Add _mutable unop.

src/comp/front/ast.rs

@@ -106,6 +106,7 @@ tag unop {
     bitnot;
     not;
     neg;
+    _mutable;
 }
 
 tag mode {

src/comp/front/parser.rs

@@ -811,6 +811,13 @@ impure fn parse_prefix_expr(parser p) -> @ast.expr {
             ex = ast.expr_unary(ast.box, e, ast.ann_none);
         }
 
+        case (token.MUTABLE) {
+            p.bump();
+            auto e = parse_prefix_expr(p);
+            hi = e.span;
+            ex = ast.expr_unary(ast._mutable, e, ast.ann_none);
+        }
+
         case (_) {
             ret parse_dot_or_call_expr(p);
         }

src/comp/middle/trans.rs

@@ -1834,6 +1834,9 @@ fn trans_unary(@block_ctxt cx, ast.unop op,
             }
             ret res(sub.bcx, val);
         }
+        case (ast._mutable) {
+            ret trans_expr(cx, e);
+        }
     }
     fail;
 }

src/comp/middle/typeck.rs

@@ -1229,6 +1229,9 @@ fn check_expr(&@fn_ctxt fcx, @ast.expr expr) -> @ast.expr {
                         }
                     }
                 }
+                case (ast._mutable) {
+                    oper_t = @rec(mut=ast.mut with *oper_t);
+                }
                 case (_) { oper_t = strip_boxes(oper_t); }
             }
             ret @fold.respan[ast.expr_](expr.span,