diff --git a/src/librustc/mir/cache.rs b/src/librustc/mir/cache.rs
index 9b75c19a875e..1ed5a22257c5 100644
--- a/src/librustc/mir/cache.rs
+++ b/src/librustc/mir/cache.rs
@@ -8,8 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use std::cell::{Ref, RefCell};
 use rustc_data_structures::indexed_vec::IndexVec;
+use rustc_data_structures::sync::{RwLock, ReadGuard};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher,
                                            StableHasherResult};
 use ich::StableHashingContext;
@@ -19,7 +19,7 @@ use rustc_serialize as serialize;
 
 #[derive(Clone, Debug)]
 pub struct Cache {
-    predecessors: RefCell<Option<IndexVec<BasicBlock, Vec<BasicBlock>>>>
+    predecessors: RwLock<Option<IndexVec<BasicBlock, Vec<BasicBlock>>>>
 }
 
 
@@ -46,7 +46,7 @@ impl<'a> HashStable<StableHashingContext<'a>> for Cache {
 impl Cache {
     pub fn new() -> Self {
         Cache {
-            predecessors: RefCell::new(None)
+            predecessors: RwLock::new(None)
         }
     }
 
@@ -55,12 +55,12 @@ impl Cache {
         *self.predecessors.borrow_mut() = None;
     }
 
-    pub fn predecessors(&self, mir: &Mir) -> Ref<IndexVec<BasicBlock, Vec<BasicBlock>>> {
+    pub fn predecessors(&self, mir: &Mir) -> ReadGuard<IndexVec<BasicBlock, Vec<BasicBlock>>> {
         if self.predecessors.borrow().is_none() {
             *self.predecessors.borrow_mut() = Some(calculate_predecessors(mir));
         }
 
-        Ref::map(self.predecessors.borrow(), |p| p.as_ref().unwrap())
+        ReadGuard::map(self.predecessors.borrow(), |p| p.as_ref().unwrap())
     }
 }
 
diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index 9ed4e6a8e00a..33f52ab09c85 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -34,7 +34,7 @@ use util::ppaux;
 use std::slice;
 use hir::{self, InlineAsm};
 use std::borrow::{Cow};
-use std::cell::Ref;
+use rustc_data_structures::sync::ReadGuard;
 use std::fmt::{self, Debug, Formatter, Write};
 use std::{iter, mem, u32};
 use std::ops::{Index, IndexMut};
@@ -187,13 +187,13 @@ impl<'tcx> Mir<'tcx> {
     }
 
     #[inline]
-    pub fn predecessors(&self) -> Ref<IndexVec<BasicBlock, Vec<BasicBlock>>> {
+    pub fn predecessors(&self) -> ReadGuard<IndexVec<BasicBlock, Vec<BasicBlock>>> {
         self.cache.predecessors(self)
     }
 
     #[inline]
-    pub fn predecessors_for(&self, bb: BasicBlock) -> Ref<Vec<BasicBlock>> {
-        Ref::map(self.predecessors(), |p| &p[bb])
+    pub fn predecessors_for(&self, bb: BasicBlock) -> ReadGuard<Vec<BasicBlock>> {
+        ReadGuard::map(self.predecessors(), |p| &p[bb])
     }
 
     #[inline]