---

yaml
---
r: 50233
b: refs/heads/auto
c: bbc4ca1
h: refs/heads/master
i:
  50231: 0cfa2ef
v: v3

Author: bors

[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: e5f8026ebab867547514677bf9d7c7870a4345ad
+refs/heads/auto: bbc4ca134900456ba2a00368ec1f19bc1a357528
 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"]

branches/auto/src/librustc/middle/typeck/check/regionck.rs

@@ -615,7 +615,7 @@ pub mod guarantor {
         // expressions, both of which always yield a region variable, so
         // mk_subr should never fail.
         let rptr_ty = rcx.resolve_node_type(id);
-        if !ty::type_is_error(rptr_ty) {
+        if !ty::type_is_error(rptr_ty) && !ty::type_is_bot(rptr_ty) {
             let tcx = rcx.fcx.ccx.tcx;
             debug!("rptr_ty=%s", ty_to_str(tcx, rptr_ty));
             let r = ty::ty_region(tcx, span, rptr_ty);