error: non-binding `let` on a future
  --> $DIR/let_underscore_future.rs:16:5
   |
LL |     let _ = some_async_fn();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: consider awaiting the future or dropping explicitly with `std::mem::drop`
   = note: `-D clippy::let-underscore-future` implied by `-D warnings`

error: non-binding `let` on a future
  --> $DIR/let_underscore_future.rs:18:5
   |
LL |     let _ = custom();
   |     ^^^^^^^^^^^^^^^^^
   |
   = help: consider awaiting the future or dropping explicitly with `std::mem::drop`

error: non-binding `let` on a future
  --> $DIR/let_underscore_future.rs:23:5
   |
LL |     let _ = future;
   |     ^^^^^^^^^^^^^^^
   |
   = help: consider awaiting the future or dropping explicitly with `std::mem::drop`

error: this argument is a mutable reference, but not used mutably
  --> $DIR/let_underscore_future.rs:11:35
   |
LL | fn do_something_to_future(future: &mut impl Future<Output = ()>) {}
   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&impl Future<Output = ()>`
   |
   = note: `-D clippy::needless-pass-by-ref-mut` implied by `-D warnings`

error: aborting due to 4 previous errors