Rollup merge of #107680 - dtolnay:docrepr, r=Amanieu

Hide repr attribute from doc of types without guaranteed repr

Rustdoc has an undesirable behavior of blindly copying `repr` into the documentation of structs and enums, even when there is no particular repr that the type guarantees to its users. This is a source of confusion for standard library users who assume the fact that a repr is documented means it must be something the standard library promises they can rely on (in transmutes, or FFI).

Some issues on the topic of rustdoc's incorrect handling of `repr`:

- rust-lang/rust#66401
- rust-lang/rust#90435

In places, the standard library currently works around this confusing rustdoc behavior by just omitting `repr(transparent)` altogether even where it should be required if equivalent code were being written outside of the standard library. See #61969.

IMO that is even more confusing, even for standard library maintainers — see rust-lang/rust#105018 (comment). It's also not something that works for other reprs like `C` or `u8` which cannot just be omitted even in standard library code.

This PR tries a different approach for some types that are being currently incorrectly documented with a repr.

> **Warning**
> This PR does not imply that every type that still has a `repr` attribute in its docs after this PR is now public for users to rely on. This PR only tries to reduce harm from this longstanding rustdoc issue.

Author: matthiaskrgr

crates/core_simd/src/masks.rs

@@ -88,7 +88,7 @@ impl_element! { isize }
 /// The layout of this type is unspecified, and may change between platforms
 /// and/or Rust versions, and code should not assume that it is equivalent to
 /// `[T; LANES]`.
-#[repr(transparent)]
+#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/90435
 pub struct Mask<T, const LANES: usize>(mask_impl::Mask<T, LANES>)
 where
     T: MaskElement,