---

yaml
---
r: 51675
b: refs/heads/incoming
c: bbc4ca1
h: refs/heads/master
i:
  51673: cd01626
  51671: a4b97fc
v: v3

Author: bors

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

branches/incoming/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);