---

yaml
---
r: 51671
b: refs/heads/incoming
c: 0dc6c41
h: refs/heads/master
i:
  51669: 2bc7597
  51667: b85e6cd
  51663: 57c5932
v: v3

Author: nikomatsakis

[refs]

@@ -6,7 +6,7 @@ refs/heads/try: 8eb2bab100b42f0ba751552d8eff00eb2134c55a
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/ndm: f3868061cd7988080c30d6d5bf352a5a5fe2460b
 refs/heads/try2: 147ecfdd8221e4a4d4e090486829a06da1e0ca3c
-refs/heads/incoming: 5f52669296bceac4907608a6ac3bfba41de7dab9
+refs/heads/incoming: 0dc6c414af158370b73bfd379bec84a1a8d480f6
 refs/heads/dist-snap: 8b98e5a296d95c5e832db0756828e5bec31c6f50
 refs/tags/release-0.2: c870d2dffb391e14efb05aa27898f1f6333a9596
 refs/tags/release-0.3: b5f0d0f648d9a6153664837026ba1be43d3e2503

branches/incoming/src/libcore/unstable/lang.rs

@@ -99,8 +99,12 @@ pub unsafe fn borrow_as_imm(a: *u8) {
 #[lang="return_to_mut"]
 #[inline(always)]
 pub unsafe fn return_to_mut(a: *u8) {
-    let a: *mut BoxRepr = transmute(a);
-    (*a).header.ref_count &= !FROZEN_BIT;
+    // Sometimes the box is null, if it is conditionally frozen.
+    // See e.g. #4904.
+    if !a.is_null() {
+        let a: *mut BoxRepr = transmute(a);
+        (*a).header.ref_count &= !FROZEN_BIT;
+    }
 }
 
 #[lang="check_not_borrowed"]