Auto merge of #42634 - Zoxc:for-desugar2, r=nikomatsakis

Change the for-loop desugar so the `break` does not affect type inference. Fixes #42618

Rewrite the `for` loop desugaring to avoid contaminating the inference results. Under the older desugaring, `for x in vec![] { .. }` would erroneously type-check, even though the type of `vec![]` is unconstrained. (written by @nikomatsakis)

src/test/compile-fail/for-loop-unconstrained-element-type.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 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 that `for` loops don't introduce artificial
+// constraints on the type of the binding (`i`).
+// Subtle changes in the desugaring can cause the
+// type of elements in the vector to (incorrectly)
+// fallback to `!` or `()`.
+
+fn main() {
+    for i in Vec::new() { } //~ ERROR type annotations needed
+}

src/test/run-pass/for-loop-lifetime-of-unbound-values.rs

@@ -0,0 +1,43 @@
+// Copyright 2017 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 when destructors run in a for loop. The intention is
+// that the value for each iteration is dropped *after* the loop
+// body has executed. This is true even when the value is assigned
+// to a `_` pattern (and hence ignored).
+
+use std::cell::Cell;
+
+struct Flag<'a>(&'a Cell<bool>);
+
+impl<'a> Drop for Flag<'a> {
+    fn drop(&mut self) {
+        self.0.set(false)
+    }
+}
+
+fn main() {
+    let alive2 = Cell::new(true);
+    for _i in std::iter::once(Flag(&alive2)) {
+        // The Flag value should be alive in the for loop body
+        assert_eq!(alive2.get(), true);
+    }
+    // The Flag value should be dead outside of the loop
+    assert_eq!(alive2.get(), false);
+
+    let alive = Cell::new(true);
+    for _ in std::iter::once(Flag(&alive)) {
+        // The Flag value should be alive in the for loop body even if it wasn't
+        // bound by the for loop
+        assert_eq!(alive.get(), true);
+    }
+    // The Flag value should be dead outside of the loop
+    assert_eq!(alive.get(), false);
+}

src/test/run-pass/for-loop-mut-ref-element.rs

@@ -0,0 +1,15 @@
+// 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.
+
+// Tests that for loops can bind elements as mutable references
+
+fn main() {
+    for ref mut _a in std::iter::once(true) {}
+}

src/test/run-pass/for-loop-unconstrained-element-type-i32-fallback.rs

@@ -0,0 +1,20 @@
+// Copyright 2017 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 that the type of `sum` falls back to `i32` here,
+// and that the for loop desugaring doesn't inferfere with
+// that.
+
+fn main() {
+    let mut sum = 0;
+    for i in Vec::new() {
+        sum += i;
+    }
+}