Incorrect "use of undeclared lifetime name" error with nested async functions

[alexcrichton]

Given this Rust code:

struct Foo<'a>(&'a u32);

impl<'a> Foo<'a> {
    async fn foo() {
        struct Bar<'b>(&'b u32);

        impl<'b> Bar<'b> {
            async fn bar() {}
        }
    }
}

the compiler errors out with:

$ rustc +nightly foo.rs --crate-type lib --edition 2021
error[E0261]: use of undeclared lifetime name `'a`
 --> foo.rs:3:6
  |
3 | impl<'a> Foo<'a> {
  |      ^^ undeclared lifetime
  |
  = help: if you want to experiment with in-band lifetime bindings, add `#![feature(in_band_lifetimes)]` to the crate attributes
help: consider introducing lifetime `'a` here
  |
7 |         impl<'a, 'b> Bar<'b> {
  |              +++
help: consider introducing lifetime `'a` here
  |
8 |             async fn bar<'a>() {}
  |                         ++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0261`.

however this code:

struct Foo<'a>(&'a u32);

impl<'a> Foo<'a> {
    async fn foo() {}
}

struct Bar<'b>(&'b u32);

impl<'b> Bar<'b> {
    async fn bar() {}
}

compiles successfully.

This seems like it might be related to the expansion of async code internally in rustc but I'm not entirely sure myself.

The nightly I tested is:

$ rustc +nightly -vV
rustc 1.60.0-nightly (e646f3d2a 2022-02-10)
binary: rustc
commit-hash: e646f3d2a9541952310778288854943678738ea9
commit-date: 2022-02-10
host: aarch64-unknown-linux-gnu
release: 1.60.0-nightly
LLVM version: 13.0.0

[ehuss]

This appears to have been fixed by #94773, but I think it would be good to add a regression test.