// The ownership of `ptr` is effectively transferred to the
1107    /// `Vec<T>` which may then deallocate, reallocate or change the
1108    /// contents of memory pointed to by the pointer at will. Ensure
1109    /// that nothing else uses the pointer after calling this
1110    /// function.
1111    ///
1112    /// [`String`]: crate::string::String
1113    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
1114    /// [*currently allocated*]: crate::alloc::Allocator#currently-allocated-memory
1115    /// [*fit*]: crate::alloc::Allocator#memory-fitting
1116    ///
1117    /// # Examples
1118    ///
1119    // FIXME Update this when vec_into_raw_parts is stabilized
1120    /// ```
1121    /// #![feature(allocator_api, box_vec_non_null)]
1122    ///
1123    /// use std::alloc::System;
1124    ///
1125    /// use std::ptr::NonNull;
1126    /// use std::mem;
1127    ///
1128    /// let mut v = Vec::with_capacity_in(3, System);
1129    /// v.push(1);
1130    /// v.push(2);
1131    /// v.push(3);
1132    ///
1133    /// // Prevent running `v`'s destructor so we are in complete control
1134    /// // of the allocation.
1135    /// let mut v = mem::ManuallyDrop::new(v);
1136    ///
1137    /// // Pull out the various important pieces of information about `v`
1138    /// let p = unsafe { NonNull::new_unchecked(v.as_mut_ptr()) };
1139    /// let len = v.len();
1140    /// let cap = v.capacity();
1141    /// let alloc = v.allocator();
1142    ///
1143    /// unsafe {
1144    ///     // Overwrite memory with 4, 5, 6
1145    ///     for i in 0..len {
1146    ///         p.add(i).write(4 + i);
1147    ///     }
1148    ///
1149    ///     // Put everything back together into a Vec
1150    ///     let rebuilt = Vec::from_parts_in(p, len, cap, alloc.clone());
1151    ///     assert_eq!(rebuilt, [4, 5, 6]);
1152    /// }
1153    /// ```
1154    ///
1155    /// Using memory that was allocated elsewhere:
1156    ///
1157    /// ```rust
1158    /// #![feature(allocator_api, box_vec_non_null)]
1159    ///
1160    /// use std::alloc::{AllocError, Allocator, Global, Layout};
1161    ///
1162    /// fn main() {
1163    ///     let layout = Layout::array::<u32>(16).expect("overflow cannot happen");
1164    ///
1165    ///     let vec = unsafe {
1166    ///         let mem = match Global.allocate(layout) {
1167    ///             Ok(mem) => mem.cast::<u32>(),
1168    ///             Err(AllocError) => return,
1169    ///         };
1170    ///
1171    ///         mem.write(1_000_000);
1172    ///
1173    ///         Vec::from_parts_in(mem, 1, 16, Global)
1174    ///     };
1175    ///
1176    ///     assert_eq!(vec, &[1_000_000]);
1177    ///     assert_eq!(vec.capacity(), 16);
1178    /// }
1179    /// ```
1180    #[inline]
1181    #[unstable(feature = "allocator_api", reason = "new API", issue = "32838")]
1182    // #[unstable(feature = "box_vec_non_null", issue = "130364")]
1183    pub unsafe fn from_parts_in(ptr: NonNull<T>, length: usize, capacity: usize, alloc: A) -> Self {
1184        ub_checks::assert_unsafe_precondition!(
1185            check_library_ub,
1186            "Vec::from_parts_in requires that length <= capacity",
1187            (length: usize = length, capacity: usize = capacity) => length <= capacity
1188        );
1189        unsafe { Vec { buf: RawVec::from_nonnull_in(ptr, capacity, alloc), len: length } }
1190    }
1191
1192    /// Decomposes a `Vec<T>` into its raw components: `(pointer, length, capacity, allocator)`.
1193    ///
1194    /// Returns the raw pointer to the underlying data, the length of the vector (in elements),
1195    /// the allocated capacity of the data (in elements), and the allocator. These are the same
1196    /// arguments in the same order as the arguments to [`from_raw_parts_in`].
1197    ///
1198    /// After calling this function, the caller is responsible for the
1199    /// memory previously managed by the `Vec`. The only way to do
1200    /// this is to convert the raw pointer, length, and capacity back
1201    /// into a `Vec` with the [`from_raw_parts_in`] function, allowing
1202    /// the destructor to perform the cleanup.
1203    ///
1204    /// [`from_raw_parts_in`]: Vec::from_raw_parts_in
1205    ///
1206    /// # Examples
1207    ///
1208    /// ```
1209    /// #![feature(allocator_api, vec_into_raw_parts)]
1210    ///
1211    /// use std::alloc::System;
1212    ///
1213    /// let mut v: Vec<i32, System> = Vec::new_in(System);
1214    /// v.push(-1);
1215    /// v.push(0);
1216    /// v.push(1);
1217    ///
1218    /// let (ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
1219    ///
1220    /// let rebuilt = unsafe {
1221    ///     // We can now make changes to the components, such as
1222    ///     // transmuting the raw pointer to a compatible type.
1223    ///     let ptr = ptr as *mut u32;
1224    ///
1225    ///     Vec::from_raw_parts_in(ptr, len, cap, alloc)
1226    /// };
1227    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
1228    /// ```
1229    #[must_use = "losing the pointer will leak memory"]
1230    #[unstable(feature = "allocator_api", issue = "32838")]
1231    // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
1232    pub fn into_raw_parts_with_alloc(self) -> (*mut T, usize, usize, A) {
1233        let mut me = ManuallyDrop::new(self);
1234        let len = me.len();
1235        let capacity = me.capacity();
1236        let ptr = me.as_mut_ptr();
1237        let alloc = unsafe { ptr::read(me.allocator()) };
1238        (ptr, len, capacity, alloc)
1239    }
1240
1241    #[doc(alias = "into_non_null_parts_with_alloc")]
1242    /// Decomposes a `Vec<T>` into its raw components: `(NonNull pointer, length, capacity, allocator)`.
1243    ///
1244    /// Returns the `NonNull` pointer to the underlying data, the length of the vector (in elements),
1245    /// the allocated capacity of the data (in elements), and the allocator. These are the same
1246    /// arguments in the same order as the arguments to [`from_parts_in`].
1247    ///
1248    /// After calling this function, the caller is responsible for the
1249    /// memory previously managed by the `Vec`. The only way to do
1250    /// this is to convert the `NonNull` pointer, length, and capacity back
1251    /// into a `Vec` with the [`from_parts_in`] function, allowing
1252    /// the destructor to perform the cleanup.
1253    ///
1254    /// [`from_parts_in`]: Vec::from_parts_in
1255    ///
1256    /// # Examples
1257    ///
1258    /// ```
1259    /// #![feature(allocator_api, vec_into_raw_parts, box_vec_non_null)]
1260    ///
1261    /// use std::alloc::System;
1262    ///
1263    /// let mut v: Vec<i32, System> = Vec::new_in(System);
1264    /// v.push(-1);
1265    /// v.push(0);
1266    /// v.push(1);
1267    ///
1268    /// let (ptr, len, cap, alloc) = v.into_parts_with_alloc();
1269    ///
1270    /// let rebuilt = unsafe {
1271    ///     // We can now make changes to the components, such as
1272    ///     // transmuting the raw pointer to a compatible type.
1273    ///     let ptr = ptr.cast::<u32>();
1274    ///
1275    ///     Vec::from_parts_in(ptr, len, cap, alloc)
1276    /// };
1277    /// assert_eq!(rebuilt, [4294967295, 0, 1]);
1278    /// ```
1279    #[must_use = "losing the pointer will leak memory"]
1280    #[unstable(feature = "allocator_api", issue = "32838")]
1281    // #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1282    // #[unstable(feature = "vec_into_raw_parts", reason = "new API", issue = "65816")]
1283    pub fn into_parts_with_alloc(self) -> (NonNull<T>, usize, usize, A) {
1284        let (ptr, len, capacity, alloc) = self.into_raw_parts_with_alloc();
1285        // SAFETY: A `Vec` always has a non-null pointer.
1286        (unsafe { NonNull::new_unchecked(ptr) }, len, capacity, alloc)
1287    }
1288
1289    /// Returns the total number of elements the vector can hold without
1290    /// reallocating.
1291    ///
1292    /// # Examples
1293    ///
1294    /// ```
1295    /// let mut vec: Vec<i32> = Vec::with_capacity(10);
1296    /// vec.push(42);
1297    /// assert!(vec.capacity() >= 10);
1298    /// ```
1299    ///
1300    /// A vector with zero-sized elements will always have a capacity of usize::MAX:
1301    ///
1302    /// ```
1303    /// #[derive(Clone)]
1304    /// struct ZeroSized;
1305    ///
1306    /// fn main() {
1307    ///     assert_eq!(std::mem::size_of::<ZeroSized>(), 0);
1308    ///     let v = vec![ZeroSized; 0];
1309    ///     assert_eq!(v.capacity(), usize::MAX);
1310    /// }
1311    /// ```
1312    #[inline]
1313    #[stable(feature = "rust1", since = "1.0.0")]
1314    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1315    pub const fn capacity(&self) -> usize {
1316        self.buf.capacity()
1317    }
1318
1319    /// Reserves capacity for at least `additional` more elements to be inserted
1320    /// in the given `Vec<T>`. The collection may reserve more space to
1321    /// speculatively avoid frequent reallocations. After calling `reserve`,
1322    /// capacity will be greater than or equal to `self.len() + additional`.
1323    /// Does nothing if capacity is already sufficient.
1324    ///
1325    /// # Panics
1326    ///
1327    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
1328    ///
1329    /// # Examples
1330    ///
1331    /// ```
1332    /// let mut vec = vec![1];
1333    /// vec.reserve(10);
1334    /// assert!(vec.capacity() >= 11);
1335    /// ```
1336    #[cfg(not(no_global_oom_handling))]
1337    #[stable(feature = "rust1", since = "1.0.0")]
1338    #[track_caller]
1339    #[rustc_diagnostic_item = "vec_reserve"]
1340    pub fn reserve(&mut self, additional: usize) {
1341        self.buf.reserve(self.len, additional);
1342    }
1343
1344    /// Reserves the minimum capacity for at least `additional` more elements to
1345    /// be inserted in the given `Vec<T>`. Unlike [`reserve`], this will not
1346    /// deliberately over-allocate to speculatively avoid frequent allocations.
1347    /// After calling `reserve_exact`, capacity will be greater than or equal to
1348    /// `self.len() + additional`. Does nothing if the capacity is already
1349    /// sufficient.
1350    ///
1351    /// Note that the allocator may give the collection more space than it
1352    /// requests. Therefore, capacity can not be relied upon to be precisely
1353    /// minimal. Prefer [`reserve`] if future insertions are expected.
1354    ///
1355    /// [`reserve`]: Vec::reserve
1356    ///
1357    /// # Panics
1358    ///
1359    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
1360    ///
1361    /// # Examples
1362    ///
1363    /// ```
1364    /// let mut vec = vec![1];
1365    /// vec.reserve_exact(10);
1366    /// assert!(vec.capacity() >= 11);
1367    /// ```
1368    #[cfg(not(no_global_oom_handling))]
1369    #[stable(feature = "rust1", since = "1.0.0")]
1370    #[track_caller]
1371    pub fn reserve_exact(&mut self, additional: usize) {
1372        self.buf.reserve_exact(self.len, additional);
1373    }
1374
1375    /// Tries to reserve capacity for at least `additional` more elements to be inserted
1376    /// in the given `Vec<T>`. The collection may reserve more space to speculatively avoid
1377    /// frequent reallocations. After calling `try_reserve`, capacity will be
1378    /// greater than or equal to `self.len() + additional` if it returns
1379    /// `Ok(())`. Does nothing if capacity is already sufficient. This method
1380    /// preserves the contents even if an error occurs.
1381    ///
1382    /// # Errors
1383    ///
1384    /// If the capacity overflows, or the allocator reports a failure, then an error
1385    /// is returned.
1386    ///
1387    /// # Examples
1388    ///
1389    /// ```
1390    /// use std::collections::TryReserveError;
1391    ///
1392    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
1393    ///     let mut output = Vec::new();
1394    ///
1395    ///     // Pre-reserve the memory, exiting if we can't
1396    ///     output.try_reserve(data.len())?;
1397    ///
1398    ///     // Now we know this can't OOM in the middle of our complex work
1399    ///     output.extend(data.iter().map(|&val| {
1400    ///         val * 2 + 5 // very complicated
1401    ///     }));
1402    ///
1403    ///     Ok(output)
1404    /// }
1405    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
1406    /// ```
1407    #[stable(feature = "try_reserve", since = "1.57.0")]
1408    pub fn try_reserve(&mut self, additional: usize) -> Result<(), TryReserveError> {
1409        self.buf.try_reserve(self.len, additional)
1410    }
1411
1412    /// Tries to reserve the minimum capacity for at least `additional`
1413    /// elements to be inserted in the given `Vec<T>`. Unlike [`try_reserve`],
1414    /// this will not deliberately over-allocate to speculatively avoid frequent
1415    /// allocations. After calling `try_reserve_exact`, capacity will be greater
1416    /// than or equal to `self.len() + additional` if it returns `Ok(())`.
1417    /// Does nothing if the capacity is already sufficient.
1418    ///
1419    /// Note that the allocator may give the collection more space than it
1420    /// requests. Therefore, capacity can not be relied upon to be precisely
1421    /// minimal. Prefer [`try_reserve`] if future insertions are expected.
1422    ///
1423    /// [`try_reserve`]: Vec::try_reserve
1424    ///
1425    /// # Errors
1426    ///
1427    /// If the capacity overflows, or the allocator reports a failure, then an error
1428    /// is returned.
1429    ///
1430    /// # Examples
1431    ///
1432    /// ```
1433    /// use std::collections::TryReserveError;
1434    ///
1435    /// fn process_data(data: &[u32]) -> Result<Vec<u32>, TryReserveError> {
1436    ///     let mut output = Vec::new();
1437    ///
1438    ///     // Pre-reserve the memory, exiting if we can't
1439    ///     output.try_reserve_exact(data.len())?;
1440    ///
1441    ///     // Now we know this can't OOM in the middle of our complex work
1442    ///     output.extend(data.iter().map(|&val| {
1443    ///         val * 2 + 5 // very complicated
1444    ///     }));
1445    ///
1446    ///     Ok(output)
1447    /// }
1448    /// # process_data(&[1, 2, 3]).expect("why is the test harness OOMing on 12 bytes?");
1449    /// ```
1450    #[stable(feature = "try_reserve", since = "1.57.0")]
1451    pub fn try_reserve_exact(&mut self, additional: usize) -> Result<(), TryReserveError> {
1452        self.buf.try_reserve_exact(self.len, additional)
1453    }
1454
1455    /// Shrinks the capacity of the vector as much as possible.
1456    ///
1457    /// The behavior of this method depends on the allocator, which may either shrink the vector
1458    /// in-place or reallocate. The resulting vector might still have some excess capacity, just as
1459    /// is the case for [`with_capacity`]. See [`Allocator::shrink`] for more details.
1460    ///
1461    /// [`with_capacity`]: Vec::with_capacity
1462    ///
1463    /// # Examples
1464    ///
1465    /// ```
1466    /// let mut vec = Vec::with_capacity(10);
1467    /// vec.extend([1, 2, 3]);
1468    /// assert!(vec.capacity() >= 10);
1469    /// vec.shrink_to_fit();
1470    /// assert!(vec.capacity() >= 3);
1471    /// ```
1472    #[cfg(not(no_global_oom_handling))]
1473    #[stable(feature = "rust1", since = "1.0.0")]
1474    #[track_caller]
1475    #[inline]
1476    pub fn shrink_to_fit(&mut self) {
1477        // The capacity is never less than the length, and there's nothing to do when
1478        // they are equal, so we can avoid the panic case in `RawVec::shrink_to_fit`
1479        // by only calling it with a greater capacity.
1480        if self.capacity() > self.len {
1481            self.buf.shrink_to_fit(self.len);
1482        }
1483    }
1484
1485    /// Shrinks the capacity of the vector with a lower bound.
1486    ///
1487    /// The capacity will remain at least as large as both the length
1488    /// and the supplied value.
1489    ///
1490    /// If the current capacity is less than the lower limit, this is a no-op.
