error: `Iterator::map` call that discard the iterator's values
  --> $DIR/issue-109152.rs:5:21
   |
LL |     vec![42].iter().map(drop);
   |                     ^^^^----^
   |                     |   |
   |                     |   this function returns `()`, which is likely not what you wanted
   |                     |   called `Iterator::map` with callable that returns `()`
   |                     after this call to map, the resulting iterator is `impl Iterator<Item = ()>`, which means the only information carried by the iterator is the number of items
   |
   = note: `Iterator::map`, like many of the methods on `Iterator`, gets executed lazily, meaning that its effects won't be visible until it is iterated
note: the lint level is defined here
  --> $DIR/issue-109152.rs:1:9
   |
LL | #![deny(map_unit_fn)]
   |         ^^^^^^^^^^^
help: you might have meant to use `Iterator::for_each`
   |
LL |     vec![42].iter().for_each(drop);
   |                     ~~~~~~~~

error: aborting due to 1 previous error