warning: the feature `return_type_notation` is incomplete and may not be safe to use and/or cause compiler crashes
  --> $DIR/basic.rs:5:12
   |
LL | #![feature(return_type_notation, async_fn_in_trait)]
   |            ^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #109417 <https://github.com/rust-lang/rust/issues/109417> for more information
   = note: `#[warn(incomplete_features)]` on by default

warning: 1 warning emitted