1491    ///
1492    /// # Examples
1493    ///
1494    /// ```
1495    /// let mut vec = Vec::with_capacity(10);
1496    /// vec.extend([1, 2, 3]);
1497    /// assert!(vec.capacity() >= 10);
1498    /// vec.shrink_to(4);
1499    /// assert!(vec.capacity() >= 4);
1500    /// vec.shrink_to(0);
1501    /// assert!(vec.capacity() >= 3);
1502    /// ```
1503    #[cfg(not(no_global_oom_handling))]
1504    #[stable(feature = "shrink_to", since = "1.56.0")]
1505    #[track_caller]
1506    pub fn shrink_to(&mut self, min_capacity: usize) {
1507        if self.capacity() > min_capacity {
1508            self.buf.shrink_to_fit(cmp::max(self.len, min_capacity));
1509        }
1510    }
1511
1512    /// Converts the vector into [`Box<[T]>`][owned slice].
1513    ///
1514    /// Before doing the conversion, this method discards excess capacity like [`shrink_to_fit`].
1515    ///
1516    /// [owned slice]: Box
1517    /// [`shrink_to_fit`]: Vec::shrink_to_fit
1518    ///
1519    /// # Examples
1520    ///
1521    /// ```
1522    /// let v = vec![1, 2, 3];
1523    ///
1524    /// let slice = v.into_boxed_slice();
1525    /// ```
1526    ///
1527    /// Any excess capacity is removed:
1528    ///
1529    /// ```
1530    /// let mut vec = Vec::with_capacity(10);
1531    /// vec.extend([1, 2, 3]);
1532    ///
1533    /// assert!(vec.capacity() >= 10);
1534    /// let slice = vec.into_boxed_slice();
1535    /// assert_eq!(slice.into_vec().capacity(), 3);
1536    /// ```
1537    #[cfg(not(no_global_oom_handling))]
1538    #[stable(feature = "rust1", since = "1.0.0")]
1539    #[track_caller]
1540    pub fn into_boxed_slice(mut self) -> Box<[T], A> {
1541        unsafe {
1542            self.shrink_to_fit();
1543            let me = ManuallyDrop::new(self);
1544            let buf = ptr::read(&me.buf);
1545            let len = me.len();
1546            buf.into_box(len).assume_init()
1547        }
1548    }
1549
1550    /// Shortens the vector, keeping the first `len` elements and dropping
1551    /// the rest.
1552    ///
1553    /// If `len` is greater or equal to the vector's current length, this has
1554    /// no effect.
1555    ///
1556    /// The [`drain`] method can emulate `truncate`, but causes the excess
1557    /// elements to be returned instead of dropped.
1558    ///
1559    /// Note that this method has no effect on the allocated capacity
1560    /// of the vector.
1561    ///
1562    /// # Examples
1563    ///
1564    /// Truncating a five element vector to two elements:
1565    ///
1566    /// ```
1567    /// let mut vec = vec![1, 2, 3, 4, 5];
1568    /// vec.truncate(2);
1569    /// assert_eq!(vec, [1, 2]);
1570    /// ```
1571    ///
1572    /// No truncation occurs when `len` is greater than the vector's current
1573    /// length:
1574    ///
1575    /// ```
1576    /// let mut vec = vec![1, 2, 3];
1577    /// vec.truncate(8);
1578    /// assert_eq!(vec, [1, 2, 3]);
1579    /// ```
1580    ///
1581    /// Truncating when `len == 0` is equivalent to calling the [`clear`]
1582    /// method.
1583    ///
1584    /// ```
1585    /// let mut vec = vec![1, 2, 3];
1586    /// vec.truncate(0);
1587    /// assert_eq!(vec, []);
1588    /// ```
1589    ///
1590    /// [`clear`]: Vec::clear
1591    /// [`drain`]: Vec::drain
1592    #[stable(feature = "rust1", since = "1.0.0")]
1593    pub fn truncate(&mut self, len: usize) {
1594        // This is safe because:
1595        //
1596        // * the slice passed to `drop_in_place` is valid; the `len > self.len`
1597        //   case avoids creating an invalid slice, and
1598        // * the `len` of the vector is shrunk before calling `drop_in_place`,
1599        //   such that no value will be dropped twice in case `drop_in_place`
1600        //   were to panic once (if it panics twice, the program aborts).
1601        unsafe {
1602            // Note: It's intentional that this is `>` and not `>=`.
1603            //       Changing it to `>=` has negative performance
1604            //       implications in some cases. See #78884 for more.
1605            if len > self.len {
1606                return;
1607            }
1608            let remaining_len = self.len - len;
1609            let s = ptr::slice_from_raw_parts_mut(self.as_mut_ptr().add(len), remaining_len);
1610            self.len = len;
1611            ptr::drop_in_place(s);
1612        }
1613    }
1614
1615    /// Extracts a slice containing the entire vector.
1616    ///
1617    /// Equivalent to `&s[..]`.
1618    ///
1619    /// # Examples
1620    ///
1621    /// ```
1622    /// use std::io::{self, Write};
1623    /// let buffer = vec![1, 2, 3, 5, 8];
1624    /// io::sink().write(buffer.as_slice()).unwrap();
1625    /// ```
1626    #[inline]
1627    #[stable(feature = "vec_as_slice", since = "1.7.0")]
1628    #[rustc_diagnostic_item = "vec_as_slice"]
1629    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1630    pub const fn as_slice(&self) -> &[T] {
1631        // SAFETY: `slice::from_raw_parts` requires pointee is a contiguous, aligned buffer of size
1632        // `len` containing properly-initialized `T`s. Data must not be mutated for the returned
1633        // lifetime. Further, `len * size_of::<T>` <= `isize::MAX`, and allocation does not
1634        // "wrap" through overflowing memory addresses.
1635        //
1636        // * Vec API guarantees that self.buf:
1637        //      * contains only properly-initialized items within 0..len
1638        //      * is aligned, contiguous, and valid for `len` reads
1639        //      * obeys size and address-wrapping constraints
1640        //
1641        // * We only construct `&mut` references to `self.buf` through `&mut self` methods; borrow-
1642        //   check ensures that it is not possible to mutably alias `self.buf` within the
1643        //   returned lifetime.
1644        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
1645    }
1646
1647    /// Extracts a mutable slice of the entire vector.
1648    ///
1649    /// Equivalent to `&mut s[..]`.
1650    ///
1651    /// # Examples
1652    ///
1653    /// ```
1654    /// use std::io::{self, Read};
1655    /// let mut buffer = vec![0; 3];
1656    /// io::repeat(0b101).read_exact(buffer.as_mut_slice()).unwrap();
1657    /// ```
1658    #[inline]
1659    #[stable(feature = "vec_as_slice", since = "1.7.0")]
1660    #[rustc_diagnostic_item = "vec_as_mut_slice"]
1661    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1662    pub const fn as_mut_slice(&mut self) -> &mut [T] {
1663        // SAFETY: `slice::from_raw_parts_mut` requires pointee is a contiguous, aligned buffer of
1664        // size `len` containing properly-initialized `T`s. Data must not be accessed through any
1665        // other pointer for the returned lifetime. Further, `len * size_of::<T>` <=
1666        // `ISIZE::MAX` and allocation does not "wrap" through overflowing memory addresses.
1667        //
1668        // * Vec API guarantees that self.buf:
1669        //      * contains only properly-initialized items within 0..len
1670        //      * is aligned, contiguous, and valid for `len` reads
1671        //      * obeys size and address-wrapping constraints
1672        //
1673        // * We only construct references to `self.buf` through `&self` and `&mut self` methods;
1674        //   borrow-check ensures that it is not possible to construct a reference to `self.buf`
1675        //   within the returned lifetime.
1676        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
1677    }
1678
1679    /// Returns a raw pointer to the vector's buffer, or a dangling raw pointer
1680    /// valid for zero sized reads if the vector didn't allocate.
1681    ///
1682    /// The caller must ensure that the vector outlives the pointer this
1683    /// function returns, or else it will end up dangling.
1684    /// Modifying the vector may cause its buffer to be reallocated,
1685    /// which would also make any pointers to it invalid.
1686    ///
1687    /// The caller must also ensure that the memory the pointer (non-transitively) points to
1688    /// is never written to (except inside an `UnsafeCell`) using this pointer or any pointer
1689    /// derived from it. If you need to mutate the contents of the slice, use [`as_mut_ptr`].
1690    ///
1691    /// This method guarantees that for the purpose of the aliasing model, this method
1692    /// does not materialize a reference to the underlying slice, and thus the returned pointer
1693    /// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`],
1694    /// and [`as_non_null`].
1695    /// Note that calling other methods that materialize mutable references to the slice,
1696    /// or mutable references to specific elements you are planning on accessing through this pointer,
1697    /// as well as writing to those elements, may still invalidate this pointer.
1698    /// See the second example below for how this guarantee can be used.
1699    ///
1700    ///
1701    /// # Examples
1702    ///
1703    /// ```
1704    /// let x = vec![1, 2, 4];
1705    /// let x_ptr = x.as_ptr();
1706    ///
1707    /// unsafe {
1708    ///     for i in 0..x.len() {
1709    ///         assert_eq!(*x_ptr.add(i), 1 << i);
1710    ///     }
1711    /// }
1712    /// ```
1713    ///
1714    /// Due to the aliasing guarantee, the following code is legal:
1715    ///
1716    /// ```rust
1717    /// unsafe {
1718    ///     let mut v = vec![0, 1, 2];
1719    ///     let ptr1 = v.as_ptr();
1720    ///     let _ = ptr1.read();
1721    ///     let ptr2 = v.as_mut_ptr().offset(2);
1722    ///     ptr2.write(2);
1723    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`
1724    ///     // because it mutated a different element:
1725    ///     let _ = ptr1.read();
1726    /// }
1727    /// ```
1728    ///
1729    /// [`as_mut_ptr`]: Vec::as_mut_ptr
1730    /// [`as_ptr`]: Vec::as_ptr
1731    /// [`as_non_null`]: Vec::as_non_null
1732    #[stable(feature = "vec_as_ptr", since = "1.37.0")]
1733    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1734    #[rustc_never_returns_null_ptr]
1735    #[rustc_as_ptr]
1736    #[inline]
1737    pub const fn as_ptr(&self) -> *const T {
1738        // We shadow the slice method of the same name to avoid going through
1739        // `deref`, which creates an intermediate reference.
1740        self.buf.ptr()
1741    }
1742
1743    /// Returns a raw mutable pointer to the vector's buffer, or a dangling
1744    /// raw pointer valid for zero sized reads if the vector didn't allocate.
1745    ///
1746    /// The caller must ensure that the vector outlives the pointer this
1747    /// function returns, or else it will end up dangling.
1748    /// Modifying the vector may cause its buffer to be reallocated,
1749    /// which would also make any pointers to it invalid.
1750    ///
1751    /// This method guarantees that for the purpose of the aliasing model, this method
1752    /// does not materialize a reference to the underlying slice, and thus the returned pointer
1753    /// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`],
1754    /// and [`as_non_null`].
1755    /// Note that calling other methods that materialize references to the slice,
1756    /// or references to specific elements you are planning on accessing through this pointer,
1757    /// may still invalidate this pointer.
1758    /// See the second example below for how this guarantee can be used.
1759    ///
1760    /// The method also guarantees that, as long as `T` is not zero-sized and the capacity is
1761    /// nonzero, the pointer may be passed into [`dealloc`] with a layout of
1762    /// `Layout::array::<T>(capacity)` in order to deallocate the backing memory. If this is done,
1763    /// be careful not to run the destructor of the `Vec`, as dropping it will result in
1764    /// double-frees. Wrapping the `Vec` in a [`ManuallyDrop`] is the typical way to achieve this.
1765    ///
1766    /// # Examples
1767    ///
1768    /// ```
1769    /// // Allocate vector big enough for 4 elements.
1770    /// let size = 4;
1771    /// let mut x: Vec<i32> = Vec::with_capacity(size);
1772    /// let x_ptr = x.as_mut_ptr();
1773    ///
1774    /// // Initialize elements via raw pointer writes, then set length.
1775    /// unsafe {
1776    ///     for i in 0..size {
1777    ///         *x_ptr.add(i) = i as i32;
1778    ///     }
1779    ///     x.set_len(size);
1780    /// }
1781    /// assert_eq!(&*x, &[0, 1, 2, 3]);
1782    /// ```
1783    ///
1784    /// Due to the aliasing guarantee, the following code is legal:
1785    ///
1786    /// ```rust
1787    /// unsafe {
1788    ///     let mut v = vec![0];
1789    ///     let ptr1 = v.as_mut_ptr();
1790    ///     ptr1.write(1);
1791    ///     let ptr2 = v.as_mut_ptr();
1792    ///     ptr2.write(2);
1793    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
1794    ///     ptr1.write(3);
1795    /// }
1796    /// ```
1797    ///
1798    /// Deallocating a vector using [`Box`] (which uses [`dealloc`] internally):
1799    ///
1800    /// ```
1801    /// use std::mem::{ManuallyDrop, MaybeUninit};
1802    ///
1803    /// let mut v = ManuallyDrop::new(vec![0, 1, 2]);
1804    /// let ptr = v.as_mut_ptr();
1805    /// let capacity = v.capacity();
1806    /// let slice_ptr: *mut [MaybeUninit<i32>] =
1807    ///     std::ptr::slice_from_raw_parts_mut(ptr.cast(), capacity);
1808    /// drop(unsafe { Box::from_raw(slice_ptr) });
1809    /// ```
1810    ///
1811    /// [`as_mut_ptr`]: Vec::as_mut_ptr
1812    /// [`as_ptr`]: Vec::as_ptr
1813    /// [`as_non_null`]: Vec::as_non_null
1814    /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
1815    /// [`ManuallyDrop`]: core::mem::ManuallyDrop
1816    #[stable(feature = "vec_as_ptr", since = "1.37.0")]
1817    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
1818    #[rustc_never_returns_null_ptr]
1819    #[rustc_as_ptr]
1820    #[inline]
1821    pub const fn as_mut_ptr(&mut self) -> *mut T {
1822        // We shadow the slice method of the same name to avoid going through
1823        // `deref_mut`, which creates an intermediate reference.
1824        self.buf.ptr()
1825    }
1826
1827    /// Returns a `NonNull` pointer to the vector's buffer, or a dangling
1828    /// `NonNull` pointer valid for zero sized reads if the vector didn't allocate.
1829    ///
1830    /// The caller must ensure that the vector outlives the pointer this
1831    /// function returns, or else it will end up dangling.
1832    /// Modifying the vector may cause its buffer to be reallocated,
1833    /// which would also make any pointers to it invalid.
1834    ///
1835    /// This method guarantees that for the purpose of the aliasing model, this method
1836    /// does not materialize a reference to the underlying slice, and thus the returned pointer
1837    /// will remain valid when mixed with other calls to [`as_ptr`], [`as_mut_ptr`],
1838    /// and [`as_non_null`].
1839    /// Note that calling other methods that materialize references to the slice,
1840    /// or references to specific elements you are planning on accessing through this pointer,
1841    /// may still invalidate this pointer.
1842    /// See the second example below for how this guarantee can be used.
1843    ///
1844    /// # Examples
1845    ///
1846    /// ```
1847    /// #![feature(box_vec_non_null)]
1848    ///
1849    /// // Allocate vector big enough for 4 elements.
1850    /// let size = 4;
1851    /// let mut x: Vec<i32> = Vec::with_capacity(size);
1852    /// let x_ptr = x.as_non_null();
1853    ///
1854    /// // Initialize elements via raw pointer writes, then set length.
1855    /// unsafe {
1856    ///     for i in 0..size {
1857    ///         x_ptr.add(i).write(i as i32);
1858    ///     }
1859    ///     x.set_len(size);
1860    /// }
1861    /// assert_eq!(&*x, &[0, 1, 2, 3]);
1862    /// ```
1863    ///
1864    /// Due to the aliasing guarantee, the following code is legal:
1865    ///
1866    /// ```rust
1867    /// #![feature(box_vec_non_null)]
1868    ///
1869    /// unsafe {
1870    ///     let mut v = vec![0];
1871    ///     let ptr1 = v.as_non_null();
1872    ///     ptr1.write(1);
1873    ///     let ptr2 = v.as_non_null();
1874    ///     ptr2.write(2);
1875    ///     // Notably, the write to `ptr2` did *not* invalidate `ptr1`:
1876    ///     ptr1.write(3);
1877    /// }
1878    /// ```
1879    ///
1880    /// [`as_mut_ptr`]: Vec::as_mut_ptr
1881    /// [`as_ptr`]: Vec::as_ptr
1882    /// [`as_non_null`]: Vec::as_non_null
1883    #[unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1884    #[rustc_const_unstable(feature = "box_vec_non_null", reason = "new API", issue = "130364")]
1885    #[inline]
1886    pub const fn as_non_null(&mut self) -> NonNull<T> {
1887        self.buf.non_null()
1888    }
1889
1890    /// Returns a reference to the underlying allocator.
