librustc: Fix the issue with labels shadowing variable names by making

the leading quote part of the identifier for the purposes of hygiene.

This adopts @jbclements' solution to #14539.

I'm not sure if this is a breaking change or not.

Closes #12512.

[breaking-change]

src/test/compile-fail/hygienic-label-1.rs

@@ -15,5 +15,5 @@ macro_rules! foo {
 }
 
 pub fn main() {
-    'x: loop { foo!() } //~ ERROR use of undeclared label `x`
+    'x: loop { foo!() } //~ ERROR use of undeclared label `'x`
 }

src/test/compile-fail/hygienic-label-2.rs

@@ -15,5 +15,5 @@ macro_rules! foo {
 }
 
 pub fn main() {
-    foo!(break 'x); //~ ERROR use of undeclared label `x`
+    foo!(break 'x); //~ ERROR use of undeclared label `'x`
 }

src/test/compile-fail/hygienic-label-3.rs

@@ -16,6 +16,6 @@ macro_rules! foo {
 
 pub fn main() {
     'x: for _ in range(0,1) {
-        foo!() //~ ERROR use of undeclared label `x`
+        foo!() //~ ERROR use of undeclared label `'x`
     };
 }

src/test/compile-fail/hygienic-label-4.rs

@@ -15,5 +15,5 @@ macro_rules! foo {
 }
 
 pub fn main() {
-    foo!(break 'x); //~ ERROR use of undeclared label `x`
+    foo!(break 'x); //~ ERROR use of undeclared label `'x`
 }

src/test/compile-fail/regions-name-static.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-struct Foo<'static> { //~ ERROR illegal lifetime parameter name: `static`
+struct Foo<'static> { //~ ERROR illegal lifetime parameter name: `'static`
     x: &'static int
 }
 

src/test/run-pass/loop-label-shadowing.rs

@@ -0,0 +1,19 @@
+// Copyright 2012 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.
+
+// Issue #12512.
+
+fn main() {
+    let mut foo = Vec::new();
+    'foo: for i in [1, 2, 3].iter() {
+        foo.push(i);
+    }
+}
+