rustdoc: Clone's implementations on CString is considered foreign

[jsha]

On https://doc.rust-lang.org/nightly/std/clone/trait.Clone.html#foreign-impls, CString, FromBytesUntilNulError, FromBytesWithNulError, FromVecWithNulError, IntoStringError, and NulError are considered "Implementations on Foreign Types", even though each of those types exists in std.

On stable (https://doc.rust-lang.org/stable/std/clone/trait.Clone.html#foreign-impls), those types are not considered "Implementations on Foreign Types."

I believe this is caused indirectly by 7f3cc2f (/cc @petrochenkov). That commit changed the export style for CString etc from pub use to pub type CString = alloc::ffi::CString.

However, this could also be considered a rustdoc bug. MCVE:

yarble.rs

pub struct Yarb;
pub struct Proust;
pub struct Dsug;

foo.rs

extern crate yarble;

pub trait Bar {}

impl Bar for yarble::Yarb{}
impl Bar for yarble::Proust{}

pub use yarble::Yarb;

pub type Proust = yarble::Proust;
pub type Dsug = yarble::Dsug;

impl Bar for Dsug{}

run:

rustc --crate-type lib yarble.rs && rm -r doc ; rustdoc -L . foo.rs && open doc/foo/trait.Bar.html

Expected result:

All three of Yarble, Dsug, and Proust are considered plain "Implementations".

Actual result:

Proust is considered an "Implementation on Foreign Type" (screenshot below).

[jsha]

With #97613 merged, the situation is better for the __m256 types, but still is not solved for CString, NulError, etc.

It occurred to me that std should actually have no cases of "Implementations on Foreign Types," so we can get a broader view of the issue by searching for that string in std docs:

$ rg "on Foreign Types" build/x86_64-unknown-linux-gnu/doc/ -l | sed 's,.*/doc/,https://doc.rust-lang.org/nightly/,'

https://doc.rust-lang.org/nightly/std/clone/trait.Clone.html
https://doc.rust-lang.org/nightly/std/fmt/trait.Display.html
https://doc.rust-lang.org/nightly/std/fmt/trait.Debug.html
https://doc.rust-lang.org/nightly/std/cmp/trait.Eq.html
https://doc.rust-lang.org/nightly/std/cmp/trait.PartialOrd.html
https://doc.rust-lang.org/nightly/std/cmp/trait.PartialEq.html
https://doc.rust-lang.org/nightly/std/cmp/trait.Ord.html
https://doc.rust-lang.org/nightly/std/borrow/trait.Borrow.html
https://doc.rust-lang.org/nightly/std/borrow/trait.ToOwned.html
https://doc.rust-lang.org/nightly/std/convert/trait.AsRef.html
https://doc.rust-lang.org/nightly/std/convert/trait.From.html
https://doc.rust-lang.org/nightly/std/default/trait.Default.html
https://doc.rust-lang.org/nightly/std/marker/trait.StructuralEq.html
https://doc.rust-lang.org/nightly/std/marker/trait.StructuralPartialEq.html
https://doc.rust-lang.org/nightly/std/hash/trait.Hash.html
https://doc.rust-lang.org/nightly/std/error/trait.Error.html
https://doc.rust-lang.org/nightly/std/ops/trait.Index.html
https://doc.rust-lang.org/nightly/std/ops/trait.Deref.html
https://doc.rust-lang.org/nightly/std/ops/trait.Drop.html
https://doc.rust-lang.org/nightly/alloc/string/trait.ToString.html
https://doc.rust-lang.org/nightly/alloc/slice/trait.Join.html
https://doc.rust-lang.org/nightly/alloc/slice/trait.Concat.html
https://doc.rust-lang.org/nightly/alloc/borrow/trait.ToOwned.html

Some interesting ones in there:

alloc::string::ToString considers the impls on char, u8, i8, and str to be foreign, while std:::string::ToString does not.

alloc::slice:::Concat considers both of its impls to be foreign, while std::slice::Concat does not.

alloc::borrow::ToOwned considers its impls on [T: Clone], str, and CStr to be foreign, while std::borrow::ToOwned only considers CStr to be foreign.

The remaining traits consider the alloc::ffi structs to be foreign.

[jsha]

This is fixed (in a hacky but okay way) by #97735.