Add test for IntoIterator pattern blocked by #20220. Fixes #20220.

src/test/run-pass/associated-types-issue-20220.rs

@@ -0,0 +1,36 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test references to `Self::Item` in the trait. Issue #20220.
+
+use std::vec;
+
+trait IntoIteratorX {
+    type Item;
+    type IntoIter: Iterator<Item=Self::Item>;
+
+    fn into_iter_x(self) -> Self::IntoIter;
+}
+
+impl<T> IntoIteratorX for Vec<T> {
+    type Item = T;
+    type IntoIter = vec::IntoIter<T>;
+
+    fn into_iter_x(self) -> vec::IntoIter<T> {
+        self.into_iter()
+    }
+}
+
+fn main() {
+    let vec = vec![1, 2, 3];
+    for (i, e) in vec.into_iter().enumerate() {
+        assert_eq!(i+1, e);
+    }
+}