1891    #[unstable(feature = "allocator_api", issue = "32838")]
1892    #[inline]
1893    pub fn allocator(&self) -> &A {
1894        self.buf.allocator()
1895    }
1896
1897    /// Forces the length of the vector to `new_len`.
1898    ///
1899    /// This is a low-level operation that maintains none of the normal
1900    /// invariants of the type. Normally changing the length of a vector
1901    /// is done using one of the safe operations instead, such as
1902    /// [`truncate`], [`resize`], [`extend`], or [`clear`].
1903    ///
1904    /// [`truncate`]: Vec::truncate
1905    /// [`resize`]: Vec::resize
1906    /// [`extend`]: Extend::extend
1907    /// [`clear`]: Vec::clear
1908    ///
1909    /// # Safety
1910    ///
1911    /// - `new_len` must be less than or equal to [`capacity()`].
1912    /// - The elements at `old_len..new_len` must be initialized.
1913    ///
1914    /// [`capacity()`]: Vec::capacity
1915    ///
1916    /// # Examples
1917    ///
1918    /// See [`spare_capacity_mut()`] for an example with safe
1919    /// initialization of capacity elements and use of this method.
1920    ///
1921    /// `set_len()` can be useful for situations in which the vector
1922    /// is serving as a buffer for other code, particularly over FFI:
1923    ///
1924    /// ```no_run
1925    /// # #![allow(dead_code)]
1926    /// # // This is just a minimal skeleton for the doc example;
1927    /// # // don't use this as a starting point for a real library.
1928    /// # pub struct StreamWrapper { strm: *mut std::ffi::c_void }
1929    /// # const Z_OK: i32 = 0;
1930    /// # unsafe extern "C" {
1931    /// #     fn deflateGetDictionary(
1932    /// #         strm: *mut std::ffi::c_void,
1933    /// #         dictionary: *mut u8,
1934    /// #         dictLength: *mut usize,
1935    /// #     ) -> i32;
1936    /// # }
1937    /// # impl StreamWrapper {
1938    /// pub fn get_dictionary(&self) -> Option<Vec<u8>> {
1939    ///     // Per the FFI method's docs, "32768 bytes is always enough".
1940    ///     let mut dict = Vec::with_capacity(32_768);
1941    ///     let mut dict_length = 0;
1942    ///     // SAFETY: When `deflateGetDictionary` returns `Z_OK`, it holds that:
1943    ///     // 1. `dict_length` elements were initialized.
1944    ///     // 2. `dict_length` <= the capacity (32_768)
1945    ///     // which makes `set_len` safe to call.
1946    ///     unsafe {
1947    ///         // Make the FFI call...
1948    ///         let r = deflateGetDictionary(self.strm, dict.as_mut_ptr(), &mut dict_length);
1949    ///         if r == Z_OK {
1950    ///             // ...and update the length to what was initialized.
1951    ///             dict.set_len(dict_length);
1952    ///             Some(dict)
1953    ///         } else {
1954    ///             None
1955    ///         }
1956    ///     }
1957    /// }
1958    /// # }
1959    /// ```
1960    ///
1961    /// While the following example is sound, there is a memory leak since
1962    /// the inner vectors were not freed prior to the `set_len` call:
1963    ///
1964    /// ```
1965    /// let mut vec = vec![vec![1, 0, 0],
1966    ///                    vec![0, 1, 0],
1967    ///                    vec![0, 0, 1]];
1968    /// // SAFETY:
1969    /// // 1. `old_len..0` is empty so no elements need to be initialized.
1970    /// // 2. `0 <= capacity` always holds whatever `capacity` is.
1971    /// unsafe {
1972    ///     vec.set_len(0);
1973    /// #   // FIXME(https://github.com/rust-lang/miri/issues/3670):
1974    /// #   // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
1975    /// #   vec.set_len(3);
1976    /// }
1977    /// ```
1978    ///
1979    /// Normally, here, one would use [`clear`] instead to correctly drop
1980    /// the contents and thus not leak memory.
1981    ///
1982    /// [`spare_capacity_mut()`]: Vec::spare_capacity_mut
1983    #[inline]
1984    #[stable(feature = "rust1", since = "1.0.0")]
1985    pub unsafe fn set_len(&mut self, new_len: usize) {
1986        ub_checks::assert_unsafe_precondition!(
1987            check_library_ub,
1988            "Vec::set_len requires that new_len <= capacity()",
1989            (new_len: usize = new_len, capacity: usize = self.capacity()) => new_len <= capacity
1990        );
1991
1992        self.len = new_len;
1993    }
1994
1995    /// Removes an element from the vector and returns it.
1996    ///
1997    /// The removed element is replaced by the last element of the vector.
1998    ///
1999    /// This does not preserve ordering of the remaining elements, but is *O*(1).
2000    /// If you need to preserve the element order, use [`remove`] instead.
2001    ///
2002    /// [`remove`]: Vec::remove
2003    ///
2004    /// # Panics
2005    ///
2006    /// Panics if `index` is out of bounds.
2007    ///
2008    /// # Examples
2009    ///
2010    /// ```
2011    /// let mut v = vec!["foo", "bar", "baz", "qux"];
2012    ///
2013    /// assert_eq!(v.swap_remove(1), "bar");
2014    /// assert_eq!(v, ["foo", "qux", "baz"]);
2015    ///
2016    /// assert_eq!(v.swap_remove(0), "foo");
2017    /// assert_eq!(v, ["baz", "qux"]);
2018    /// ```
2019    #[inline]
2020    #[stable(feature = "rust1", since = "1.0.0")]
2021    pub fn swap_remove(&mut self, index: usize) -> T {
2022        #[cold]
2023        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2024        #[track_caller]
2025        #[optimize(size)]
2026        fn assert_failed(index: usize, len: usize) -> ! {
2027            panic!("swap_remove index (is {index}) should be < len (is {len})");
2028        }
2029
2030        let len = self.len();
2031        if index >= len {
2032            assert_failed(index, len);
2033        }
2034        unsafe {
2035            // We replace self[index] with the last element. Note that if the
2036            // bounds check above succeeds there must be a last element (which
2037            // can be self[index] itself).
2038            let value = ptr::read(self.as_ptr().add(index));
2039            let base_ptr = self.as_mut_ptr();
2040            ptr::copy(base_ptr.add(len - 1), base_ptr.add(index), 1);
2041            self.set_len(len - 1);
2042            value
2043        }
2044    }
2045
2046    /// Inserts an element at position `index` within the vector, shifting all
2047    /// elements after it to the right.
2048    ///
2049    /// # Panics
2050    ///
2051    /// Panics if `index > len`.
2052    ///
2053    /// # Examples
2054    ///
2055    /// ```
2056    /// let mut vec = vec!['a', 'b', 'c'];
2057    /// vec.insert(1, 'd');
2058    /// assert_eq!(vec, ['a', 'd', 'b', 'c']);
2059    /// vec.insert(4, 'e');
2060    /// assert_eq!(vec, ['a', 'd', 'b', 'c', 'e']);
2061    /// ```
2062    ///
2063    /// # Time complexity
2064    ///
2065    /// Takes *O*([`Vec::len`]) time. All items after the insertion index must be
2066    /// shifted to the right. In the worst case, all elements are shifted when
2067    /// the insertion index is 0.
2068    #[cfg(not(no_global_oom_handling))]
2069    #[stable(feature = "rust1", since = "1.0.0")]
2070    #[track_caller]
2071    pub fn insert(&mut self, index: usize, element: T) {
2072        let _ = self.insert_mut(index, element);
2073    }
2074
2075    /// Inserts an element at position `index` within the vector, shifting all
2076    /// elements after it to the right, and returning a reference to the new
2077    /// element.
2078    ///
2079    /// # Panics
2080    ///
2081    /// Panics if `index > len`.
2082    ///
2083    /// # Examples
2084    ///
2085    /// ```
2086    /// #![feature(push_mut)]
2087    /// let mut vec = vec![1, 3, 5, 9];
2088    /// let x = vec.insert_mut(3, 6);
2089    /// *x += 1;
2090    /// assert_eq!(vec, [1, 3, 5, 7, 9]);
2091    /// ```
2092    ///
2093    /// # Time complexity
2094    ///
2095    /// Takes *O*([`Vec::len`]) time. All items after the insertion index must be
2096    /// shifted to the right. In the worst case, all elements are shifted when
2097    /// the insertion index is 0.
2098    #[cfg(not(no_global_oom_handling))]
2099    #[inline]
2100    #[unstable(feature = "push_mut", issue = "135974")]
2101    #[track_caller]
2102    #[must_use = "if you don't need a reference to the value, use `Vec::insert` instead"]
2103    pub fn insert_mut(&mut self, index: usize, element: T) -> &mut T {
2104        #[cold]
2105        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2106        #[track_caller]
2107        #[optimize(size)]
2108        fn assert_failed(index: usize, len: usize) -> ! {
2109            panic!("insertion index (is {index}) should be <= len (is {len})");
2110        }
2111
2112        let len = self.len();
2113        if index > len {
2114            assert_failed(index, len);
2115        }
2116
2117        // space for the new element
2118        if len == self.buf.capacity() {
2119            self.buf.grow_one();
2120        }
2121
2122        unsafe {
2123            // infallible
2124            // The spot to put the new value
2125            let p = self.as_mut_ptr().add(index);
2126            {
2127                if index < len {
2128                    // Shift everything over to make space. (Duplicating the
2129                    // `index`th element into two consecutive places.)
2130                    ptr::copy(p, p.add(1), len - index);
2131                }
2132                // Write it in, overwriting the first copy of the `index`th
2133                // element.
2134                ptr::write(p, element);
2135            }
2136            self.set_len(len + 1);
2137            &mut *p
2138        }
2139    }
2140
2141    /// Removes and returns the element at position `index` within the vector,
2142    /// shifting all elements after it to the left.
2143    ///
2144    /// Note: Because this shifts over the remaining elements, it has a
2145    /// worst-case performance of *O*(*n*). If you don't need the order of elements
2146    /// to be preserved, use [`swap_remove`] instead. If you'd like to remove
2147    /// elements from the beginning of the `Vec`, consider using
2148    /// [`VecDeque::pop_front`] instead.
2149    ///
2150    /// [`swap_remove`]: Vec::swap_remove
2151    /// [`VecDeque::pop_front`]: crate::collections::VecDeque::pop_front
2152    ///
2153    /// # Panics
2154    ///
2155    /// Panics if `index` is out of bounds.
2156    ///
2157    /// # Examples
2158    ///
2159    /// ```
2160    /// let mut v = vec!['a', 'b', 'c'];
2161    /// assert_eq!(v.remove(1), 'b');
2162    /// assert_eq!(v, ['a', 'c']);
2163    /// ```
2164    #[stable(feature = "rust1", since = "1.0.0")]
2165    #[track_caller]
2166    #[rustc_confusables("delete", "take")]
2167    pub fn remove(&mut self, index: usize) -> T {
2168        #[cold]
2169        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2170        #[track_caller]
2171        #[optimize(size)]
2172        fn assert_failed(index: usize, len: usize) -> ! {
2173            panic!("removal index (is {index}) should be < len (is {len})");
2174        }
2175
2176        let len = self.len();
2177        if index >= len {
2178            assert_failed(index, len);
2179        }
2180        unsafe {
2181            // infallible
2182            let ret;
2183            {
2184                // the place we are taking from.
2185                let ptr = self.as_mut_ptr().add(index);
2186                // copy it out, unsafely having a copy of the value on
2187                // the stack and in the vector at the same time.
2188                ret = ptr::read(ptr);
2189
2190                // Shift everything down to fill in that spot.
2191                ptr::copy(ptr.add(1), ptr, len - index - 1);
2192            }
2193            self.set_len(len - 1);
2194            ret
2195        }
2196    }
2197
2198    /// Retains only the elements specified by the predicate.
2199    ///
2200    /// In other words, remove all elements `e` for which `f(&e)` returns `false`.
2201    /// This method operates in place, visiting each element exactly once in the
2202    /// original order, and preserves the order of the retained elements.
2203    ///
2204    /// # Examples
2205    ///
2206    /// ```
2207    /// let mut vec = vec![1, 2, 3, 4];
2208    /// vec.retain(|&x| x % 2 == 0);
2209    /// assert_eq!(vec, [2, 4]);
2210    /// ```
2211    ///
2212    /// Because the elements are visited exactly once in the original order,
2213    /// external state may be used to decide which elements to keep.
2214    ///
2215    /// ```
2216    /// let mut vec = vec![1, 2, 3, 4, 5];
2217    /// let keep = [false, true, true, false, true];
2218    /// let mut iter = keep.iter();
2219    /// vec.retain(|_| *iter.next().unwrap());
2220    /// assert_eq!(vec, [2, 3, 5]);
2221    /// ```
2222    #[stable(feature = "rust1", since = "1.0.0")]
2223    pub fn retain<F>(&mut self, mut f: F)
2224    where
2225        F: FnMut(&T) -> bool,
2226    {
2227        self.retain_mut(|elem| f(elem));
2228    }
2229
2230    /// Retains only the elements specified by the predicate, passing a mutable reference to it.
2231    ///
2232    /// In other words, remove all elements `e` such that `f(&mut e)` returns `false`.
2233    /// This method operates in place, visiting each element exactly once in the
2234    /// original order, and preserves the order of the retained elements.
2235    ///
2236    /// # Examples
2237    ///
2238    /// ```
2239    /// let mut vec = vec![1, 2, 3, 4];
2240    /// vec.retain_mut(|x| if *x <= 3 {
2241    ///     *x += 1;
2242    ///     true
2243    /// } else {
2244    ///     false
2245    /// });
2246    /// assert_eq!(vec, [2, 3, 4]);
2247    /// ```
2248    #[stable(feature = "vec_retain_mut", since = "1.61.0")]
2249    pub fn retain_mut<F>(&mut self, mut f: F)
2250    where
2251        F: FnMut(&mut T) -> bool,
2252    {
2253        let original_len = self.len();
2254
2255        if original_len == 0 {
2256            // Empty case: explicit return allows better optimization, vs letting compiler infer it
2257            return;
2258        }
2259
2260        // Avoid double drop if the drop guard is not executed,
2261        // since we may make some holes during the process.
2262        unsafe { self.set_len(0) };
2263
2264        // Vec: [Kept, Kept, Hole, Hole, Hole, Hole, Unchecked, Unchecked]
2265        //      |<-              processed len   ->| ^- next to check
2266        //                  |<-  deleted cnt     ->|
2267        //      |<-              original_len                          ->|
2268        // Kept: Elements which predicate returns true on.
2269        // Hole: Moved or dropped element slot.
2270        // Unchecked: Unchecked valid elements.
2271        //
2272        // This drop guard will be invoked when predicate or `drop` of element panicked.
2273        // It shifts unchecked elements to cover holes and `set_len` to the correct length.
2274        // In cases when predicate and `drop` never panick, it will be optimized out.
2275        struct BackshiftOnDrop<'a, T, A: Allocator> {
2276            v: &'a mut Vec<T, A>,
2277            processed_len: usize,
2278            deleted_cnt: usize,
2279            original_len: usize,
2280        }
2281
2282        impl<T, A: Allocator> Drop for BackshiftOnDrop<'_, T, A> {
2283            fn drop(&mut self) {
2284                if self.deleted_cnt > 0 {
2285                    // SAFETY: Trailing unchecked items must be valid since we never touch them.
2286                    unsafe {
2287                        ptr::copy(
2288                            self.v.as_ptr().add(self.processed_len),
2289                            self.v.as_mut_ptr().add(self.processed_len - self.deleted_cnt),
2290                            self.original_len - self.processed_len,
2291                        );
2292                    }
2293                }
2294                // SAFETY: After filling holes, all items are in contiguous memory.
