Add test

Author: HMPerson1

tests/ui/issue_2356.rs

@@ -0,0 +1,24 @@
+#![deny(while_let_on_iterator)]
+
+use std::iter::Iterator;
+
+struct Foo;
+
+impl Foo {
+    fn foo1<I: Iterator<Item = usize>>(mut it: I) {
+        while let Some(_) = it.next() {
+            println!("{:?}", it.size_hint());
+        }
+    }
+
+    fn foo2<I: Iterator<Item = usize>>(mut it: I) {
+        while let Some(e) = it.next() {
+            println!("{:?}", e);
+        }
+    }
+}
+
+fn main() {
+    Foo::foo1(vec![].into_iter());
+    Foo::foo2(vec![].into_iter());
+}

tests/ui/issue_2356.stderr

@@ -0,0 +1,14 @@
+error: this loop could be written as a `for` loop
+  --> $DIR/issue_2356.rs:15:29
+   |
+15 |         while let Some(e) = it.next() {
+   |                             ^^^^^^^^^ help: try: `for e in it { .. }`
+   |
+note: lint level defined here
+  --> $DIR/issue_2356.rs:1:9
+   |
+1  | #![deny(while_let_on_iterator)]
+   |         ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+