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error: `Iterator::map` call that discard the iterator's values
  --> $DIR/lint_map_unit_fn.rs:11:18
   |
11 |         x.iter_mut().map(|items| {
   |                      ^   -------
   |                      |   |
   |  ____________________|___this function returns `()`, which is likely not what you wanted
   | |  __________________|
   | | |
12 | | |     //~^ ERROR `Iterator::map` call that discard the iterator's values
13 | | |         items.sort();
14 | | |     });
   | | |     -^ 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
   | | |_____||
   | |_______|
   |         called `Iterator::map` with callable that returns `()`
   |
   = 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
help: you might have meant to use `Iterator::for_each`
   |
11 -     x.iter_mut().map(|items| {
11 +     x.iter_mut().for_each(|items| {
   |

After:

error: `Iterator::map` call that discard the iterator's values
  --> $DIR/lint_map_unit_fn.rs:11:18
   |
11 |         x.iter_mut().map(|items| {
   |                      ^   -------
   |                      |   |
   |  ____________________|___this function returns `()`, which is likely not
   | |                    |   what you wanted
   | |  __________________|
   | | |
12 | | |     //~^ ERROR `Iterator::map` call that discard the iterator's values
13 | | |         items.sort();
14 | | |     });
   | | |     -^
   | | |_____||
   | | |_____||
   | |       |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
   |         called `Iterator::map` with callable that returns `()`
   |
   = 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
help: you might have meant to use `Iterator::for_each`
   |
11 -     x.iter_mut().map(|items| {
11 +     x.iter_mut().for_each(|items| {
   |