error: unused pinned boxed `Future` trait object that must be used
  --> tests/ui/must-use.rs:18:5
   |
18 |     Thing.f();
   |     ^^^^^^^^^
   |
   = note: futures do nothing unless you `.await` or poll them
note: the lint level is defined here
  --> tests/ui/must-use.rs:1:9
   |
1  | #![deny(unused_must_use)]
   |         ^^^^^^^^^^^^^^^

error: unused return value of `Interface::f` that must be used
  --> tests/ui/must-use.rs:18:5
   |
18 |     Thing.f();
   |     ^^^^^^^^^
   |
help: use `let _ = ...` to ignore the resulting value
   |
18 |     let _ = Thing.f();
   |     +++++++