Rustdoc does not escape GAT arguments when it fails to compute the href link for the trait

[fmease]

Given

pub type A = <S as Tr>::P<Option<i32>>; // `i32` instead of `Option<i32>` would work too but it's not as pronounced

pub struct S;
/*priv*/ trait Tr { type P<T>; }
impl Tr for S { type P<T> = (); }

Rustdoc does not escape the ASCII angle brackets for the generic argument list of the generic associated type projection <S as Tr>::P leading to the following butchered HTML output:

pub type A = <S as Tr>::P>; 

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I've noticed this while working on rustdoc integration for AliasKind::Inherent in #109410. It's possible that the refactoring PR #109246 will fix this (need to check) or at least make it more pleasant to fix. Assigning myself for now since I'm gonna fix this otherwise.

@rustbot claim
@rustbot label T-rustdoc A-rustdoc-ui F-generic_associated_types

[jsha]

Copying my comment from #109246:

rust/src/librustdoc/html/format.rs

Line 1158 in 4c0f500

_ => write!(f, "{}{:#}", assoc.name, assoc.args.print(cx))?,

{:#} is requesting alternate formatting (i.e. plaintext), but in this branch of things, alternate formatting hasn't been requested by the caller. So this is likely emitting < into an HTML context. If you change it to {} in your branch I'm guessing that will fix things.

[jsha]

Here's a live example: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_trait_selection/infer/trait.ObligationEmittingRelation.html#tymethod.register_predicates

As it turns out, this bug does appear to be what's crashing rustdoc in #109246. So I will wind up fixing it. But feel free to investigate and fix if you prefer not to wait.

[jsha]

This wound up being the fix: 236c18f

[fmease]

This wound up being the fix: 236c18f

On master at least I cannot reproduce your fix that involves adding Escape(…) in branch Generic (maybe that only works on your branch). In any case, the assoc.args.print(cx) lines in format.rs are definitely the culprit. I've already fixed it that way in #109410 but I've now also opened a separate PR.

Here's a live example: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_trait_selection/infer/trait.ObligationEmittingRelation.html#tymethod.register_predicates

Thanks for the live example!