Add ui test for `map_unit_fn` lint in closure case

tests/ui/lint/lint_map_unit_fn.rs

@@ -8,4 +8,13 @@ fn main() {
     let mut x: Vec<Vec<u8>> = vec![vec![0, 2, 1], vec![5, 4, 3]];
     x.iter_mut().map(foo);
     //~^ ERROR `Iterator::map` call that discard the iterator's values
+    x.iter_mut().map(|items| {
+    //~^ ERROR `Iterator::map` call that discard the iterator's values
+        items.sort();
+    });
+    let f = |items: &mut Vec<u8>| {
+        items.sort();
+    };
+    x.iter_mut().map(f);
+    //~^ ERROR `Iterator::map` call that discard the iterator's values
 }

tests/ui/lint/lint_map_unit_fn.stderr

@@ -21,5 +21,46 @@ help: you might have meant to use `Iterator::for_each`
 LL |     x.iter_mut().for_each(foo);
    |                  ~~~~~~~~
 
-error: aborting due to previous error
+error: `Iterator::map` call that discard the iterator's values
+  --> $DIR/lint_map_unit_fn.rs:11:18
+   |
+LL |         x.iter_mut().map(|items| {
+   |                      ^   -------
+   |                      |   |
+   |  ____________________|___this function returns `()`, which is likely not what you wanted
+   | |  __________________|
+   | | |
+LL | | |
+LL | | |         items.sort();
+LL | | |     });
+   | | |     -^ 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`
+   |
+LL |     x.iter_mut().for_each(|items| {
+   |                  ~~~~~~~~
+
+error: `Iterator::map` call that discard the iterator's values
+  --> $DIR/lint_map_unit_fn.rs:18:18
+   |
+LL |     let f = |items: &mut Vec<u8>| {
+   |             --------------------- this function returns `()`, which is likely not what you wanted
+...
+LL |     x.iter_mut().map(f);
+   |                  ^^^^-^
+   |                  |   |
+   |                  |   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
+help: you might have meant to use `Iterator::for_each`
+   |
+LL |     x.iter_mut().for_each(f);
+   |                  ~~~~~~~~
+
+error: aborting due to 3 previous errors