rollup merge of #21392: japaric/iter

closes #20953
closes #21361

---

In the future, we will likely derive these `impl`s via syntax extensions or using compiler magic (see #20617). For the time being we can use these manual `impl`s.

r? @aturon
cc @burntsushi @Kroisse

src/test/run-pass/issue-20953.rs

@@ -0,0 +1,19 @@
+// Copyright 2014 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.
+
+fn main() {
+    let mut shrinker: Box<Iterator<Item=i32>> = Box::new(vec![1].into_iter());
+    println!("{:?}", shrinker.next());
+    for v in shrinker { assert!(false); }
+
+    let mut shrinker: &mut Iterator<Item=i32> = &mut vec![1].into_iter();
+    println!("{:?}", shrinker.next());
+    for v in shrinker { assert!(false); }
+}

src/test/run-pass/issue-21361.rs

@@ -0,0 +1,19 @@
+// Copyright 2014 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.
+
+fn main() {
+    let v = vec![1, 2, 3];
+    let boxed: Box<Iterator<Item=i32>> = Box::new(v.into_iter());
+    assert_eq!(boxed.max(), Some(3));
+
+    let v = vec![1, 2, 3];
+    let boxed: &mut Iterator<Item=i32> = &mut v.into_iter();
+    assert_eq!(boxed.max(), Some(3));
+}