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

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;
}

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)

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.

Auto merge of rust-lang#85479 - Stupremee:render-Self_as-type-casts, …

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

@Stupremee

@Stupremee #84579 might be related.

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

@Stupremee

@Stupremee #84579 (comment) might be interesting.

Auto merge of rust-lang#86449 - Stupremee:render-self-cast-in-type-bo…

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rustdoc removes Try from <Self as Try>::Residual in std::ops::FromResidual #85454

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

rustdoc removes Try from <Self as Try>::Residual in std::ops::FromResidual #85454

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scottmcm commented on May 18, 2021

Stupremee commented on May 19, 2021

Stupremee commented on May 24, 2021

cynecx commented on May 25, 2021

Stupremee commented on May 25, 2021

cynecx commented on May 25, 2021

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