error: unused return value of `apples` that must be used
 --> tests/ui/must_use.rs:9:5
  |
9 |     apples(3);
  |     ^^^^^^^^^
  |
note: the lint level is defined here
 --> tests/ui/must_use.rs:1:9
  |
1 | #![deny(unused_must_use)]
  |         ^^^^^^^^^^^^^^^
help: use `let _ = ...` to ignore the resulting value
  |
9 |     let _ = apples(3);
  |     +++++++