rustdoc removes Try from <Self as Try>::Residual in std::ops::FromResidual

[veber-alex]

In the docs for the new FromResidual trait here:
https://doc.rust-lang.org/nightly/std/ops/trait.FromResidual.html

The trait is shown as:

pub trait FromResidual<R = Self::Residual> {
    fn from_residual(residual: R) -> Self;
}

which is not valid and will not compile, in the source it looks like this:

pub trait FromResidual<R = <Self as Try>::Residual> {
    fn from_residual(residual: R) -> Self;
}

[scottmcm]

Hypothesis: this a a weird case in the associated type normalization, since in just about any other position this would only be allowed because there'd be a where Self: Try in scope, but there's isn't here.

(It's a very unusual case, and also broke R-A: rust-lang/rust-analyzer#8692)

[Stupremee]

I researched a bit into this issue. The problem is that rustdoc never renders a <Self as X>::Y cast (where Self is the type to be cast) because of this check:

rust/src/librustdoc/html/format.rs

Lines 840 to 845 in 9f8012e

	let should_show_cast = match *trait_ {
	box clean::ResolvedPath { ref path, .. } => {
	!path.segments.is_empty() && !self_type.is_self_type()
	}
	_ => true,
	};

This is another example where the cast is stripped away by rustdoc:

pub trait A {
    type Tar;
}

pub struct Foo {}

impl Foo {
    pub fn foo() -> <Self as A>::Tar {}
}

impl A for Foo { type Tar = u32; }

would be shown as:

pub fn foo() -> Self::Tar

However, it's a bit harder to fix this because simply removing this check would lead to false-positive where Self is cast to the type of itself (<Self as Deref>::Target in a Deref impl for example), which is wrong too.

[Stupremee]

This needs to be re-opened because it does not work if the trait is re-exported.
Compare https://doc.rust-lang.org/nightly/std/ops/trait.FromResidual.html with https://doc.rust-lang.org/nightly/core/ops/trait.FromResidual.html

I will try to investigate and fix it in the next few days

[cynecx]

@Stupremee #84579 might be related.

[Stupremee]

Cross Crate re-exports are really weird in general. They already caused me so much pain

[cynecx]

@Stupremee #84579 (comment) might be interesting.