error: `async` trait bounds are only allowed in Rust 2018 or later
  --> $DIR/edition-2015.rs:1:16
   |
LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
   |                ^^^^^
   |
   = help: pass `--edition 2021` to `rustc`
   = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error: `async` trait bounds are only allowed in Rust 2018 or later
  --> $DIR/edition-2015.rs:1:36
   |
LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
   |                                    ^^^^^
   |
   = help: pass `--edition 2021` to `rustc`
   = note: for more on editions, read https://doc.rust-lang.org/edition-guide

error[E0658]: async closures are unstable
  --> $DIR/edition-2015.rs:1:16
   |
LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
   |                ^^^^^
   |
   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
   = help: add `#![feature(async_closure)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: to use an async block, remove the `||`: `async {`

error[E0658]: async closures are unstable
  --> $DIR/edition-2015.rs:1:36
   |
LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
   |                                    ^^^^^
   |
   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
   = help: add `#![feature(async_closure)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: to use an async block, remove the `||`: `async {`

error[E0658]: use of unstable library feature 'async_closure'
  --> $DIR/edition-2015.rs:1:42
   |
LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
   |                                          ^^^^
   |
   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
   = help: add `#![feature(async_closure)]` 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   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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[E0658]: use of unstable library feature 'async_closure'
  --> $DIR/edition-2015.rs:1:22
   |
LL | fn foo(x: impl async Fn()) -> impl async Fn() { x }
   |                      ^^^^
   |
   = note: see issue #62290 <https://github.com/rust-lang/rust/issues/62290> for more information
   = help: add `#![feature(async_closure)]` 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 6 previous errors

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