Emit error on async trait functions

[P-E-P]

          you can add the same check for `async` functions, even if that is about to change soon.

Originally posted by @CohenArthur in #2753 (comment)

error[E0706]: functions in traits cannot be declared `async`
 --> src/lib.rs:2:5
  |
2 |     async fn titi();
  |     -----^^^^^^^^^^^
  |     |
  |     `async` because of this
  |
  = note: `async` trait functions are not currently supported
  = note: consider using the `async-trait` crate: https://crates.io/crates/async-trait
  = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information

[braw-lee]

I want to fix this

[braw-lee]

I added a check and test for async function inside a trait but gccrs fails to recognize async as a token inside trait impl
test.rs:6:5: error: unrecognised token ‘async’ for item in trait impl

6 | async fn f() -> u32 {
| ^~~~~
test.rs:6:5: error: failed to parse trait impl item in trait impl
test.rs:6:5: error: failed to parse item in crate

[braw-lee]

is async token not fully supported yet? I was trying to step through the lexer with gdb to find out but for some reason it skips over some code even with -O0 flag enabled

[P-E-P]

is async token not fully supported yet? I was trying to step through the lexer with gdb to find out but for some reason it skips over some code even with -O0 flag enabled

AFAIK async token is supported by the lexer, we already have multiple async nodes in our ast. For example the parse_async_item function make use of it indirectly.

[braw-lee]

yes, looks like async is supported

this makes me think there is an unhandled case for async token when we are inside a trait
I will look into it in a few days

[tschwinge]

@P-E-P, @braw-lee, @CohenArthur, is it correct that this issue got closed via #2944 "Merge upstream, 2024-03-09" (why wasn't it then already closed via a GCC/Rust master branch commit?), or remains there work to be done here?

[braw-lee]

#2779
the second commit of this PR fixed this issue

[braw-lee]

I think it wasn't closed because I didn't mention the issue #2767 in the message