error[E0658]: gen blocks are experimental
  --> $DIR/async_gen_fn.rs:4:1
   |
LL | async gen fn foo() {}
   | ^^^^^^^^^
   |
   = note: see issue #117078 <https://github.com/rust-lang/rust/issues/117078> for more information
   = help: add `#![feature(gen_blocks)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0658`.