2295                unsafe {
2296                    self.v.set_len(self.original_len - self.deleted_cnt);
2297                }
2298            }
2299        }
2300
2301        let mut g = BackshiftOnDrop { v: self, processed_len: 0, deleted_cnt: 0, original_len };
2302
2303        fn process_loop<F, T, A: Allocator, const DELETED: bool>(
2304            original_len: usize,
2305            f: &mut F,
2306            g: &mut BackshiftOnDrop<'_, T, A>,
2307        ) where
2308            F: FnMut(&mut T) -> bool,
2309        {
2310            while g.processed_len != original_len {
2311                // SAFETY: Unchecked element must be valid.
2312                let cur = unsafe { &mut *g.v.as_mut_ptr().add(g.processed_len) };
2313                if !f(cur) {
2314                    // Advance early to avoid double drop if `drop_in_place` panicked.
2315                    g.processed_len += 1;
2316                    g.deleted_cnt += 1;
2317                    // SAFETY: We never touch this element again after dropped.
2318                    unsafe { ptr::drop_in_place(cur) };
2319                    // We already advanced the counter.
2320                    if DELETED {
2321                        continue;
2322                    } else {
2323                        break;
2324                    }
2325                }
2326                if DELETED {
2327                    // SAFETY: `deleted_cnt` > 0, so the hole slot must not overlap with current element.
2328                    // We use copy for move, and never touch this element again.
2329                    unsafe {
2330                        let hole_slot = g.v.as_mut_ptr().add(g.processed_len - g.deleted_cnt);
2331                        ptr::copy_nonoverlapping(cur, hole_slot, 1);
2332                    }
2333                }
2334                g.processed_len += 1;
2335            }
2336        }
2337
2338        // Stage 1: Nothing was deleted.
2339        process_loop::<F, T, A, false>(original_len, &mut f, &mut g);
2340
2341        // Stage 2: Some elements were deleted.
2342        process_loop::<F, T, A, true>(original_len, &mut f, &mut g);
2343
2344        // All item are processed. This can be optimized to `set_len` by LLVM.
2345        drop(g);
2346    }
2347
2348    /// Removes all but the first of consecutive elements in the vector that resolve to the same
2349    /// key.
2350    ///
2351    /// If the vector is sorted, this removes all duplicates.
2352    ///
2353    /// # Examples
2354    ///
2355    /// ```
2356    /// let mut vec = vec![10, 20, 21, 30, 20];
2357    ///
2358    /// vec.dedup_by_key(|i| *i / 10);
2359    ///
2360    /// assert_eq!(vec, [10, 20, 30, 20]);
2361    /// ```
2362    #[stable(feature = "dedup_by", since = "1.16.0")]
2363    #[inline]
2364    pub fn dedup_by_key<F, K>(&mut self, mut key: F)
2365    where
2366        F: FnMut(&mut T) -> K,
2367        K: PartialEq,
2368    {
2369        self.dedup_by(|a, b| key(a) == key(b))
2370    }
2371
2372    /// Removes all but the first of consecutive elements in the vector satisfying a given equality
2373    /// relation.
2374    ///
2375    /// The `same_bucket` function is passed references to two elements from the vector and
2376    /// must determine if the elements compare equal. The elements are passed in opposite order
2377    /// from their order in the slice, so if `same_bucket(a, b)` returns `true`, `a` is removed.
2378    ///
2379    /// If the vector is sorted, this removes all duplicates.
2380    ///
2381    /// # Examples
2382    ///
2383    /// ```
2384    /// let mut vec = vec!["foo", "bar", "Bar", "baz", "bar"];
2385    ///
2386    /// vec.dedup_by(|a, b| a.eq_ignore_ascii_case(b));
2387    ///
2388    /// assert_eq!(vec, ["foo", "bar", "baz", "bar"]);
2389    /// ```
2390    #[stable(feature = "dedup_by", since = "1.16.0")]
2391    pub fn dedup_by<F>(&mut self, mut same_bucket: F)
2392    where
2393        F: FnMut(&mut T, &mut T) -> bool,
2394    {
2395        let len = self.len();
2396        if len <= 1 {
2397            return;
2398        }
2399
2400        // Check if we ever want to remove anything.
2401        // This allows to use copy_non_overlapping in next cycle.
2402        // And avoids any memory writes if we don't need to remove anything.
2403        let mut first_duplicate_idx: usize = 1;
2404        let start = self.as_mut_ptr();
2405        while first_duplicate_idx != len {
2406            let found_duplicate = unsafe {
2407                // SAFETY: first_duplicate always in range [1..len)
2408                // Note that we start iteration from 1 so we never overflow.
2409                let prev = start.add(first_duplicate_idx.wrapping_sub(1));
2410                let current = start.add(first_duplicate_idx);
2411                // We explicitly say in docs that references are reversed.
2412                same_bucket(&mut *current, &mut *prev)
2413            };
2414            if found_duplicate {
2415                break;
2416            }
2417            first_duplicate_idx += 1;
2418        }
2419        // Don't need to remove anything.
2420        // We cannot get bigger than len.
2421        if first_duplicate_idx == len {
2422            return;
2423        }
2424
2425        /* INVARIANT: vec.len() > read > write > write-1 >= 0 */
2426        struct FillGapOnDrop<'a, T, A: core::alloc::Allocator> {
2427            /* Offset of the element we want to check if it is duplicate */
2428            read: usize,
2429
2430            /* Offset of the place where we want to place the non-duplicate
2431             * when we find it. */
2432            write: usize,
2433
2434            /* The Vec that would need correction if `same_bucket` panicked */
2435            vec: &'a mut Vec<T, A>,
2436        }
2437
2438        impl<'a, T, A: core::alloc::Allocator> Drop for FillGapOnDrop<'a, T, A> {
2439            fn drop(&mut self) {
2440                /* This code gets executed when `same_bucket` panics */
2441
2442                /* SAFETY: invariant guarantees that `read - write`
2443                 * and `len - read` never overflow and that the copy is always
2444                 * in-bounds. */
2445                unsafe {
2446                    let ptr = self.vec.as_mut_ptr();
2447                    let len = self.vec.len();
2448
2449                    /* How many items were left when `same_bucket` panicked.
2450                     * Basically vec[read..].len() */
2451                    let items_left = len.wrapping_sub(self.read);
2452
2453                    /* Pointer to first item in vec[write..write+items_left] slice */
2454                    let dropped_ptr = ptr.add(self.write);
2455                    /* Pointer to first item in vec[read..] slice */
2456                    let valid_ptr = ptr.add(self.read);
2457
2458                    /* Copy `vec[read..]` to `vec[write..write+items_left]`.
2459                     * The slices can overlap, so `copy_nonoverlapping` cannot be used */
2460                    ptr::copy(valid_ptr, dropped_ptr, items_left);
2461
2462                    /* How many items have been already dropped
2463                     * Basically vec[read..write].len() */
2464                    let dropped = self.read.wrapping_sub(self.write);
2465
2466                    self.vec.set_len(len - dropped);
2467                }
2468            }
2469        }
2470
2471        /* Drop items while going through Vec, it should be more efficient than
2472         * doing slice partition_dedup + truncate */
2473
2474        // Construct gap first and then drop item to avoid memory corruption if `T::drop` panics.
2475        let mut gap =
2476            FillGapOnDrop { read: first_duplicate_idx + 1, write: first_duplicate_idx, vec: self };
2477        unsafe {
2478            // SAFETY: we checked that first_duplicate_idx in bounds before.
2479            // If drop panics, `gap` would remove this item without drop.
2480            ptr::drop_in_place(start.add(first_duplicate_idx));
2481        }
2482
2483        /* SAFETY: Because of the invariant, read_ptr, prev_ptr and write_ptr
2484         * are always in-bounds and read_ptr never aliases prev_ptr */
2485        unsafe {
2486            while gap.read < len {
2487                let read_ptr = start.add(gap.read);
2488                let prev_ptr = start.add(gap.write.wrapping_sub(1));
2489
2490                // We explicitly say in docs that references are reversed.
2491                let found_duplicate = same_bucket(&mut *read_ptr, &mut *prev_ptr);
2492                if found_duplicate {
2493                    // Increase `gap.read` now since the drop may panic.
2494                    gap.read += 1;
2495                    /* We have found duplicate, drop it in-place */
2496                    ptr::drop_in_place(read_ptr);
2497                } else {
2498                    let write_ptr = start.add(gap.write);
2499
2500                    /* read_ptr cannot be equal to write_ptr because at this point
2501                     * we guaranteed to skip at least one element (before loop starts).
2502                     */
2503                    ptr::copy_nonoverlapping(read_ptr, write_ptr, 1);
2504
2505                    /* We have filled that place, so go further */
2506                    gap.write += 1;
2507                    gap.read += 1;
2508                }
2509            }
2510
2511            /* Technically we could let `gap` clean up with its Drop, but
2512             * when `same_bucket` is guaranteed to not panic, this bloats a little
2513             * the codegen, so we just do it manually */
2514            gap.vec.set_len(gap.write);
2515            mem::forget(gap);
2516        }
2517    }
2518
2519    /// Appends an element to the back of a collection.
2520    ///
2521    /// # Panics
2522    ///
2523    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
2524    ///
2525    /// # Examples
2526    ///
2527    /// ```
2528    /// let mut vec = vec![1, 2];
2529    /// vec.push(3);
2530    /// assert_eq!(vec, [1, 2, 3]);
2531    /// ```
2532    ///
2533    /// # Time complexity
2534    ///
2535    /// Takes amortized *O*(1) time. If the vector's length would exceed its
2536    /// capacity after the push, *O*(*capacity*) time is taken to copy the
2537    /// vector's elements to a larger allocation. This expensive operation is
2538    /// offset by the *capacity* *O*(1) insertions it allows.
2539    #[cfg(not(no_global_oom_handling))]
2540    #[inline]
2541    #[stable(feature = "rust1", since = "1.0.0")]
2542    #[rustc_confusables("push_back", "put", "append")]
2543    #[track_caller]
2544    pub fn push(&mut self, value: T) {
2545        let _ = self.push_mut(value);
2546    }
2547
2548    /// Appends an element if there is sufficient spare capacity, otherwise an error is returned
2549    /// with the element.
2550    ///
2551    /// Unlike [`push`] this method will not reallocate when there's insufficient capacity.
2552    /// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
2553    ///
2554    /// [`push`]: Vec::push
2555    /// [`reserve`]: Vec::reserve
2556    /// [`try_reserve`]: Vec::try_reserve
2557    ///
2558    /// # Examples
2559    ///
2560    /// A manual, panic-free alternative to [`FromIterator`]:
2561    ///
2562    /// ```
2563    /// #![feature(vec_push_within_capacity)]
2564    ///
2565    /// use std::collections::TryReserveError;
2566    /// fn from_iter_fallible<T>(iter: impl Iterator<Item=T>) -> Result<Vec<T>, TryReserveError> {
2567    ///     let mut vec = Vec::new();
2568    ///     for value in iter {
2569    ///         if let Err(value) = vec.push_within_capacity(value) {
2570    ///             vec.try_reserve(1)?;
2571    ///             // this cannot fail, the previous line either returned or added at least 1 free slot
2572    ///             let _ = vec.push_within_capacity(value);
2573    ///         }
2574    ///     }
2575    ///     Ok(vec)
2576    /// }
2577    /// assert_eq!(from_iter_fallible(0..100), Ok(Vec::from_iter(0..100)));
2578    /// ```
2579    ///
2580    /// # Time complexity
2581    ///
2582    /// Takes *O*(1) time.
2583    #[inline]
2584    #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
2585    pub fn push_within_capacity(&mut self, value: T) -> Result<(), T> {
2586        self.push_mut_within_capacity(value).map(|_| ())
2587    }
2588
2589    /// Appends an element to the back of a collection, returning a reference to it.
2590    ///
2591    /// # Panics
2592    ///
2593    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
2594    ///
2595    /// # Examples
2596    ///
2597    /// ```
2598    /// #![feature(push_mut)]
2599    ///
2600    ///
2601    /// let mut vec = vec![1, 2];
2602    /// let last = vec.push_mut(3);
2603    /// assert_eq!(*last, 3);
2604    /// assert_eq!(vec, [1, 2, 3]);
2605    ///
2606    /// let last = vec.push_mut(3);
2607    /// *last += 1;
2608    /// assert_eq!(vec, [1, 2, 3, 4]);
2609    /// ```
2610    ///
2611    /// # Time complexity
2612    ///
2613    /// Takes amortized *O*(1) time. If the vector's length would exceed its
2614    /// capacity after the push, *O*(*capacity*) time is taken to copy the
2615    /// vector's elements to a larger allocation. This expensive operation is
2616    /// offset by the *capacity* *O*(1) insertions it allows.
2617    #[cfg(not(no_global_oom_handling))]
2618    #[inline]
2619    #[unstable(feature = "push_mut", issue = "135974")]
2620    #[track_caller]
2621    #[must_use = "if you don't need a reference to the value, use `Vec::push` instead"]
2622    pub fn push_mut(&mut self, value: T) -> &mut T {
2623        // Inform codegen that the length does not change across grow_one().
2624        let len = self.len;
2625        // This will panic or abort if we would allocate > isize::MAX bytes
2626        // or if the length increment would overflow for zero-sized types.
2627        if len == self.buf.capacity() {
2628            self.buf.grow_one();
2629        }
2630        unsafe {
2631            let end = self.as_mut_ptr().add(len);
2632            ptr::write(end, value);
2633            self.len = len + 1;
2634            // SAFETY: We just wrote a value to the pointer that will live the lifetime of the reference.
2635            &mut *end
2636        }
2637    }
2638
2639    /// Appends an element and returns a reference to it if there is sufficient spare capacity,
2640    /// otherwise an error is returned with the element.
2641    ///
2642    /// Unlike [`push_mut`] this method will not reallocate when there's insufficient capacity.
2643    /// The caller should use [`reserve`] or [`try_reserve`] to ensure that there is enough capacity.
2644    ///
2645    /// [`push_mut`]: Vec::push_mut
2646    /// [`reserve`]: Vec::reserve
2647    /// [`try_reserve`]: Vec::try_reserve
2648    ///
2649    /// # Time complexity
2650    ///
2651    /// Takes *O*(1) time.
2652    #[unstable(feature = "push_mut", issue = "135974")]
2653    // #[unstable(feature = "vec_push_within_capacity", issue = "100486")]
2654    #[inline]
2655    #[must_use = "if you don't need a reference to the value, use `Vec::push_within_capacity` instead"]
2656    pub fn push_mut_within_capacity(&mut self, value: T) -> Result<&mut T, T> {
2657        if self.len == self.buf.capacity() {
2658            return Err(value);
2659        }
2660        unsafe {
2661            let end = self.as_mut_ptr().add(self.len);
2662            ptr::write(end, value);
2663            self.len += 1;
2664            // SAFETY: We just wrote a value to the pointer that will live the lifetime of the reference.
2665            Ok(&mut *end)
2666        }
2667    }
2668
2669    /// Removes the last element from a vector and returns it, or [`None`] if it
2670    /// is empty.
2671    ///
2672    /// If you'd like to pop the first element, consider using
2673    /// [`VecDeque::pop_front`] instead.
2674    ///
2675    /// [`VecDeque::pop_front`]: crate::collections::VecDeque::pop_front
2676    ///
2677    /// # Examples
2678    ///
2679    /// ```
2680    /// let mut vec = vec![1, 2, 3];
2681    /// assert_eq!(vec.pop(), Some(3));
2682    /// assert_eq!(vec, [1, 2]);
2683    /// ```
2684    ///
2685    /// # Time complexity
2686    ///
2687    /// Takes *O*(1) time.
2688    #[inline]
2689    #[stable(feature = "rust1", since = "1.0.0")]
2690    #[rustc_diagnostic_item = "vec_pop"]
2691    pub fn pop(&mut self) -> Option<T> {
2692        if self.len == 0 {
2693            None
2694        } else {
2695            unsafe {
2696                self.len -= 1;
2697                core::hint::assert_unchecked(self.len < self.capacity());
2698                Some(ptr::read(self.as_ptr().add(self.len())))
2699            }
2700        }
2701    }
2702
2703    /// Removes and returns the last element from a vector if the predicate
2704    /// returns `true`, or [`None`] if the predicate returns false or the vector
2705    /// is empty (the predicate will not be called in that case).
