error[E0046]: not all trait items implemented, missing: `foo`
  --> $DIR/missing-feature-flag.rs:11:1
   |
LL |     async fn foo(_: T) -> &'static str;
   |     ----------------------------------- `foo` from trait
...
LL | impl<T> MyTrait<T> for MyStruct {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation

error[E0308]: mismatched types
  --> $DIR/missing-feature-flag.rs:15:42
   |
LL |     async fn foo(_: i32) -> &'static str {}
   |                                          ^^ expected `&str`, found `()`

error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
  --> $DIR/missing-feature-flag.rs:15:5
   |
LL | impl<T> MyTrait<T> for MyStruct {}
   | ------------------------------- parent `impl` is here
...
LL |     async fn foo(_: i32) -> &'static str {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot specialize default item `foo`
   |
   = note: to specialize, `foo` in the parent `impl` must be marked `default`

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0046, E0308, E0520.
For more information about an error, try `rustc --explain E0046`.