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`:

- https://github.com/rust-lang/rust/issues/66401
- https://github.com/rust-lang/rust/issues/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 https://github.com/rust-lang/rust/pull/105018#discussion_r1058400997. 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.

library/core/src/any.rs

@@ -866,7 +866,7 @@ fn request_by_type_tag<'a, I>(provider: &'a (impl Provider + ?Sized)) -> Option<
 ///
 /// A data provider provides values by calling this type's provide methods.
 #[unstable(feature = "provide_any", issue = "96024")]
-#[repr(transparent)]
+#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/90435
 pub struct Demand<'a>(dyn Erased<'a> + 'a);
 
 impl<'a> Demand<'a> {

library/core/src/ffi/mod.rs

@@ -203,7 +203,7 @@ mod c_long_definition {
 //     be UB.
 #[doc = include_str!("c_void.md")]
 #[cfg_attr(not(bootstrap), lang = "c_void")]
-#[repr(u8)]
+#[cfg_attr(not(doc), repr(u8))] // work around https://github.com/rust-lang/rust/issues/90435
 #[stable(feature = "core_c_void", since = "1.30.0")]
 pub enum c_void {
     #[unstable(
@@ -244,7 +244,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     target_os = "uefi",
     windows,
 ))]
-#[repr(transparent)]
+#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/90435
 #[unstable(
     feature = "c_variadic",
     reason = "the `c_variadic` feature has not been properly tested on \
@@ -296,7 +296,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     not(target_os = "uefi"),
     not(windows),
 ))]
-#[repr(C)]
+#[cfg_attr(not(doc), repr(C))] // work around https://github.com/rust-lang/rust/issues/66401
 #[derive(Debug)]
 #[unstable(
     feature = "c_variadic",
@@ -316,7 +316,7 @@ pub struct VaListImpl<'f> {
 
 /// PowerPC ABI implementation of a `va_list`.
 #[cfg(all(target_arch = "powerpc", not(target_os = "uefi"), not(windows)))]
-#[repr(C)]
+#[cfg_attr(not(doc), repr(C))] // work around https://github.com/rust-lang/rust/issues/66401
 #[derive(Debug)]
 #[unstable(
     feature = "c_variadic",
@@ -336,7 +336,7 @@ pub struct VaListImpl<'f> {
 
 /// s390x ABI implementation of a `va_list`.
 #[cfg(target_arch = "s390x")]
-#[repr(C)]
+#[cfg_attr(not(doc), repr(C))] // work around https://github.com/rust-lang/rust/issues/66401
 #[derive(Debug)]
 #[unstable(
     feature = "c_variadic",
@@ -355,7 +355,7 @@ pub struct VaListImpl<'f> {
 
 /// x86_64 ABI implementation of a `va_list`.
 #[cfg(all(target_arch = "x86_64", not(target_os = "uefi"), not(windows)))]
-#[repr(C)]
+#[cfg_attr(not(doc), repr(C))] // work around https://github.com/rust-lang/rust/issues/66401
 #[derive(Debug)]
 #[unstable(
     feature = "c_variadic",
@@ -373,7 +373,7 @@ pub struct VaListImpl<'f> {
 }
 
 /// A wrapper for a `va_list`
-#[repr(transparent)]
+#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/90435
 #[derive(Debug)]
 #[unstable(
     feature = "c_variadic",

library/core/src/task/wake.rs

@@ -232,7 +232,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 ///
 /// [`Future::poll()`]: core::future::Future::poll
 /// [`Poll::Pending`]: core::task::Poll::Pending
-#[repr(transparent)]
+#[cfg_attr(not(doc), repr(transparent))] // work around https://github.com/rust-lang/rust/issues/66401
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub struct Waker {
     waker: RawWaker,

library/portable-simd/crates/core_simd/src/masks.rs

@@ -88,7 +88,7 @@ unsafe impl MaskElement for $ty {}
 /// 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,