auto merge of #10123 : klutzy/rust/fix-7507, r=alexcrichton

This patch fixes rustc to emit explicit error if LHS of assignment is
not allowed.

Fixes #7507
Fixes #7508

src/test/compile-fail/bad-expr-lhs.rs

@@ -0,0 +1,20 @@
+// Copyright 2012-2013 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() {
+    1 = 2; //~ ERROR illegal left-hand side expression
+    1 += 2; //~ ERROR illegal left-hand side expression
+    (1, 2) = (3, 4); //~ ERROR illegal left-hand side expression
+
+    let (a, b) = (1, 2);
+    (a, b) = (3, 4); //~ ERROR illegal left-hand side expression
+
+    None = Some(3); //~ ERROR illegal left-hand side expression
+}

src/test/compile-fail/borrowck-assign-to-constants.rs

@@ -12,6 +12,5 @@ static foo: int = 5;
 
 fn main() {
     // assigning to various global constants
-    None = Some(3); //~ ERROR cannot assign to immutable static item
     foo = 6; //~ ERROR cannot assign to immutable static item
 }