---

yaml
---
r: 50229
b: refs/heads/auto
c: 0dc6c41
h: refs/heads/master
i:
  50227: c0e81ec
v: v3

Author: nikomatsakis

[refs]

@@ -14,5 +14,5 @@ refs/heads/try3: 9387340aab40a73e8424c48fd42f0c521a4875c0
 refs/tags/release-0.3.1: 495bae036dfe5ec6ceafd3312b4dca48741e845b
 refs/tags/release-0.4: e828ea2080499553b97dfe33b3f4d472b4562ad7
 refs/tags/release-0.5: 7e3bcfbf21278251ee936ad53e92e9b719702d73
-refs/heads/auto: 5f52669296bceac4907608a6ac3bfba41de7dab9
+refs/heads/auto: 0dc6c414af158370b73bfd379bec84a1a8d480f6
 refs/heads/servo: af82457af293e2a842ba6b7759b70288da276167

branches/auto/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"]