---

yaml
---
r: 49590
b: refs/heads/master
c: 0dc6c41
h: refs/heads/master
v: v3

Author: nikomatsakis

[refs]

@@ -1,5 +1,5 @@
 ---
-refs/heads/master: 5f52669296bceac4907608a6ac3bfba41de7dab9
+refs/heads/master: 0dc6c414af158370b73bfd379bec84a1a8d480f6
 refs/heads/snap-stage1: e33de59e47c5076a89eadeb38f4934f58a3618a6
 refs/heads/snap-stage3: f7a2371c176663d59062ec5158f39faecba45768
 refs/heads/try: 2a8fb58d79e685d5ca07b039badcf2ae3ef077ea

trunk/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"]