Rollup merge of rust-lang#120107 - shepmaster:dead-code-repr-transparent, r=Nilstrieb

dead_code treats #[repr(transparent)] the same as #[repr(C)]

In rust-lang#92972 we enabled linting on unused fields in tuple structs. In rust-lang#118297 that lint was enabled by default. That exposed issues like rust-lang#119659, where the fields of a struct marked `#[repr(transparent)]` were reported by the `dead_code` lint. The language team [decided](rust-lang#119659 (comment)) that the lint should treat `repr(transparent)` the same as `#[repr(C)]`.

Fixes rust-lang#119659

Author: matthiaskrgr

alloc/src/boxed/thin.rs

@@ -171,7 +171,6 @@ struct WithHeader<H>(NonNull<u8>, PhantomData<H>);
 /// An opaque representation of `WithHeader<H>` to avoid the
 /// projection invariance of `<T as Pointee>::Metadata`.
 #[repr(transparent)]
-#[allow(dead_code)] // Field only used through `WithHeader` type above.
 struct WithOpaqueHeader(NonNull<u8>);
 
 impl WithOpaqueHeader {