Ignore 'mutable foo' in plain field contexts.

src/comp/front/parser.rs

@@ -1003,6 +1003,12 @@ fn is_ident(token.token t) -> bool {
 
 impure fn parse_prefix_expr(parser p) -> @ast.expr {
 
+    if (p.peek() == token.MUTABLE) {
+        p.bump();
+        p.get_session().span_warn(p.get_span(),
+            "ignoring deprecated 'mutable' prefix operator");
+    }
+
     auto lo = p.get_span();
     auto hi = lo;