error[E0107]: enum takes 2 generic arguments but 1 generic argument was supplied
  --> $DIR/issue-65159.rs:5:20
   |
LL | async fn copy() -> Result<()>
   |                    ^^^^^^ -- supplied 1 generic argument
   |                    |
   |                    expected 2 generic arguments
   |
help: add missing generic argument
   |
LL | async fn copy() -> Result<(), E>
   |                             +++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0107`.