From 57c75b6b10c329ed1c9c1957e98a990dd04608f2 Mon Sep 17 00:00:00 2001
From: Niko Matsakis <niko@alum.mit.edu>
Date: Thu, 28 May 2015 15:18:47 -0400
Subject: [PATCH] permit const-fn in macro expansions

---
 src/librustc/middle/check_const.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/librustc/middle/check_const.rs b/src/librustc/middle/check_const.rs
index 390b3142e93..ff7c570284f 100644
--- a/src/librustc/middle/check_const.rs
+++ b/src/librustc/middle/check_const.rs
@@ -205,7 +205,16 @@ impl<'a, 'tcx> CheckCrateVisitor<'a, 'tcx> {
                             ret_ty: Ty<'tcx>)
                             -> bool {
         if let Some(fn_like) = const_eval::lookup_const_fn_by_id(self.tcx, def_id) {
-            if self.mode != Mode::Var && !self.tcx.sess.features.borrow().const_fn {
+            if
+                // we are in a static/const initializer
+                self.mode != Mode::Var &&
+
+                // feature-gate is not enabled
+                !self.tcx.sess.features.borrow().const_fn &&
+
+                // this doesn't come from a macro that has #[allow_internal_unstable]
+                !self.tcx.sess.codemap().span_allows_unstable(expr.span)
+            {
                 self.tcx.sess.span_err(
                     expr.span,
                     &format!("const fns are an unstable feature"));