Rustdoc lists empty opaque type for async fn

[est31]

Since beta 1.69.0 (and also in nightly), the rustdoc output has an empty opaque type listed. Take this code:

fn main() {}
async fn foo() {}

Then save it as main.rs and do:

rustdoc +beta main.rs --crate-type bin --edition=2021 --document-private-items

There will be an empty opaque type listed, which is not clickable like the other things (probably due to being empty):

If you navigate to the opaque..html file manually, you get this:

It also reproduces on current nightly as of filing this bug. And also for cargo init --bin followed by adding an async function and then cargo +beta doc. The option to document private items is important.

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged +T-rustdoc

[jyn514]

cc @compiler-errors

[compiler-errors]

Not sure why I was preemptively pinged, but @GuillaumeGomez regressed this in #104889.

searched nightlies: from nightly-2022-11-01 to nightly-2023-04-04
regressed nightly: nightly-2022-12-23
searched commit range: b569c9d...8a97b48
regressed commit: cce9e72

[compiler-errors]

Anyways I can fix this since I already looked at the code.

@rustbot claim

[GuillaumeGomez]

I wasn't but it's great if you already have a solution. :)