Add ui test for map_unit_fn lint

tests/ui/lint/lint_map_unit_fn.rs

@@ -0,0 +1,11 @@
+#![deny(map_unit_fn)]
+
+fn foo(items: &mut Vec<u8>) {
+    items.sort();
+}
+
+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
+}

tests/ui/lint/lint_map_unit_fn.stderr

@@ -0,0 +1,25 @@
+error: `Iterator::map` call that discard the iterator's values
+  --> $DIR/lint_map_unit_fn.rs:9:18
+   |
+LL | fn foo(items: &mut Vec<u8>) {
+   | --------------------------- this function returns `()`, which is likely not what you wanted
+...
+LL |     x.iter_mut().map(foo);
+   |                  ^^^^---^
+   |                  |   |
+   |                  |   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/lint_map_unit_fn.rs:1:9
+   |
+LL | #![deny(map_unit_fn)]
+   |         ^^^^^^^^^^^
+help: you might have meant to use `Iterator::for_each`
+   |
+LL |     x.iter_mut().for_each(foo);
+   |                  ~~~~~~~~
+
+error: aborting due to previous error
+