librustc: Disallow "mut" from distributing over bindings.

This is the backwards-incompatible part of per-binding-site "mut".

src/libsyntax/ast_util.rs

@@ -619,6 +619,15 @@ pub enum Privacy {
     Public
 }
 
+/// Returns true if the given pattern consists solely of an identifier
+/// and false otherwise.
+pub fn pat_is_ident(pat: @ast::pat) -> bool {
+    match pat.node {
+        ast::pat_ident(*) => true,
+        _ => false,
+    }
+}
+
 // HYGIENE FUNCTIONS
 
 /// Construct an identifier with the given name and an empty context: