rustc: When encountering "mutable" as a tycon, parse it, drop it on the floor, and emit a warning

src/comp/driver/session.rs

@@ -40,6 +40,14 @@ obj session(cfg targ) {
         fail;
     }
 
+    fn span_warn(span sp, str msg) {
+        log #fmt("%s:%u:%u:%u:%u: warning: %s",
+                 sp.filename,
+                 sp.lo.line, sp.lo.col,
+                 sp.hi.line, sp.hi.col,
+                 msg);
+    }
+
     fn bug(str msg) {
         log #fmt("error: internal compiler error %s", msg);
         fail;

src/comp/front/parser.rs

@@ -452,6 +452,15 @@ impure fn parse_ty(parser p) -> @ast.ty {
             t = ast.ty_path(parse_path(p, GREEDY), none[ast.def]);
         }
 
+        case (token.MUTABLE) {
+            p.bump();
+            p.get_session().span_warn(p.get_span(),
+                "ignoring deprecated 'mutable' type constructor");
+            auto typ = parse_ty(p);
+            t = typ.node;
+            hi = typ.span;
+        }
+
         case (_) {
             p.err("expecting type");
             t = ast.ty_nil;