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`, 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