2706    ///
2707    /// # Examples
2708    ///
2709    /// ```
2710    /// let mut vec = vec![1, 2, 3, 4];
2711    /// let pred = |x: &mut i32| *x % 2 == 0;
2712    ///
2713    /// assert_eq!(vec.pop_if(pred), Some(4));
2714    /// assert_eq!(vec, [1, 2, 3]);
2715    /// assert_eq!(vec.pop_if(pred), None);
2716    /// ```
2717    #[stable(feature = "vec_pop_if", since = "1.86.0")]
2718    pub fn pop_if(&mut self, predicate: impl FnOnce(&mut T) -> bool) -> Option<T> {
2719        let last = self.last_mut()?;
2720        if predicate(last) { self.pop() } else { None }
2721    }
2722
2723    /// Returns a mutable reference to the last item in the vector, or
2724    /// `None` if it is empty.
2725    ///
2726    /// # Examples
2727    ///
2728    /// Basic usage:
2729    ///
2730    /// ```
2731    /// #![feature(vec_peek_mut)]
2732    /// let mut vec = Vec::new();
2733    /// assert!(vec.peek_mut().is_none());
2734    ///
2735    /// vec.push(1);
2736    /// vec.push(5);
2737    /// vec.push(2);
2738    /// assert_eq!(vec.last(), Some(&2));
2739    /// if let Some(mut val) = vec.peek_mut() {
2740    ///     *val = 0;
2741    /// }
2742    /// assert_eq!(vec.last(), Some(&0));
2743    /// ```
2744    #[inline]
2745    #[unstable(feature = "vec_peek_mut", issue = "122742")]
2746    pub fn peek_mut(&mut self) -> Option<PeekMut<'_, T, A>> {
2747        PeekMut::new(self)
2748    }
2749
2750    /// Moves all the elements of `other` into `self`, leaving `other` empty.
2751    ///
2752    /// # Panics
2753    ///
2754    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
2755    ///
2756    /// # Examples
2757    ///
2758    /// ```
2759    /// let mut vec = vec![1, 2, 3];
2760    /// let mut vec2 = vec![4, 5, 6];
2761    /// vec.append(&mut vec2);
2762    /// assert_eq!(vec, [1, 2, 3, 4, 5, 6]);
2763    /// assert_eq!(vec2, []);
2764    /// ```
2765    #[cfg(not(no_global_oom_handling))]
2766    #[inline]
2767    #[stable(feature = "append", since = "1.4.0")]
2768    #[track_caller]
2769    pub fn append(&mut self, other: &mut Self) {
2770        unsafe {
2771            self.append_elements(other.as_slice() as _);
2772            other.set_len(0);
2773        }
2774    }
2775
2776    /// Appends elements to `self` from other buffer.
2777    #[cfg(not(no_global_oom_handling))]
2778    #[inline]
2779    #[track_caller]
2780    unsafe fn append_elements(&mut self, other: *const [T]) {
2781        let count = other.len();
2782        self.reserve(count);
2783        let len = self.len();
2784        unsafe { ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count) };
2785        self.len += count;
2786    }
2787
2788    /// Removes the subslice indicated by the given range from the vector,
2789    /// returning a double-ended iterator over the removed subslice.
2790    ///
2791    /// If the iterator is dropped before being fully consumed,
2792    /// it drops the remaining removed elements.
2793    ///
2794    /// The returned iterator keeps a mutable borrow on the vector to optimize
2795    /// its implementation.
2796    ///
2797    /// # Panics
2798    ///
2799    /// Panics if the range has `start_bound > end_bound`, or, if the range is
2800    /// bounded on either end and past the length of the vector.
2801    ///
2802    /// # Leaking
2803    ///
2804    /// If the returned iterator goes out of scope without being dropped (due to
2805    /// [`mem::forget`], for example), the vector may have lost and leaked
2806    /// elements arbitrarily, including elements outside the range.
2807    ///
2808    /// # Examples
2809    ///
2810    /// ```
2811    /// let mut v = vec![1, 2, 3];
2812    /// let u: Vec<_> = v.drain(1..).collect();
2813    /// assert_eq!(v, &[1]);
2814    /// assert_eq!(u, &[2, 3]);
2815    ///
2816    /// // A full range clears the vector, like `clear()` does
2817    /// v.drain(..);
2818    /// assert_eq!(v, &[]);
2819    /// ```
2820    #[stable(feature = "drain", since = "1.6.0")]
2821    pub fn drain<R>(&mut self, range: R) -> Drain<'_, T, A>
2822    where
2823        R: RangeBounds<usize>,
2824    {
2825        // Memory safety
2826        //
2827        // When the Drain is first created, it shortens the length of
2828        // the source vector to make sure no uninitialized or moved-from elements
2829        // are accessible at all if the Drain's destructor never gets to run.
2830        //
2831        // Drain will ptr::read out the values to remove.
2832        // When finished, remaining tail of the vec is copied back to cover
2833        // the hole, and the vector length is restored to the new length.
2834        //
2835        let len = self.len();
2836        let Range { start, end } = slice::range(range, ..len);
2837
2838        unsafe {
2839            // set self.vec length's to start, to be safe in case Drain is leaked
2840            self.set_len(start);
2841            let range_slice = slice::from_raw_parts(self.as_ptr().add(start), end - start);
2842            Drain {
2843                tail_start: end,
2844                tail_len: len - end,
2845                iter: range_slice.iter(),
2846                vec: NonNull::from(self),
2847            }
2848        }
2849    }
2850
2851    /// Clears the vector, removing all values.
2852    ///
2853    /// Note that this method has no effect on the allocated capacity
2854    /// of the vector.
2855    ///
2856    /// # Examples
2857    ///
2858    /// ```
2859    /// let mut v = vec![1, 2, 3];
2860    ///
2861    /// v.clear();
2862    ///
2863    /// assert!(v.is_empty());
2864    /// ```
2865    #[inline]
2866    #[stable(feature = "rust1", since = "1.0.0")]
2867    pub fn clear(&mut self) {
2868        let elems: *mut [T] = self.as_mut_slice();
2869
2870        // SAFETY:
2871        // - `elems` comes directly from `as_mut_slice` and is therefore valid.
2872        // - Setting `self.len` before calling `drop_in_place` means that,
2873        //   if an element's `Drop` impl panics, the vector's `Drop` impl will
2874        //   do nothing (leaking the rest of the elements) instead of dropping
2875        //   some twice.
2876        unsafe {
2877            self.len = 0;
2878            ptr::drop_in_place(elems);
2879        }
2880    }
2881
2882    /// Returns the number of elements in the vector, also referred to
2883    /// as its 'length'.
2884    ///
2885    /// # Examples
2886    ///
2887    /// ```
2888    /// let a = vec![1, 2, 3];
2889    /// assert_eq!(a.len(), 3);
2890    /// ```
2891    #[inline]
2892    #[stable(feature = "rust1", since = "1.0.0")]
2893    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
2894    #[rustc_confusables("length", "size")]
2895    pub const fn len(&self) -> usize {
2896        let len = self.len;
2897
2898        // SAFETY: The maximum capacity of `Vec<T>` is `isize::MAX` bytes, so the maximum value can
2899        // be returned is `usize::checked_div(size_of::<T>()).unwrap_or(usize::MAX)`, which
2900        // matches the definition of `T::MAX_SLICE_LEN`.
2901        unsafe { intrinsics::assume(len <= T::MAX_SLICE_LEN) };
2902
2903        len
2904    }
2905
2906    /// Returns `true` if the vector contains no elements.
2907    ///
2908    /// # Examples
2909    ///
2910    /// ```
2911    /// let mut v = Vec::new();
2912    /// assert!(v.is_empty());
2913    ///
2914    /// v.push(1);
2915    /// assert!(!v.is_empty());
2916    /// ```
2917    #[stable(feature = "rust1", since = "1.0.0")]
2918    #[rustc_diagnostic_item = "vec_is_empty"]
2919    #[rustc_const_stable(feature = "const_vec_string_slice", since = "1.87.0")]
2920    pub const fn is_empty(&self) -> bool {
2921        self.len() == 0
2922    }
2923
2924    /// Splits the collection into two at the given index.
2925    ///
2926    /// Returns a newly allocated vector containing the elements in the range
2927    /// `[at, len)`. After the call, the original vector will be left containing
2928    /// the elements `[0, at)` with its previous capacity unchanged.
2929    ///
2930    /// - If you want to take ownership of the entire contents and capacity of
2931    ///   the vector, see [`mem::take`] or [`mem::replace`].
2932    /// - If you don't need the returned vector at all, see [`Vec::truncate`].
2933    /// - If you want to take ownership of an arbitrary subslice, or you don't
2934    ///   necessarily want to store the removed items in a vector, see [`Vec::drain`].
2935    ///
2936    /// # Panics
2937    ///
2938    /// Panics if `at > len`.
2939    ///
2940    /// # Examples
2941    ///
2942    /// ```
2943    /// let mut vec = vec!['a', 'b', 'c'];
2944    /// let vec2 = vec.split_off(1);
2945    /// assert_eq!(vec, ['a']);
2946    /// assert_eq!(vec2, ['b', 'c']);
2947    /// ```
2948    #[cfg(not(no_global_oom_handling))]
2949    #[inline]
2950    #[must_use = "use `.truncate()` if you don't need the other half"]
2951    #[stable(feature = "split_off", since = "1.4.0")]
2952    #[track_caller]
2953    pub fn split_off(&mut self, at: usize) -> Self
2954    where
2955        A: Clone,
2956    {
2957        #[cold]
2958        #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
2959        #[track_caller]
2960        #[optimize(size)]
2961        fn assert_failed(at: usize, len: usize) -> ! {
2962            panic!("`at` split index (is {at}) should be <= len (is {len})");
2963        }
2964
2965        if at > self.len() {
2966            assert_failed(at, self.len());
2967        }
2968
2969        let other_len = self.len - at;
2970        let mut other = Vec::with_capacity_in(other_len, self.allocator().clone());
2971
2972        // Unsafely `set_len` and copy items to `other`.
2973        unsafe {
2974            self.set_len(at);
2975            other.set_len(other_len);
2976
2977            ptr::copy_nonoverlapping(self.as_ptr().add(at), other.as_mut_ptr(), other.len());
2978        }
2979        other
2980    }
2981
2982    /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
2983    ///
2984    /// If `new_len` is greater than `len`, the `Vec` is extended by the
2985    /// difference, with each additional slot filled with the result of
2986    /// calling the closure `f`. The return values from `f` will end up
2987    /// in the `Vec` in the order they have been generated.
2988    ///
2989    /// If `new_len` is less than `len`, the `Vec` is simply truncated.
2990    ///
2991    /// This method uses a closure to create new values on every push. If
2992    /// you'd rather [`Clone`] a given value, use [`Vec::resize`]. If you
2993    /// want to use the [`Default`] trait to generate values, you can
2994    /// pass [`Default::default`] as the second argument.
2995    ///
2996    /// # Panics
2997    ///
2998    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
2999    ///
3000    /// # Examples
3001    ///
3002    /// ```
3003    /// let mut vec = vec![1, 2, 3];
3004    /// vec.resize_with(5, Default::default);
3005    /// assert_eq!(vec, [1, 2, 3, 0, 0]);
3006    ///
3007    /// let mut vec = vec![];
3008    /// let mut p = 1;
3009    /// vec.resize_with(4, || { p *= 2; p });
3010    /// assert_eq!(vec, [2, 4, 8, 16]);
3011    /// ```
3012    #[cfg(not(no_global_oom_handling))]
3013    #[stable(feature = "vec_resize_with", since = "1.33.0")]
3014    #[track_caller]
3015    pub fn resize_with<F>(&mut self, new_len: usize, f: F)
3016    where
3017        F: FnMut() -> T,
3018    {
3019        let len = self.len();
3020        if new_len > len {
3021            self.extend_trusted(iter::repeat_with(f).take(new_len - len));
3022        } else {
3023            self.truncate(new_len);
3024        }
3025    }
3026
3027    /// Consumes and leaks the `Vec`, returning a mutable reference to the contents,
3028    /// `&'a mut [T]`.
3029    ///
3030    /// Note that the type `T` must outlive the chosen lifetime `'a`. If the type
3031    /// has only static references, or none at all, then this may be chosen to be
3032    /// `'static`.
3033    ///
3034    /// As of Rust 1.57, this method does not reallocate or shrink the `Vec`,
3035    /// so the leaked allocation may include unused capacity that is not part
3036    /// of the returned slice.
3037    ///
3038    /// This function is mainly useful for data that lives for the remainder of
3039    /// the program's life. Dropping the returned reference will cause a memory
3040    /// leak.
3041    ///
3042    /// # Examples
3043    ///
3044    /// Simple usage:
3045    ///
3046    /// ```
3047    /// let x = vec![1, 2, 3];
3048    /// let static_ref: &'static mut [usize] = x.leak();
3049    /// static_ref[0] += 1;
3050    /// assert_eq!(static_ref, &[2, 2, 3]);
3051    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
3052    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
3053    /// # drop(unsafe { Box::from_raw(static_ref) });
3054    /// ```
3055    #[stable(feature = "vec_leak", since = "1.47.0")]
3056    #[inline]
3057    pub fn leak<'a>(self) -> &'a mut [T]
3058    where
3059        A: 'a,
3060    {
3061        let mut me = ManuallyDrop::new(self);
3062        unsafe { slice::from_raw_parts_mut(me.as_mut_ptr(), me.len) }
3063    }
3064
3065    /// Returns the remaining spare capacity of the vector as a slice of
3066    /// `MaybeUninit<T>`.
3067    ///
3068    /// The returned slice can be used to fill the vector with data (e.g. by
3069    /// reading from a file) before marking the data as initialized using the
3070    /// [`set_len`] method.
3071    ///
3072    /// [`set_len`]: Vec::set_len
3073    ///
3074    /// # Examples
3075    ///
3076    /// ```
3077    /// // Allocate vector big enough for 10 elements.
3078    /// let mut v = Vec::with_capacity(10);
3079    ///
3080    /// // Fill in the first 3 elements.
3081    /// let uninit = v.spare_capacity_mut();
3082    /// uninit[0].write(0);
3083    /// uninit[1].write(1);
3084    /// uninit[2].write(2);
3085    ///
3086    /// // Mark the first 3 elements of the vector as being initialized.
3087    /// unsafe {
3088    ///     v.set_len(3);
3089    /// }
3090    ///
3091    /// assert_eq!(&v, &[0, 1, 2]);
3092    /// ```
3093    #[stable(feature = "vec_spare_capacity", since = "1.60.0")]
3094    #[inline]
3095    pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
3096        // Note:
3097        // This method is not implemented in terms of `split_at_spare_mut`,
3098        // to prevent invalidation of pointers to the buffer.
3099        unsafe {
3100            slice::from_raw_parts_mut(
3101                self.as_mut_ptr().add(self.len) as *mut MaybeUninit<T>,
3102                self.buf.capacity() - self.len,
3103            )
3104        }
3105    }
3106
3107    /// Returns vector content as a slice of `T`, along with the remaining spare
3108    /// capacity of the vector as a slice of `MaybeUninit<T>`.
3109    ///
3110    /// The returned spare capacity slice can be used to fill the vector with data
3111    /// (e.g. by reading from a file) before marking the data as initialized using
3112    /// the [`set_len`] method.
3113    ///
3114    /// [`set_len`]: Vec::set_len
3115    ///
3116    /// Note that this is a low-level API, which should be used with care for
3117    /// optimization purposes. If you need to append data to a `Vec`
3118    /// you can use [`push`], [`extend`], [`extend_from_slice`],
3119    /// [`extend_from_within`], [`insert`], [`append`], [`resize`] or
3120    /// [`resize_with`], depending on your exact needs.
3121    ///
3122    /// [`push`]: Vec::push
3123    /// [`extend`]: Vec::extend
3124    /// [`extend_from_slice`]: Vec::extend_from_slice
3125    /// [`extend_from_within`]: Vec::extend_from_within
3126    /// [`insert`]: Vec::insert
3127    /// [`append`]: Vec::append
3128    /// [`resize`]: Vec::resize
3129    /// [`resize_with`]: Vec::resize_with
3130    ///
3131    /// # Examples
3132    ///
3133    /// ```
3134    /// #![feature(vec_split_at_spare)]
3135    ///
3136    /// let mut v = vec![1, 1, 2];
3137    ///
3138    /// // Reserve additional space big enough for 10 elements.
