Rustdoc makes trait bounds with associated types overly verbose

[joseph-gio]

Location

https://doc.rust-lang.org/std/future/trait.IntoFuture.html

https://doc.rust-lang.org/std/iter/trait.IntoIterator.html

Summary

The definition of the IntoFuture trait looks like:

pub trait IntoFuture {
    type Output;
    type IntoFuture: Future<Output = Self::Output>;
    fn into_future(self) -> Self::IntoFuture;
}

Rustdoc renders this as:

pub trait IntoFuture {
    type Output;
    type IntoFuture: Future
    where
        <Self::IntoFuture as Future>::Output == Self::Output;
    fn into_future(self) -> Self::IntoFuture;
}

This makes the trait bound for the IntoFuture associated type verbose and cryptic.

[fmease]

@rustbot label -A-docs C-bug T-rustdoc A-rustdoc-ui A-associated-items

[fmease]

I did a quick investigation: This reproduces on stable and nightly but only if the corresponding trait comes from an extern crate (which is the case with std & core). Thus marking this issue as
@rustbot label A-cross-crate-reexports

Minimal reproducer:

// rustc one.rs --crate-type=lib
pub trait Future {
    type Output;
}

pub trait IntoFuture {
    type Output;
    type IntoFuture: Future<Output = Self::Output>;
}

// rustdoc two.rs --extern=one=libone.rlib --edition=2021
pub use one::IntoFuture;

@rustbot claim

[fmease]

Highly related to / “duplicate” of #84579 although the latter is quite outdated.