Feature gate trace_macros.

Fixes #11631

src/test/compile-fail/gated-trace_macros.rs

@@ -0,0 +1,15 @@
+// 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.
+
+// xfail-fast feature doesn't work.
+
+fn main() {
+    trace_macros!(true); //~ ERROR: `trace_macros` is not stable
+}

src/test/compile-fail/macros-nonfatal-errors.rs

@@ -12,6 +12,7 @@
 // immediately, so that we get more errors listed at a time.
 
 #[feature(asm)];
+#[feature(trace_macros)];
 
 #[deriving(Default, //~ ERROR
            Rand, //~ ERROR