3139    /// v.reserve(10);
3140    ///
3141    /// let (init, uninit) = v.split_at_spare_mut();
3142    /// let sum = init.iter().copied().sum::<u32>();
3143    ///
3144    /// // Fill in the next 4 elements.
3145    /// uninit[0].write(sum);
3146    /// uninit[1].write(sum * 2);
3147    /// uninit[2].write(sum * 3);
3148    /// uninit[3].write(sum * 4);
3149    ///
3150    /// // Mark the 4 elements of the vector as being initialized.
3151    /// unsafe {
3152    ///     let len = v.len();
3153    ///     v.set_len(len + 4);
3154    /// }
3155    ///
3156    /// assert_eq!(&v, &[1, 1, 2, 4, 8, 12, 16]);
3157    /// ```
3158    #[unstable(feature = "vec_split_at_spare", issue = "81944")]
3159    #[inline]
3160    pub fn split_at_spare_mut(&mut self) -> (&mut [T], &mut [MaybeUninit<T>]) {
3161        // SAFETY:
3162        // - len is ignored and so never changed
3163        let (init, spare, _) = unsafe { self.split_at_spare_mut_with_len() };
3164        (init, spare)
3165    }
3166
3167    /// Safety: changing returned .2 (&mut usize) is considered the same as calling `.set_len(_)`.
3168    ///
3169    /// This method provides unique access to all vec parts at once in `extend_from_within`.
3170    unsafe fn split_at_spare_mut_with_len(
3171        &mut self,
3172    ) -> (&mut [T], &mut [MaybeUninit<T>], &mut usize) {
3173        let ptr = self.as_mut_ptr();
3174        // SAFETY:
3175        // - `ptr` is guaranteed to be valid for `self.len` elements
3176        // - but the allocation extends out to `self.buf.capacity()` elements, possibly
3177        // uninitialized
3178        let spare_ptr = unsafe { ptr.add(self.len) };
3179        let spare_ptr = spare_ptr.cast_uninit();
3180        let spare_len = self.buf.capacity() - self.len;
3181
3182        // SAFETY:
3183        // - `ptr` is guaranteed to be valid for `self.len` elements
3184        // - `spare_ptr` is pointing one element past the buffer, so it doesn't overlap with `initialized`
3185        unsafe {
3186            let initialized = slice::from_raw_parts_mut(ptr, self.len);
3187            let spare = slice::from_raw_parts_mut(spare_ptr, spare_len);
3188
3189            (initialized, spare, &mut self.len)
3190        }
3191    }
3192
3193    /// Groups every `N` elements in the `Vec<T>` into chunks to produce a `Vec<[T; N]>`, dropping
3194    /// elements in the remainder. `N` must be greater than zero.
3195    ///
3196    /// If the capacity is not a multiple of the chunk size, the buffer will shrink down to the
3197    /// nearest multiple with a reallocation or deallocation.
3198    ///
3199    /// This function can be used to reverse [`Vec::into_flattened`].
3200    ///
3201    /// # Examples
3202    ///
3203    /// ```
3204    /// #![feature(vec_into_chunks)]
3205    ///
3206    /// let vec = vec![0, 1, 2, 3, 4, 5, 6, 7];
3207    /// assert_eq!(vec.into_chunks::<3>(), [[0, 1, 2], [3, 4, 5]]);
3208    ///
3209    /// let vec = vec![0, 1, 2, 3];
3210    /// let chunks: Vec<[u8; 10]> = vec.into_chunks();
3211    /// assert!(chunks.is_empty());
3212    ///
3213    /// let flat = vec![0; 8 * 8 * 8];
3214    /// let reshaped: Vec<[[[u8; 8]; 8]; 8]> = flat.into_chunks().into_chunks().into_chunks();
3215    /// assert_eq!(reshaped.len(), 1);
3216    /// ```
3217    #[cfg(not(no_global_oom_handling))]
3218    #[unstable(feature = "vec_into_chunks", issue = "142137")]
3219    pub fn into_chunks<const N: usize>(mut self) -> Vec<[T; N], A> {
3220        const {
3221            assert!(N != 0, "chunk size must be greater than zero");
3222        }
3223
3224        let (len, cap) = (self.len(), self.capacity());
3225
3226        let len_remainder = len % N;
3227        if len_remainder != 0 {
3228            self.truncate(len - len_remainder);
3229        }
3230
3231        let cap_remainder = cap % N;
3232        if !T::IS_ZST && cap_remainder != 0 {
3233            self.buf.shrink_to_fit(cap - cap_remainder);
3234        }
3235
3236        let (ptr, _, _, alloc) = self.into_raw_parts_with_alloc();
3237
3238        // SAFETY:
3239        // - `ptr` and `alloc` were just returned from `self.into_raw_parts_with_alloc()`
3240        // - `[T; N]` has the same alignment as `T`
3241        // - `size_of::<[T; N]>() * cap / N == size_of::<T>() * cap`
3242        // - `len / N <= cap / N` because `len <= cap`
3243        // - the allocated memory consists of `len / N` valid values of type `[T; N]`
3244        // - `cap / N` fits the size of the allocated memory after shrinking
3245        unsafe { Vec::from_raw_parts_in(ptr.cast(), len / N, cap / N, alloc) }
3246    }
3247}
3248
3249impl<T: Clone, A: Allocator> Vec<T, A> {
3250    /// Resizes the `Vec` in-place so that `len` is equal to `new_len`.
3251    ///
3252    /// If `new_len` is greater than `len`, the `Vec` is extended by the
3253    /// difference, with each additional slot filled with `value`.
3254    /// If `new_len` is less than `len`, the `Vec` is simply truncated.
3255    ///
3256    /// This method requires `T` to implement [`Clone`],
3257    /// in order to be able to clone the passed value.
3258    /// If you need more flexibility (or want to rely on [`Default`] instead of
3259    /// [`Clone`]), use [`Vec::resize_with`].
3260    /// If you only need to resize to a smaller size, use [`Vec::truncate`].
3261    ///
3262    /// # Panics
3263    ///
3264    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
3265    ///
3266    /// # Examples
3267    ///
3268    /// ```
3269    /// let mut vec = vec!["hello"];
3270    /// vec.resize(3, "world");
3271    /// assert_eq!(vec, ["hello", "world", "world"]);
3272    ///
3273    /// let mut vec = vec!['a', 'b', 'c', 'd'];
3274    /// vec.resize(2, '_');
3275    /// assert_eq!(vec, ['a', 'b']);
3276    /// ```
3277    #[cfg(not(no_global_oom_handling))]
3278    #[stable(feature = "vec_resize", since = "1.5.0")]
3279    #[track_caller]
3280    pub fn resize(&mut self, new_len: usize, value: T) {
3281        let len = self.len();
3282
3283        if new_len > len {
3284            self.extend_with(new_len - len, value)
3285        } else {
3286            self.truncate(new_len);
3287        }
3288    }
3289
3290    /// Clones and appends all elements in a slice to the `Vec`.
3291    ///
3292    /// Iterates over the slice `other`, clones each element, and then appends
3293    /// it to this `Vec`. The `other` slice is traversed in-order.
3294    ///
3295    /// Note that this function is the same as [`extend`],
3296    /// except that it also works with slice elements that are Clone but not Copy.
3297    /// If Rust gets specialization this function may be deprecated.
3298    ///
3299    /// # Examples
3300    ///
3301    /// ```
3302    /// let mut vec = vec![1];
3303    /// vec.extend_from_slice(&[2, 3, 4]);
3304    /// assert_eq!(vec, [1, 2, 3, 4]);
3305    /// ```
3306    ///
3307    /// [`extend`]: Vec::extend
3308    #[cfg(not(no_global_oom_handling))]
3309    #[stable(feature = "vec_extend_from_slice", since = "1.6.0")]
3310    #[track_caller]
3311    pub fn extend_from_slice(&mut self, other: &[T]) {
3312        self.spec_extend(other.iter())
3313    }
3314
3315    /// Given a range `src`, clones a slice of elements in that range and appends it to the end.
3316    ///
3317    /// `src` must be a range that can form a valid subslice of the `Vec`.
3318    ///
3319    /// # Panics
3320    ///
3321    /// Panics if starting index is greater than the end index
3322    /// or if the index is greater than the length of the vector.
3323    ///
3324    /// # Examples
3325    ///
3326    /// ```
3327    /// let mut characters = vec!['a', 'b', 'c', 'd', 'e'];
3328    /// characters.extend_from_within(2..);
3329    /// assert_eq!(characters, ['a', 'b', 'c', 'd', 'e', 'c', 'd', 'e']);
3330    ///
3331    /// let mut numbers = vec![0, 1, 2, 3, 4];
3332    /// numbers.extend_from_within(..2);
3333    /// assert_eq!(numbers, [0, 1, 2, 3, 4, 0, 1]);
3334    ///
3335    /// let mut strings = vec![String::from("hello"), String::from("world"), String::from("!")];
3336    /// strings.extend_from_within(1..=2);
3337    /// assert_eq!(strings, ["hello", "world", "!", "world", "!"]);
3338    /// ```
3339    #[cfg(not(no_global_oom_handling))]
3340    #[stable(feature = "vec_extend_from_within", since = "1.53.0")]
3341    #[track_caller]
3342    pub fn extend_from_within<R>(&mut self, src: R)
3343    where
3344        R: RangeBounds<usize>,
3345    {
3346        let range = slice::range(src, ..self.len());
3347        self.reserve(range.len());
3348
3349        // SAFETY:
3350        // - `slice::range` guarantees that the given range is valid for indexing self
3351        unsafe {
3352            self.spec_extend_from_within(range);
3353        }
3354    }
3355}
3356
3357impl<T, A: Allocator, const N: usize> Vec<[T; N], A> {
3358    /// Takes a `Vec<[T; N]>` and flattens it into a `Vec<T>`.
3359    ///
3360    /// # Panics
3361    ///
3362    /// Panics if the length of the resulting vector would overflow a `usize`.
3363    ///
3364    /// This is only possible when flattening a vector of arrays of zero-sized
3365    /// types, and thus tends to be irrelevant in practice. If
3366    /// `size_of::<T>() > 0`, this will never panic.
3367    ///
3368    /// # Examples
3369    ///
3370    /// ```
3371    /// let mut vec = vec![[1, 2, 3], [4, 5, 6], [7, 8, 9]];
3372    /// assert_eq!(vec.pop(), Some([7, 8, 9]));
3373    ///
3374    /// let mut flattened = vec.into_flattened();
3375    /// assert_eq!(flattened.pop(), Some(6));
3376    /// ```
3377    #[stable(feature = "slice_flatten", since = "1.80.0")]
3378    pub fn into_flattened(self) -> Vec<T, A> {
3379        let (ptr, len, cap, alloc) = self.into_raw_parts_with_alloc();
3380        let (new_len, new_cap) = if T::IS_ZST {
3381            (len.checked_mul(N).expect("vec len overflow"), usize::MAX)
3382        } else {
3383            // SAFETY:
3384            // - `cap * N` cannot overflow because the allocation is already in
3385            // the address space.
3386            // - Each `[T; N]` has `N` valid elements, so there are `len * N`
3387            // valid elements in the allocation.
3388            unsafe { (len.unchecked_mul(N), cap.unchecked_mul(N)) }
3389        };
3390        // SAFETY:
3391        // - `ptr` was allocated by `self`
3392        // - `ptr` is well-aligned because `[T; N]` has the same alignment as `T`.
3393        // - `new_cap` refers to the same sized allocation as `cap` because
3394        // `new_cap * size_of::<T>()` == `cap * size_of::<[T; N]>()`
3395        // - `len` <= `cap`, so `len * N` <= `cap * N`.
3396        unsafe { Vec::<T, A>::from_raw_parts_in(ptr.cast(), new_len, new_cap, alloc) }
3397    }
3398}
3399
3400impl<T: Clone, A: Allocator> Vec<T, A> {
3401    #[cfg(not(no_global_oom_handling))]
3402    #[track_caller]
3403    /// Extend the vector by `n` clones of value.
3404    fn extend_with(&mut self, n: usize, value: T) {
3405        self.reserve(n);
3406
3407        unsafe {
3408            let mut ptr = self.as_mut_ptr().add(self.len());
3409            // Use SetLenOnDrop to work around bug where compiler
3410            // might not realize the store through `ptr` through self.set_len()
3411            // don't alias.
3412            let mut local_len = SetLenOnDrop::new(&mut self.len);
3413
3414            // Write all elements except the last one
3415            for _ in 1..n {
3416                ptr::write(ptr, value.clone());
3417                ptr = ptr.add(1);
3418                // Increment the length in every step in case clone() panics
3419                local_len.increment_len(1);
3420            }
3421
3422            if n > 0 {
3423                // We can write the last element directly without cloning needlessly
3424                ptr::write(ptr, value);
3425                local_len.increment_len(1);
3426            }
3427
3428            // len set by scope guard
3429        }
3430    }
3431}
3432
3433impl<T: PartialEq, A: Allocator> Vec<T, A> {
3434    /// Removes consecutive repeated elements in the vector according to the
3435    /// [`PartialEq`] trait implementation.
3436    ///
3437    /// If the vector is sorted, this removes all duplicates.
3438    ///
3439    /// # Examples
3440    ///
3441    /// ```
3442    /// let mut vec = vec![1, 2, 2, 3, 2];
3443    ///
3444    /// vec.dedup();
3445    ///
3446    /// assert_eq!(vec, [1, 2, 3, 2]);
3447    /// ```
3448    #[stable(feature = "rust1", since = "1.0.0")]
3449    #[inline]
3450    pub fn dedup(&mut self) {
3451        self.dedup_by(|a, b| a == b)
3452    }
3453}
3454
3455////////////////////////////////////////////////////////////////////////////////
3456// Internal methods and functions
3457////////////////////////////////////////////////////////////////////////////////
3458
3459#[doc(hidden)]
3460#[cfg(not(no_global_oom_handling))]
3461#[stable(feature = "rust1", since = "1.0.0")]
3462#[rustc_diagnostic_item = "vec_from_elem"]
3463#[track_caller]
3464pub fn from_elem<T: Clone>(elem: T, n: usize) -> Vec<T> {
3465    <T as SpecFromElem>::from_elem(elem, n, Global)
3466}
3467
3468#[doc(hidden)]
3469#[cfg(not(no_global_oom_handling))]
3470#[unstable(feature = "allocator_api", issue = "32838")]
3471#[track_caller]
3472pub fn from_elem_in<T: Clone, A: Allocator>(elem: T, n: usize, alloc: A) -> Vec<T, A> {
3473    <T as SpecFromElem>::from_elem(elem, n, alloc)
3474}
3475
3476#[cfg(not(no_global_oom_handling))]
3477trait ExtendFromWithinSpec {
3478    /// # Safety
3479    ///
3480    /// - `src` needs to be valid index
3481    /// - `self.capacity() - self.len()` must be `>= src.len()`
3482    unsafe fn spec_extend_from_within(&mut self, src: Range<usize>);
3483}
3484
3485#[cfg(not(no_global_oom_handling))]
3486impl<T: Clone, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
3487    default unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
3488        // SAFETY:
3489        // - len is increased only after initializing elements
3490        let (this, spare, len) = unsafe { self.split_at_spare_mut_with_len() };
3491
3492        // SAFETY:
3493        // - caller guarantees that src is a valid index
3494        let to_clone = unsafe { this.get_unchecked(src) };
3495
3496        iter::zip(to_clone, spare)
3497            .map(|(src, dst)| dst.write(src.clone()))
3498            // Note:
3499            // - Element was just initialized with `MaybeUninit::write`, so it's ok to increase len
3500            // - len is increased after each element to prevent leaks (see issue #82533)
3501            .for_each(|_| *len += 1);
3502    }
3503}
3504
3505#[cfg(not(no_global_oom_handling))]
3506impl<T: Copy, A: Allocator> ExtendFromWithinSpec for Vec<T, A> {
3507    unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
3508        let count = src.len();
3509        {
3510            let (init, spare) = self.split_at_spare_mut();
3511
3512            // SAFETY:
3513            // - caller guarantees that `src` is a valid index
3514            let source = unsafe { init.get_unchecked(src) };
3515
3516            // SAFETY:
3517            // - Both pointers are created from unique slice references (`&mut [_]`)
3518            //   so they are valid and do not overlap.
