error[E0670]: `async fn` is not permitted in Rust 2015
  --> $DIR/async_gen_fn.rs:4:1
   |
LL | async gen fn foo() {}
   | ^^^^^ to use `async fn`, switch to Rust 2018 or later
   |
   = help: pass `--edition 2021` to `rustc`
   = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error: expected one of `extern`, `fn`, or `unsafe`, found `gen`
  --> $DIR/async_gen_fn.rs:4:7
   |
LL | async gen fn foo() {}
   |       ^^^ expected one of `extern`, `fn`, or `unsafe`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0670`.