rustdoc doesn't handle generic associated types properly

[JanBeh]

I tried this code:

pub trait Machine<'a> {
    /// Data type representing values passed to or returned from machine
    type Datum<'b>
    where
        Self: 'b;
    /// Function (for example returned by [`Compile::compile`])
    /// which can be executed by the machine
    type Function<'b>: Function<Datum = Self::Datum<'b>>
    where
        Self: 'b;
}

I expected rustdoc to create proper documentation for these GATs.

Instead, rustdoc created the following output:

pub trait Machine<'a> {
    type Datum;
    type Function: Function<Datum = Self::Datum>;
}

and

Associated Types
[src]
type Datum

Data type representing values passed to or returned from machine
[src]
type Function: Function<Datum = Self::Datum>

Function (for example returned by Compile::compile) which can be executed by the machine

The lifetime argument 'b is missing in the documentation.

Meta

rustc --version --verbose:

rustc 1.59.0-nightly (f8abed9ed 2021-12-26)
binary: rustc
commit-hash: f8abed9ed48bace6be0087bcd44ed534e239b8d8
commit-date: 2021-12-26
host: x86_64-unknown-freebsd
release: 1.59.0-nightly
LLVM version: 13.0.0

[BGR360]

@rustbot label +T-rustdoc +F-generic_associated_types

[jackh726]

GATs issue triage: blocking. I'm going to go ahead and mark this as blocking precisely because I want to elevate it's priority: in theory, this isn't a hard fix, but having proper documentation of code with GATs is kind of a big ergonomics win. In actuality, this can be demoted to not blocking if it actually turns out to be a really difficult problem or something.

[compiler-errors]

@rustbot claim

having proper documentation of code with GATs is kind of a big ergonomics win.

Strongly agree. Stabilizing GATs when rustdocs don't properly render associated types' own generics is kinda a big problem, IMO.

this can be demoted to not blocking if it actually turns out to be a really difficult problem or something.

As far as I can tell, this doesn't seem too hard. I'm working through it right now. I've got GATs rendering in trait definitions fine right now, working through some rendering issues, but I can probably get help in the rustdoc zulip.