3519            // - Elements are :Copy so it's OK to copy them, without doing
3520            //   anything with the original values
3521            // - `count` is equal to the len of `source`, so source is valid for
3522            //   `count` reads
3523            // - `.reserve(count)` guarantees that `spare.len() >= count` so spare
3524            //   is valid for `count` writes
3525            unsafe { ptr::copy_nonoverlapping(source.as_ptr(), spare.as_mut_ptr() as _, count) };
3526        }
3527
3528        // SAFETY:
3529        // - The elements were just initialized by `copy_nonoverlapping`
3530        self.len += count;
3531    }
3532}
3533
3534////////////////////////////////////////////////////////////////////////////////
3535// Common trait implementations for Vec
3536////////////////////////////////////////////////////////////////////////////////
3537
3538#[stable(feature = "rust1", since = "1.0.0")]
3539impl<T, A: Allocator> ops::Deref for Vec<T, A> {
3540    type Target = [T];
3541
3542    #[inline]
3543    fn deref(&self) -> &[T] {
3544        self.as_slice()
3545    }
3546}
3547
3548#[stable(feature = "rust1", since = "1.0.0")]
3549impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
3550    #[inline]
3551    fn deref_mut(&mut self) -> &mut [T] {
3552        self.as_mut_slice()
3553    }
3554}
3555
3556#[unstable(feature = "deref_pure_trait", issue = "87121")]
3557unsafe impl<T, A: Allocator> ops::DerefPure for Vec<T, A> {}
3558
3559#[cfg(not(no_global_oom_handling))]
3560#[stable(feature = "rust1", since = "1.0.0")]
3561impl<T: Clone, A: Allocator + Clone> Clone for Vec<T, A> {
3562    #[track_caller]
3563    fn clone(&self) -> Self {
3564        let alloc = self.allocator().clone();
3565        <[T]>::to_vec_in(&**self, alloc)
3566    }
3567
3568    /// Overwrites the contents of `self` with a clone of the contents of `source`.
3569    ///
3570    /// This method is preferred over simply assigning `source.clone()` to `self`,
3571    /// as it avoids reallocation if possible. Additionally, if the element type
3572    /// `T` overrides `clone_from()`, this will reuse the resources of `self`'s
3573    /// elements as well.
3574    ///
3575    /// # Examples
3576    ///
3577    /// ```
3578    /// let x = vec![5, 6, 7];
3579    /// let mut y = vec![8, 9, 10];
3580    /// let yp: *const i32 = y.as_ptr();
3581    ///
3582    /// y.clone_from(&x);
3583    ///
3584    /// // The value is the same
3585    /// assert_eq!(x, y);
3586    ///
3587    /// // And no reallocation occurred
3588    /// assert_eq!(yp, y.as_ptr());
3589    /// ```
3590    #[track_caller]
3591    fn clone_from(&mut self, source: &Self) {
3592        crate::slice::SpecCloneIntoVec::clone_into(source.as_slice(), self);
3593    }
3594}
3595
3596/// The hash of a vector is the same as that of the corresponding slice,
3597/// as required by the `core::borrow::Borrow` implementation.
3598///
3599/// ```
3600/// use std::hash::BuildHasher;
3601///
3602/// let b = std::hash::RandomState::new();
3603/// let v: Vec<u8> = vec![0xa8, 0x3c, 0x09];
3604/// let s: &[u8] = &[0xa8, 0x3c, 0x09];
3605/// assert_eq!(b.hash_one(v), b.hash_one(s));
3606/// ```
3607#[stable(feature = "rust1", since = "1.0.0")]
3608impl<T: Hash, A: Allocator> Hash for Vec<T, A> {
3609    #[inline]
3610    fn hash<H: Hasher>(&self, state: &mut H) {
3611        Hash::hash(&**self, state)
3612    }
3613}
3614
3615#[stable(feature = "rust1", since = "1.0.0")]
3616impl<T, I: SliceIndex<[T]>, A: Allocator> Index<I> for Vec<T, A> {
3617    type Output = I::Output;
3618
3619    #[inline]
3620    fn index(&self, index: I) -> &Self::Output {
3621        Index::index(&**self, index)
3622    }
3623}
3624
3625#[stable(feature = "rust1", since = "1.0.0")]
3626impl<T, I: SliceIndex<[T]>, A: Allocator> IndexMut<I> for Vec<T, A> {
3627    #[inline]
3628    fn index_mut(&mut self, index: I) -> &mut Self::Output {
3629        IndexMut::index_mut(&mut **self, index)
3630    }
3631}
3632
3633/// Collects an iterator into a Vec, commonly called via [`Iterator::collect()`]
3634///
3635/// # Allocation behavior
3636///
3637/// In general `Vec` does not guarantee any particular growth or allocation strategy.
3638/// That also applies to this trait impl.
3639///
3640/// **Note:** This section covers implementation details and is therefore exempt from
3641/// stability guarantees.
3642///
3643/// Vec may use any or none of the following strategies,
3644/// depending on the supplied iterator:
3645///
3646/// * preallocate based on [`Iterator::size_hint()`]
3647///   * and panic if the number of items is outside the provided lower/upper bounds
3648/// * use an amortized growth strategy similar to `pushing` one item at a time
3649/// * perform the iteration in-place on the original allocation backing the iterator
3650///
3651/// The last case warrants some attention. It is an optimization that in many cases reduces peak memory
3652/// consumption and improves cache locality. But when big, short-lived allocations are created,
3653/// only a small fraction of their items get collected, no further use is made of the spare capacity
3654/// and the resulting `Vec` is moved into a longer-lived structure, then this can lead to the large
3655/// allocations having their lifetimes unnecessarily extended which can result in increased memory
3656/// footprint.
3657///
3658/// In cases where this is an issue, the excess capacity can be discarded with [`Vec::shrink_to()`],
3659/// [`Vec::shrink_to_fit()`] or by collecting into [`Box<[T]>`][owned slice] instead, which additionally reduces
3660/// the size of the long-lived struct.
3661///
3662/// [owned slice]: Box
3663///
3664/// ```rust
3665/// # use std::sync::Mutex;
3666/// static LONG_LIVED: Mutex<Vec<Vec<u16>>> = Mutex::new(Vec::new());
3667///
3668/// for i in 0..10 {
3669///     let big_temporary: Vec<u16> = (0..1024).collect();
3670///     // discard most items
3671///     let mut result: Vec<_> = big_temporary.into_iter().filter(|i| i % 100 == 0).collect();
3672///     // without this a lot of unused capacity might be moved into the global
3673///     result.shrink_to_fit();
3674///     LONG_LIVED.lock().unwrap().push(result);
3675/// }
3676/// ```
3677#[cfg(not(no_global_oom_handling))]
3678#[stable(feature = "rust1", since = "1.0.0")]
3679impl<T> FromIterator<T> for Vec<T> {
3680    #[inline]
3681    #[track_caller]
3682    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Vec<T> {
3683        <Self as SpecFromIter<T, I::IntoIter>>::from_iter(iter.into_iter())
3684    }
3685}
3686
3687#[stable(feature = "rust1", since = "1.0.0")]
3688impl<T, A: Allocator> IntoIterator for Vec<T, A> {
3689    type Item = T;
3690    type IntoIter = IntoIter<T, A>;
3691
3692    /// Creates a consuming iterator, that is, one that moves each value out of
3693    /// the vector (from start to end). The vector cannot be used after calling
3694    /// this.
3695    ///
3696    /// # Examples
3697    ///
3698    /// ```
3699    /// let v = vec!["a".to_string(), "b".to_string()];
3700    /// let mut v_iter = v.into_iter();
3701    ///
3702    /// let first_element: Option<String> = v_iter.next();
3703    ///
3704    /// assert_eq!(first_element, Some("a".to_string()));
3705    /// assert_eq!(v_iter.next(), Some("b".to_string()));
3706    /// assert_eq!(v_iter.next(), None);
3707    /// ```
3708    #[inline]
3709    fn into_iter(self) -> Self::IntoIter {
3710        unsafe {
3711            let me = ManuallyDrop::new(self);
3712            let alloc = ManuallyDrop::new(ptr::read(me.allocator()));
3713            let buf = me.buf.non_null();
3714            let begin = buf.as_ptr();
3715            let end = if T::IS_ZST {
3716                begin.wrapping_byte_add(me.len())
3717            } else {
3718                begin.add(me.len()) as *const T
3719            };
3720            let cap = me.buf.capacity();
3721            IntoIter { buf, phantom: PhantomData, cap, alloc, ptr: buf, end }
3722        }
3723    }
3724}
3725
3726#[stable(feature = "rust1", since = "1.0.0")]
3727impl<'a, T, A: Allocator> IntoIterator for &'a Vec<T, A> {
3728    type Item = &'a T;
3729    type IntoIter = slice::Iter<'a, T>;
3730
3731    fn into_iter(self) -> Self::IntoIter {
3732        self.iter()
3733    }
3734}
3735
3736#[stable(feature = "rust1", since = "1.0.0")]
3737impl<'a, T, A: Allocator> IntoIterator for &'a mut Vec<T, A> {
3738    type Item = &'a mut T;
3739    type IntoIter = slice::IterMut<'a, T>;
3740
3741    fn into_iter(self) -> Self::IntoIter {
3742        self.iter_mut()
3743    }
3744}
3745
3746#[cfg(not(no_global_oom_handling))]
3747#[stable(feature = "rust1", since = "1.0.0")]
3748impl<T, A: Allocator> Extend<T> for Vec<T, A> {
3749    #[inline]
3750    #[track_caller]
3751    fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
3752        <Self as SpecExtend<T, I::IntoIter>>::spec_extend(self, iter.into_iter())
3753    }
3754
3755    #[inline]
3756    #[track_caller]
3757    fn extend_one(&mut self, item: T) {
3758        self.push(item);
3759    }
3760
3761    #[inline]
3762    #[track_caller]
3763    fn extend_reserve(&mut self, additional: usize) {
3764        self.reserve(additional);
3765    }
3766
3767    #[inline]
3768    unsafe fn extend_one_unchecked(&mut self, item: T) {
3769        // SAFETY: Our preconditions ensure the space has been reserved, and `extend_reserve` is implemented correctly.
3770        unsafe {
3771            let len = self.len();
3772            ptr::write(self.as_mut_ptr().add(len), item);
3773            self.set_len(len + 1);
3774        }
3775    }
3776}
3777
3778impl<T, A: Allocator> Vec<T, A> {
3779    // leaf method to which various SpecFrom/SpecExtend implementations delegate when
3780    // they have no further optimizations to apply
3781    #[cfg(not(no_global_oom_handling))]
3782    #[track_caller]
3783    fn extend_desugared<I: Iterator<Item = T>>(&mut self, mut iterator: I) {
3784        // This is the case for a general iterator.
3785        //
3786        // This function should be the moral equivalent of:
3787        //
3788        //      for item in iterator {
3789        //          self.push(item);
3790        //      }
3791        while let Some(element) = iterator.next() {
3792            let len = self.len();
3793            if len == self.capacity() {
3794                let (lower, _) = iterator.size_hint();
3795                self.reserve(lower.saturating_add(1));
3796            }
3797            unsafe {
3798                ptr::write(self.as_mut_ptr().add(len), element);
3799                // Since next() executes user code which can panic we have to bump the length
3800                // after each step.
3801                // NB can't overflow since we would have had to alloc the address space
3802                self.set_len(len + 1);
3803            }
3804        }
3805    }
3806
3807    // specific extend for `TrustedLen` iterators, called both by the specializations
3808    // and internal places where resolving specialization makes compilation slower
3809    #[cfg(not(no_global_oom_handling))]
3810    #[track_caller]
3811    fn extend_trusted(&mut self, iterator: impl iter::TrustedLen<Item = T>) {
3812        let (low, high) = iterator.size_hint();
3813        if let Some(additional) = high {
3814            debug_assert_eq!(
3815                low,
3816                additional,
3817                "TrustedLen iterator's size hint is not exact: {:?}",
3818                (low, high)
3819            );
3820            self.reserve(additional);
3821            unsafe {
3822                let ptr = self.as_mut_ptr();
3823                let mut local_len = SetLenOnDrop::new(&mut self.len);
3824                iterator.for_each(move |element| {
3825                    ptr::write(ptr.add(local_len.current_len()), element);
3826                    // Since the loop executes user code which can panic we have to update
3827                    // the length every step to correctly drop what we've written.
3828                    // NB can't overflow since we would have had to alloc the address space
3829                    local_len.increment_len(1);
3830                });
3831            }
3832        } else {
3833            // Per TrustedLen contract a `None` upper bound means that the iterator length
3834            // truly exceeds usize::MAX, which would eventually lead to a capacity overflow anyway.
3835            // Since the other branch already panics eagerly (via `reserve()`) we do the same here.
3836            // This avoids additional codegen for a fallback code path which would eventually
3837            // panic anyway.
3838            panic!("capacity overflow");
3839        }
3840    }
3841
3842    /// Creates a splicing iterator that replaces the specified range in the vector
3843    /// with the given `replace_with` iterator and yields the removed items.
3844    /// `replace_with` does not need to be the same length as `range`.
3845    ///
3846    /// `range` is removed even if the `Splice` iterator is not consumed before it is dropped.
3847    ///
3848    /// It is unspecified how many elements are removed from the vector
3849    /// if the `Splice` value is leaked.
3850    ///
3851    /// The input iterator `replace_with` is only consumed when the `Splice` value is dropped.
3852    ///
3853    /// This is optimal if:
3854    ///
3855    /// * The tail (elements in the vector after `range`) is empty,
3856    /// * or `replace_with` yields fewer or equal elements than `range`'s length
3857    /// * or the lower bound of its `size_hint()` is exact.
3858    ///
3859    /// Otherwise, a temporary vector is allocated and the tail is moved twice.
3860    ///
3861    /// # Panics
3862    ///
3863    /// Panics if the range has `start_bound > end_bound`, or, if the range is
3864    /// bounded on either end and past the length of the vector.
3865    ///
3866    /// # Examples
3867    ///
3868    /// ```
3869    /// let mut v = vec![1, 2, 3, 4];
3870    /// let new = [7, 8, 9];
3871    /// let u: Vec<_> = v.splice(1..3, new).collect();
3872    /// assert_eq!(v, [1, 7, 8, 9, 4]);
3873    /// assert_eq!(u, [2, 3]);
3874    /// ```
3875    ///
3876    /// Using `splice` to insert new items into a vector efficiently at a specific position
3877    /// indicated by an empty range:
3878    ///
3879    /// ```
3880    /// let mut v = vec![1, 5];
3881    /// let new = [2, 3, 4];
3882    /// v.splice(1..1, new);
3883    /// assert_eq!(v, [1, 2, 3, 4, 5]);
3884    /// ```
3885    #[cfg(not(no_global_oom_handling))]
3886    #[inline]
3887    #[stable(feature = "vec_splice", since = "1.21.0")]
3888    pub fn splice<R, I>(&mut self, range: R, replace_with: I) -> Splice<'_, I::IntoIter, A>
3889    where
3890        R: RangeBounds<usize>,
3891        I: IntoIterator<Item = T>,
3892    {
3893        Splice { drain: self.drain(range), replace_with: replace_with.into_iter() }
3894    }
3895
3896    /// Creates an iterator which uses a closure to determine if an element in the range should be removed.
3897    ///
3898    /// If the closure returns `true`, the element is removed from the vector
3899    /// and yielded. If the closure returns `false`, or panics, the element
3900    /// remains in the vector and will not be yielded.
3901    ///
3902    /// Only elements that fall in the provided range are considered for extraction, but any elements
3903    /// after the range will still have to be moved if any element has been extracted.
3904    ///
3905    /// If the returned `ExtractIf` is not exhausted, e.g. because it is dropped without iterating
3906    /// or the iteration short-circuits, then the remaining elements will be retained.
3907    /// Use [`retain_mut`] with a negated predicate if you do not need the returned iterator.
