error: `async` trait implementations are unsupported
  --> $DIR/impl-header.rs:5:6
   |
LL | impl async Fn<()> for F {}
   |      ^^^^^

error[E0658]: the precise format of `Fn`-family traits' type parameters is subject to change
  --> $DIR/impl-header.rs:5:12
   |
LL | impl async Fn<()> for F {}
   |            ^^^^^^
   |
   = note: see issue #29625 <https://github.com/rust-lang/rust/issues/29625> for more information
   = help: add `#![feature(unboxed_closures)]` 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   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date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[E0183]: manual implementations of `Fn` are experimental
  --> $DIR/impl-header.rs:5:12
   |
LL | impl async Fn<()> for F {}
   |            ^^^^^^ manual implementations of `Fn` are experimental
   |
   = help: add `#![feature(unboxed_closures)]` to the crate attributes to enable

error[E0046]: not all trait items implemented, missing: `call`
  --> $DIR/impl-header.rs:5:1
   |
LL | impl async Fn<()> for F {}
   | ^^^^^^^^^^^^^^^^^^^^^^^ missing `call` in implementation
   |
   = help: implement the missing item: `fn call(&self, _: ()) -> <Self as FnOnce<()>>::Output { todo!() }`

error[E0277]: expected a `FnMut()` closure, found `F`
  --> $DIR/impl-header.rs:5:23
   |
LL | impl async Fn<()> for F {}
   |                       ^ expected an `FnMut()` closure, found `F`
   |
help: the trait `FnMut()` is not implemented for `F`
  --> $DIR/impl-header.rs:3:1
   |
LL | struct F;
   | ^^^^^^^^
   = note: wrap the `F` in a closure with no arguments: `|| { /* code */ }`
note: required by a bound in `Fn`
  --> $SRC_DIR/core/src/ops/function.rs:LL:COL

error: aborting due to 5 previous errors

Some errors have detailed explanations: E0046, E0183, E0277, E0658.
For more information about an error, try `rustc --explain E0046`.