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Improve docs for module

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Alice Ryhl 2025-09-25 08:39:35 +00:00
parent 7e1c00dbce
commit cd44cd0f63
1 changed files with 16 additions and 2 deletions
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18
library/core/src/pin.rs
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@ -1692,8 +1692,22 @@ impl<Ptr: [const] Deref> const Deref for Pin<Ptr> {
mod helper {
/// Helper that prevents downstream crates from implementing `DerefMut` for `Pin`.
///
/// This type is not `#[fundamental]`, so it's possible to relax its `DerefMut` impl bounds in
/// the future, so the orphan rules reject downstream impls of `DerefMut` of `Pin`.
/// The `Pin` type implements the unsafe trait `PinCoerceUnsized`, which essentially requires
/// that the type does not have a malicious `Deref` or `DerefMut` impl. However, without this
/// helper module, downstream crates are able to write `impl DerefMut for Pin<LocalType>` as
/// long as it does not overlap with the impl provided by stdlib. This is because `Pin` is
/// `#[fundamental]`, so stdlib promises to never implement traits for `Pin` that it does not
/// implement today.
///
/// However, this is problematic. Downstream crates could implement `DerefMut` for
/// `Pin<&LocalType>`, and they could do so maliciously. To prevent this, the implementation for
/// `Pin` delegates to this helper module. Since `helper::Pin` is not `#[fundamental]`, the
/// orphan rules assume that stdlib might implement `helper::DerefMut` for `helper::Pin<&_>` in
/// the future. Because of this, downstream crates can no longer provide an implementation of
/// `DerefMut` for `Pin<&_>`, as it might overlap with a trait impl that, according to the
/// orphan rules, the stdlib could introduce without a breaking change in a future release.
///
/// See <https://github.com/rust-lang/rust/issues/85099> for the issue this fixes.
#[repr(transparent)]
#[unstable(feature = "pin_derefmut_internals", issue = "none")]
#[allow(missing_debug_implementations)]