missing_iterator_fold: test the lint

Author: Philippe-Cholet

Diff for: tests/ui/missing_iterator_fold.rs

@@ -1,5 +1,12 @@
 #![warn(clippy::missing_iterator_fold)]
 
-fn main() {
-    // test code goes here
+struct Countdown(u8);
+
+impl Iterator for Countdown {
+    type Item = u8;
+
+    fn next(&mut self) -> Option<u8> {
+        self.0 = self.0.checked_sub(1)?;
+        Some(self.0)
+    }
 }

Diff for: tests/ui/missing_iterator_fold.stderr

@@ -0,0 +1,17 @@
+error: you are implementing `Iterator` without specializing its `fold` method
+  --> $DIR/missing_iterator_fold.rs:5:1
+   |
+LL | / impl Iterator for Countdown {
+LL | |     type Item = u8;
+LL | |
+LL | |     fn next(&mut self) -> Option<u8> {
+...  |
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = note: `-D clippy::missing-iterator-fold` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::missing_iterator_fold)]`
+
+error: aborting due to 1 previous error
+