3908    ///
3909    /// [`retain_mut`]: Vec::retain_mut
3910    ///
3911    /// Using this method is equivalent to the following code:
3912    ///
3913    /// ```
3914    /// # let some_predicate = |x: &mut i32| { *x % 2 == 1 };
3915    /// # let mut vec = vec![0, 1, 2, 3, 4, 5, 6];
3916    /// # let mut vec2 = vec.clone();
3917    /// # let range = 1..5;
3918    /// let mut i = range.start;
3919    /// let end_items = vec.len() - range.end;
3920    /// # let mut extracted = vec![];
3921    ///
3922    /// while i < vec.len() - end_items {
3923    ///     if some_predicate(&mut vec[i]) {
3924    ///         let val = vec.remove(i);
3925    ///         // your code here
3926    /// #         extracted.push(val);
3927    ///     } else {
3928    ///         i += 1;
3929    ///     }
3930    /// }
3931    ///
3932    /// # let extracted2: Vec<_> = vec2.extract_if(range, some_predicate).collect();
3933    /// # assert_eq!(vec, vec2);
3934    /// # assert_eq!(extracted, extracted2);
3935    /// ```
3936    ///
3937    /// But `extract_if` is easier to use. `extract_if` is also more efficient,
3938    /// because it can backshift the elements of the array in bulk.
3939    ///
3940    /// The iterator also lets you mutate the value of each element in the
3941    /// closure, regardless of whether you choose to keep or remove it.
3942    ///
3943    /// # Panics
3944    ///
3945    /// If `range` is out of bounds.
3946    ///
3947    /// # Examples
3948    ///
3949    /// Splitting a vector into even and odd values, reusing the original vector:
3950    ///
3951    /// ```
3952    /// let mut numbers = vec![1, 2, 3, 4, 5, 6, 8, 9, 11, 13, 14, 15];
3953    ///
3954    /// let evens = numbers.extract_if(.., |x| *x % 2 == 0).collect::<Vec<_>>();
3955    /// let odds = numbers;
3956    ///
3957    /// assert_eq!(evens, vec![2, 4, 6, 8, 14]);
3958    /// assert_eq!(odds, vec![1, 3, 5, 9, 11, 13, 15]);
3959    /// ```
3960    ///
3961    /// Using the range argument to only process a part of the vector:
3962    ///
3963    /// ```
3964    /// let mut items = vec![0, 0, 0, 0, 0, 0, 0, 1, 2, 1, 2, 1, 2];
3965    /// let ones = items.extract_if(7.., |x| *x == 1).collect::<Vec<_>>();
3966    /// assert_eq!(items, vec![0, 0, 0, 0, 0, 0, 0, 2, 2, 2]);
3967    /// assert_eq!(ones.len(), 3);
3968    /// ```
3969    #[stable(feature = "extract_if", since = "1.87.0")]
3970    pub fn extract_if<F, R>(&mut self, range: R, filter: F) -> ExtractIf<'_, T, F, A>
3971    where
3972        F: FnMut(&mut T) -> bool,
3973        R: RangeBounds<usize>,
3974    {
3975        ExtractIf::new(self, filter, range)
3976    }
3977}
3978
3979/// Extend implementation that copies elements out of references before pushing them onto the Vec.
3980///
3981/// This implementation is specialized for slice iterators, where it uses [`copy_from_slice`] to
3982/// append the entire slice at once.
3983///
3984/// [`copy_from_slice`]: slice::copy_from_slice
3985#[cfg(not(no_global_oom_handling))]
3986#[stable(feature = "extend_ref", since = "1.2.0")]
3987impl<'a, T: Copy + 'a, A: Allocator> Extend<&'a T> for Vec<T, A> {
3988    #[track_caller]
3989    fn extend<I: IntoIterator<Item = &'a T>>(&mut self, iter: I) {
3990        self.spec_extend(iter.into_iter())
3991    }
3992
3993    #[inline]
3994    #[track_caller]
3995    fn extend_one(&mut self, &item: &'a T) {
3996        self.push(item);
3997    }
3998
3999    #[inline]
4000    #[track_caller]
4001    fn extend_reserve(&mut self, additional: usize) {
4002        self.reserve(additional);
4003    }
4004
4005    #[inline]
4006    unsafe fn extend_one_unchecked(&mut self, &item: &'a T) {
4007        // SAFETY: Our preconditions ensure the space has been reserved, and `extend_reserve` is implemented correctly.
4008        unsafe {
4009            let len = self.len();
4010            ptr::write(self.as_mut_ptr().add(len), item);
4011            self.set_len(len + 1);
4012        }
4013    }
4014}
4015
4016/// Implements comparison of vectors, [lexicographically](Ord#lexicographical-comparison).
4017#[stable(feature = "rust1", since = "1.0.0")]
4018impl<T, A1, A2> PartialOrd<Vec<T, A2>> for Vec<T, A1>
4019where
4020    T: PartialOrd,
4021    A1: Allocator,
4022    A2: Allocator,
4023{
4024    #[inline]
4025    fn partial_cmp(&self, other: &Vec<T, A2>) -> Option<Ordering> {
4026        PartialOrd::partial_cmp(&**self, &**other)
4027    }
4028}
4029
4030#[stable(feature = "rust1", since = "1.0.0")]
4031impl<T: Eq, A: Allocator> Eq for Vec<T, A> {}
4032
4033/// Implements ordering of vectors, [lexicographically](Ord#lexicographical-comparison).
4034#[stable(feature = "rust1", since = "1.0.0")]
4035impl<T: Ord, A: Allocator> Ord for Vec<T, A> {
4036    #[inline]
4037    fn cmp(&self, other: &Self) -> Ordering {
4038        Ord::cmp(&**self, &**other)
4039    }
4040}
4041
4042#[stable(feature = "rust1", since = "1.0.0")]
4043unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
4044    fn drop(&mut self) {
4045        unsafe {
4046            // use drop for [T]
4047            // use a raw slice to refer to the elements of the vector as weakest necessary type;
4048            // could avoid questions of validity in certain cases
4049            ptr::drop_in_place(ptr::slice_from_raw_parts_mut(self.as_mut_ptr(), self.len))
4050        }
4051        // RawVec handles deallocation
4052    }
4053}
4054
4055#[stable(feature = "rust1", since = "1.0.0")]
4056#[rustc_const_unstable(feature = "const_default", issue = "143894")]
4057impl<T> const Default for Vec<T> {
4058    /// Creates an empty `Vec<T>`.
4059    ///
4060    /// The vector will not allocate until elements are pushed onto it.
4061    fn default() -> Vec<T> {
4062        Vec::new()
4063    }
4064}
4065
4066#[stable(feature = "rust1", since = "1.0.0")]
4067impl<T: fmt::Debug, A: Allocator> fmt::Debug for Vec<T, A> {
4068    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4069        fmt::Debug::fmt(&**self, f)
4070    }
4071}
4072
4073#[stable(feature = "rust1", since = "1.0.0")]
4074impl<T, A: Allocator> AsRef<Vec<T, A>> for Vec<T, A> {
4075    fn as_ref(&self) -> &Vec<T, A> {
4076        self
4077    }
4078}
4079
4080#[stable(feature = "vec_as_mut", since = "1.5.0")]
4081impl<T, A: Allocator> AsMut<Vec<T, A>> for Vec<T, A> {
4082    fn as_mut(&mut self) -> &mut Vec<T, A> {
4083        self
4084    }
4085}
4086
4087#[stable(feature = "rust1", since = "1.0.0")]
4088impl<T, A: Allocator> AsRef<[T]> for Vec<T, A> {
4089    fn as_ref(&self) -> &[T] {
4090        self
4091    }
4092}
4093
4094#[stable(feature = "vec_as_mut", since = "1.5.0")]
4095impl<T, A: Allocator> AsMut<[T]> for Vec<T, A> {
4096    fn as_mut(&mut self) -> &mut [T] {
4097        self
4098    }
4099}
4100
4101#[cfg(not(no_global_oom_handling))]
4102#[stable(feature = "rust1", since = "1.0.0")]
4103impl<T: Clone> From<&[T]> for Vec<T> {
4104    /// Allocates a `Vec<T>` and fills it by cloning `s`'s items.
4105    ///
4106    /// # Examples
4107    ///
4108    /// ```
4109    /// assert_eq!(Vec::from(&[1, 2, 3][..]), vec![1, 2, 3]);
4110    /// ```
4111    #[track_caller]
4112    fn from(s: &[T]) -> Vec<T> {
4113        s.to_vec()
4114    }
4115}
4116
4117#[cfg(not(no_global_oom_handling))]
4118#[stable(feature = "vec_from_mut", since = "1.19.0")]
4119impl<T: Clone> From<&mut [T]> for Vec<T> {
4120    /// Allocates a `Vec<T>` and fills it by cloning `s`'s items.
4121    ///
4122    /// # Examples
4123    ///
4124    /// ```
4125    /// assert_eq!(Vec::from(&mut [1, 2, 3][..]), vec![1, 2, 3]);
4126    /// ```
4127    #[track_caller]
4128    fn from(s: &mut [T]) -> Vec<T> {
4129        s.to_vec()
4130    }
4131}
4132
4133#[cfg(not(no_global_oom_handling))]
4134#[stable(feature = "vec_from_array_ref", since = "1.74.0")]
4135impl<T: Clone, const N: usize> From<&[T; N]> for Vec<T> {
4136    /// Allocates a `Vec<T>` and fills it by cloning `s`'s items.
4137    ///
4138    /// # Examples
4139    ///
4140    /// ```
4141    /// assert_eq!(Vec::from(&[1, 2, 3]), vec![1, 2, 3]);
4142    /// ```
4143    #[track_caller]
4144    fn from(s: &[T; N]) -> Vec<T> {
4145        Self::from(s.as_slice())
4146    }
4147}
4148
4149#[cfg(not(no_global_oom_handling))]
4150#[stable(feature = "vec_from_array_ref", since = "1.74.0")]
4151impl<T: Clone, const N: usize> From<&mut [T; N]> for Vec<T> {
4152    /// Allocates a `Vec<T>` and fills it by cloning `s`'s items.
4153    ///
4154    /// # Examples
4155    ///
4156    /// ```
4157    /// assert_eq!(Vec::from(&mut [1, 2, 3]), vec![1, 2, 3]);
4158    /// ```
4159    #[track_caller]
4160    fn from(s: &mut [T; N]) -> Vec<T> {
4161        Self::from(s.as_mut_slice())
4162    }
4163}
4164
4165#[cfg(not(no_global_oom_handling))]
4166#[stable(feature = "vec_from_array", since = "1.44.0")]
4167impl<T, const N: usize> From<[T; N]> for Vec<T> {
4168    /// Allocates a `Vec<T>` and moves `s`'s items into it.
4169    ///
4170    /// # Examples
4171    ///
4172    /// ```
4173    /// assert_eq!(Vec::from([1, 2, 3]), vec![1, 2, 3]);
4174    /// ```
4175    #[track_caller]
4176    fn from(s: [T; N]) -> Vec<T> {
4177        <[T]>::into_vec(Box::new(s))
4178    }
4179}
4180
4181#[stable(feature = "vec_from_cow_slice", since = "1.14.0")]
4182impl<'a, T> From<Cow<'a, [T]>> for Vec<T>
4183where
4184    [T]: ToOwned<Owned = Vec<T>>,
4185{
4186    /// Converts a clone-on-write slice into a vector.
4187    ///
4188    /// If `s` already owns a `Vec<T>`, it will be returned directly.
4189    /// If `s` is borrowing a slice, a new `Vec<T>` will be allocated and
4190    /// filled by cloning `s`'s items into it.
4191    ///
4192    /// # Examples
4193    ///
4194    /// ```
4195    /// # use std::borrow::Cow;
4196    /// let o: Cow<'_, [i32]> = Cow::Owned(vec![1, 2, 3]);
4197    /// let b: Cow<'_, [i32]> = Cow::Borrowed(&[1, 2, 3]);
4198    /// assert_eq!(Vec::from(o), Vec::from(b));
4199    /// ```
4200    #[track_caller]
4201    fn from(s: Cow<'a, [T]>) -> Vec<T> {
4202        s.into_owned()
4203    }
4204}
4205
4206// note: test pulls in std, which causes errors here
4207#[stable(feature = "vec_from_box", since = "1.18.0")]
4208impl<T, A: Allocator> From<Box<[T], A>> for Vec<T, A> {
4209    /// Converts a boxed slice into a vector by transferring ownership of
4210    /// the existing heap allocation.
4211    ///
4212    /// # Examples
4213    ///
4214    /// ```
4215    /// let b: Box<[i32]> = vec![1, 2, 3].into_boxed_slice();
4216    /// assert_eq!(Vec::from(b), vec![1, 2, 3]);
4217    /// ```
4218    fn from(s: Box<[T], A>) -> Self {
4219        s.into_vec()
4220    }
4221}
4222
4223// note: test pulls in std, which causes errors here
4224#[cfg(not(no_global_oom_handling))]
4225#[stable(feature = "box_from_vec", since = "1.20.0")]
4226impl<T, A: Allocator> From<Vec<T, A>> for Box<[T], A> {
4227    /// Converts a vector into a boxed slice.
4228    ///
4229    /// Before doing the conversion, this method discards excess capacity like [`Vec::shrink_to_fit`].
4230    ///
4231    /// [owned slice]: Box
4232    /// [`Vec::shrink_to_fit`]: Vec::shrink_to_fit
4233    ///
4234    /// # Examples
4235    ///
4236    /// ```
4237    /// assert_eq!(Box::from(vec![1, 2, 3]), vec![1, 2, 3].into_boxed_slice());
4238    /// ```
4239    ///
4240    /// Any excess capacity is removed:
4241    /// ```
4242    /// let mut vec = Vec::with_capacity(10);
4243    /// vec.extend([1, 2, 3]);
4244    ///
4245    /// assert_eq!(Box::from(vec), vec![1, 2, 3].into_boxed_slice());
4246    /// ```
4247    #[track_caller]
4248    fn from(v: Vec<T, A>) -> Self {
4249        v.into_boxed_slice()
4250    }
4251}
4252
4253#[cfg(not(no_global_oom_handling))]
4254#[stable(feature = "rust1", since = "1.0.0")]
4255impl From<&str> for Vec<u8> {
4256    /// Allocates a `Vec<u8>` and fills it with a UTF-8 string.
4257    ///
4258    /// # Examples
4259    ///
4260    /// ```
4261    /// assert_eq!(Vec::from("123"), vec![b'1', b'2', b'3']);
4262    /// ```
4263    #[track_caller]
4264    fn from(s: &str) -> Vec<u8> {
4265        From::from(s.as_bytes())
4266    }
4267}
4268
4269#[stable(feature = "array_try_from_vec", since = "1.48.0")]
4270impl<T, A: Allocator, const N: usize> TryFrom<Vec<T, A>> for [T; N] {
4271    type Error = Vec<T, A>;
4272
4273    /// Gets the entire contents of the `Vec<T>` as an array,
4274    /// if its size exactly matches that of the requested array.
4275    ///
4276    /// # Examples
4277    ///
4278    /// ```
4279    /// assert_eq!(vec![1, 2, 3].try_into(), Ok([1, 2, 3]));
4280    /// assert_eq!(<Vec<i32>>::new().try_into(), Ok([]));
4281    /// ```
4282    ///
4283    /// If the length doesn't match, the input comes back in `Err`:
4284    /// ```
4285    /// let r: Result<[i32; 4], _> = (0..10).collect::<Vec<_>>().try_into();
4286    /// assert_eq!(r, Err(vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]));
4287    /// ```
4288    ///
4289    /// If you're fine with just getting a prefix of the `Vec<T>`,
4290    /// you can call [`.truncate(N)`](Vec::truncate) first.
4291    /// ```
4292    /// let mut v = String::from("hello world").into_bytes();
4293    /// v.sort();
4294    /// v.truncate(2);
4295    /// let [a, b]: [_; 2] = v.try_into().unwrap();
4296    /// assert_eq!(a, b' ');
4297    /// assert_eq!(b, b'd');
4298    /// ```
4299    fn try_from(mut vec: Vec<T, A>) -> Result<[T; N], Vec<T, A>> {
4300        if vec.len() != N {
4301            return Err(vec);
4302        }
4303
4304        // SAFETY: `.set_len(0)` is always sound.
4305        unsafe { vec.set_len(0) };
4306
4307        // SAFETY: A `Vec`'s pointer is always aligned properly, and
4308        // the alignment the array needs is the same as the items.
4309        // We checked earlier that we have sufficient items.
4310        // The items will not double-drop as the `set_len`
4311        // tells the `Vec` not to also drop them.
4312        let array = unsafe { ptr::read(vec.as_ptr() as *const [T; N]) };
4313        Ok(array)
4314    }
4315}