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Re-stabilize Weak::as_ptr and friends for unsized T #80764

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Merged
merged 8 commits into from
Jan 16, 2021
Merged

Re-stabilize Weak::as_ptr and friends for unsized T #80764

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6bc772c
Re-stabilize Weak::as_ptr &friends for unsized T
CAD97 Jan 6, 2021
f00b458
Tighten/clarify documentation of rc data_offset
CAD97 Jan 7, 2021
b10b9e2
Remove "pointer describes" terminology
CAD97 Jan 7, 2021
1e578c9
Reclarify Weak<->raw pointer safety comments
CAD97 Jan 7, 2021
4901c55
Replace set_data_ptr with pointer::set_ptr_value
CAD97 Jan 7, 2021
747dbcb
Provide reasoning for rc data_offset safety
CAD97 Jan 9, 2021
b5b6760
Weak::into_raw shouldn't translate sentinel value
CAD97 Jan 11, 2021
c14e919
Apply suggestions from code review
CAD97 Jan 13, 2021
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Remove "pointer describes" terminology
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@CAD97
CAD97 committed Jan 7, 2021
commit b10b9e25ffcb06077375960f07a0c6443184f07a
9 changes: 4 additions & 5 deletions library/alloc/src/rc.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1899,7 +1899,7 @@ impl<T: ?Sized> Weak<T> {
// a valid pointer, so that `from_raw` can reverse this transformation.
(ptr as *mut T).set_ptr_value(ptr::null_mut())
} else {
// SAFETY: If the pointer is not dangling, it describes to a valid allocation.
// SAFETY: If the pointer is not dangling, it references a valid allocation.
// The payload may be dropped at this point, and we have to maintain provenance,
// so use raw pointer manipulation.
unsafe { &raw mut (*ptr).value }
Expand Down Expand Up @@ -1991,8 +1991,8 @@ impl<T: ?Sized> Weak<T> {
// SAFETY: this is the same sentinel as used in Weak::new and is_dangling
(ptr as *mut RcBox<T>).set_ptr_value(usize::MAX as *mut _)
} else {
// Otherwise, this describes a real allocation.
// SAFETY: data_offset is safe to call, as ptr describes a real allocation.
// Otherwise, this references a real allocation.
// SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
let offset = unsafe { data_offset(ptr) };
// Thus, we reverse the offset to get the whole RcBox.
// SAFETY: the pointer originated from a Weak, so this offset is safe.
Expand Down Expand Up @@ -2320,8 +2320,7 @@ impl<T: ?Sized> AsRef<T> for Rc<T> {
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Rc<T> {}

/// Get the offset within an `RcBox` for
/// a payload of type described by a pointer.
/// Get the offset within an `RcBox` for the payload behind a pointer.
///
/// # Safety
///
Expand Down
9 changes: 4 additions & 5 deletions library/alloc/src/sync.rs
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Expand Up @@ -1685,7 +1685,7 @@ impl<T: ?Sized> Weak<T> {
// a valid pointer, so that `from_raw` can reverse this transformation.
(ptr as *mut T).set_ptr_value(ptr::null_mut())
} else {
// SAFETY: If the pointer is not dangling, it describes to a valid allocation.
// SAFETY: If the pointer is not dangling, it references a valid allocation.
// The payload may be dropped at this point, and we have to maintain provenance,
// so use raw pointer manipulation.
unsafe { &raw mut (*ptr).data }
Expand Down Expand Up @@ -1777,8 +1777,8 @@ impl<T: ?Sized> Weak<T> {
// SAFETY: this is the same sentinel as used in Weak::new and is_dangling
(ptr as *mut ArcInner<T>).set_ptr_value(usize::MAX as *mut _)
} else {
// Otherwise, this describes a real allocation.
// SAFETY: data_offset is safe to call, as ptr describes a real allocation.
// Otherwise, this references a real allocation.
// SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
let offset = unsafe { data_offset(ptr) };
// Thus, we reverse the offset to get the whole RcBox.
// SAFETY: the pointer originated from a Weak, so this offset is safe.
Expand Down Expand Up @@ -2471,8 +2471,7 @@ impl<T: ?Sized> AsRef<T> for Arc<T> {
#[stable(feature = "pin", since = "1.33.0")]
impl<T: ?Sized> Unpin for Arc<T> {}

/// Get the offset within an `ArcInner` for
/// a payload of type described by a pointer.
/// Get the offset within an `ArcInner` for the payload behind a pointer.
///
/// # Safety
///
Expand Down