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Change pedantically incorrect OnceCell/OnceLock wording #125504

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Merged
merged 1 commit into from
Jun 4, 2024
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Change pedantically incorrect OnceCell/OnceLock wording #125504

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Change pedantically incorrect OnceCell/OnceLock wording 
While the semantic intent of a OnceCell/OnceLock is that it can only be written
to once (upon init), the fact of the matter is that both these types offer a
`take(&mut self) -> Option<T>` mechanism that, when successful, resets the cell
to its initial state, thereby technically allowing it to be written to again.

Despite the fact that this can only happen with a mutable reference (generally
only used during the construction of the OnceCell/OnceLock), it would be
incorrect to say that the type itself as a whole categorically prevents being
initialized or written to more than once (since it is possible to imagine an
identical type only without the `take()` method that actually fulfills that
contract).

To clarify, change "that cannot be.." to "that nominally cannot.." and add a
note to OnceCell about what can be done with an `&mut Self` reference.
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@mqudsi
mqudsi committed May 24, 2024
commit 65dffc1990f91b470f23175ef243bdd5e0b5313a
4 changes: 2 additions & 2 deletions library/core/src/cell/once.rs
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Expand Up @@ -2,12 +2,12 @@ use crate::cell::UnsafeCell;
use crate::fmt;
use crate::mem;

/// A cell which can be written to only once.
/// A cell which can nominally be written to only once.
///
/// This allows obtaining a shared `&T` reference to its inner value without copying or replacing
/// it (unlike [`Cell`]), and without runtime borrow checks (unlike [`RefCell`]). However,
/// only immutable references can be obtained unless one has a mutable reference to the cell
/// itself.
/// itself. In the same vein, the cell can only be re-initialized with such a mutable reference.
///
/// For a thread-safe version of this struct, see [`std::sync::OnceLock`].
///
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2 changes: 1 addition & 1 deletion library/std/src/sync/once_lock.rs
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Expand Up @@ -5,7 +5,7 @@ use crate::mem::MaybeUninit;
use crate::panic::{RefUnwindSafe, UnwindSafe};
use crate::sync::Once;

/// A synchronization primitive which can be written to only once.
/// A synchronization primitive which can nominally be written to only once.
///
/// This type is a thread-safe [`OnceCell`], and can be used in statics.
///
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