rustc: provide more precise information about refutable patterns.

The compiler now points exactly which part(s) of a pattern are
refutable, rather than just highlighting the whole pattern.

src/test/compile-fail/precise-refutable-pattern-errors.rs

@@ -0,0 +1,32 @@
+// 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 func(
+    (
+        1, //~ ERROR refutable pattern in function argument
+        (
+            Some( //~ ERROR refutable pattern in function argument
+                1), // nested, so no warning.
+            2..3 //~ ERROR refutable pattern in function argument
+            )
+        ): (int, (Option<int>, int))
+        ) {}
+
+fn main() {
+    let (
+        1, //~ ERROR refutable pattern in local binding
+        (
+            Some( //~ ERROR refutable pattern in local binding
+                1), // nested, so no warning.
+            2..3 //~ ERROR refutable pattern in local binding
+            )
+        ) = (1, (None, 2));
+}