From 3405c548e8f65c9955dfc89188f3cd9db4432bba Mon Sep 17 00:00:00 2001
From: Elliott Slaughter <eslaughter@mozilla.com>
Date: Mon, 20 Aug 2012 10:09:22 -0700
Subject: [PATCH] gc: Avoid freeing null pointers.

---
 src/libcore/gc.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/libcore/gc.rs b/src/libcore/gc.rs
index 066fa0b2e43..697c6fcdfb4 100644
--- a/src/libcore/gc.rs
+++ b/src/libcore/gc.rs
@@ -102,6 +102,12 @@ unsafe fn walk_gc_roots(mem: Memory, visitor: Visitor) {
                 match sp {
                   Some(sp_info) => {
                     for walk_safe_point(frame.fp, sp_info) |root, tydesc| {
+                        // Skip null pointers, which can occur when a
+                        // unique pointer has already been freed.
+                        if ptr::is_null(*root) {
+                            again;
+                        }
+
                         if ptr::is_null(tydesc) {
                             // Root is a generic box.
                             let refcount = **root;
-- 
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