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Rollup merge of #134602 - kpreid:pointerlike-doc, r=tgross35

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Document `PointerLike` implementation restrictions.

Since <https://github.com/rust-lang/rust/pull/133226>, it is no longer automatically implemented, but must be manually implemented, with special restrictions. The documentation now (roughly) explains those special restrictions.

cc `@compiler-errors` (author of the previous PR)
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Matthias Krüger 2024-12-22 09:12:11 +01:00 • committed by GitHub
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10
library/core/src/marker.rs
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@ -982,8 +982,14 @@ pub trait Tuple {}
/// A marker for pointer-like types.
///
/// This trait can only be implemented for types that have the same size and alignment
/// as a `usize` or `*const ()`.
/// This trait can only be implemented for types that are certain to have
/// the same size and alignment as a [`usize`] or [`*const ()`](pointer).
/// To ensure this, there are special requirements on implementations
/// of `PointerLike` (other than the already-provided implementations
/// for built-in types):
///
/// * The type must have `#[repr(transparent)]`.
/// * The type’s sole non-zero-sized field must itself implement `PointerLike`.
#[unstable(feature = "pointer_like_trait", issue = "none")]
#[lang = "pointer_like"]
#[diagnostic::on_unimplemented(