#[rustc_pub_transparent] does not shows up as #[repr(transparent)] in docs

[orlp]

Location (URL)

https://doc.rust-lang.org/nightly/std/mem/struct.ManuallyDrop.html

Summary

In 1.65 this struct was shown as #[repr(transparent)] in the docs: https://doc.rust-lang.org/1.65/std/mem/struct.ManuallyDrop.html.

In 1.66 this regressed and the attribute disappeared: https://doc.rust-lang.org/1.66/std/mem/struct.ManuallyDrop.html.

It is now marked as #[rustc_pub_transparent] so I think we should show #[repr(transparent)] again (as well as for any other structs marked #[rustc_pub_transparent]).

[eggyal]

Is it meaningful to show the attribute in the documentation for a type whose non-zero sized field is private? What exactly does it tell the reader?

[orlp]

In the most absolute theoretically correct sense, with the current hiding of private fields, nothing. In practical terms a type marked like this (Exclusive, ManuallyDrop, Cell, etc), will be understood to simply contain the generic type.

This could be made explicit by showing the private field, should the type be marked rustc_pub_transparent. In this case the type is still private from a Rust access perspective, but public from a documentation perspective. I personally think this makes a lot of sense for rustc_pub_transparent. For example for ManuallyDrop the documentation could show:

#[repr(transparent)]
pub struct ManuallyDrop<T>(/* private */ T)
where
    T: ?Sized,

[Hezuikn-2]

taking a pointer of inner without moving out of ManuallyDrop with into_inner relies on repr transparent as there is no as_mut_ptr on it

it has DerefMut however taking &mut to cast to *mut is a statement that all the rules of references are respected, this is why &raw mut and its wrapper addr_of_mut!, which was stabilized first, exist