diff --git a/src/librustc/mir/cache.rs b/src/librustc/mir/cache.rs
index 9b4136674187..52848cc435a6 100644
--- a/src/librustc/mir/cache.rs
+++ b/src/librustc/mir/cache.rs
@@ -1,5 +1,4 @@
 use rustc_index::vec::IndexVec;
-use rustc_data_structures::sync::{RwLock, MappedReadGuard, ReadGuard};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_serialize::{Encodable, Encoder, Decodable, Decoder};
 use crate::ich::StableHashingContext;
@@ -7,7 +6,7 @@ use crate::mir::{Body, BasicBlock};
 
 #[derive(Clone, Debug)]
 pub struct Cache {
-    predecessors: RwLock<Option<IndexVec<BasicBlock, Vec<BasicBlock>>>>
+    predecessors: Option<IndexVec<BasicBlock, Vec<BasicBlock>>>
 }
 
 
@@ -32,24 +31,29 @@ impl<'a> HashStable<StableHashingContext<'a>> for Cache {
 impl Cache {
     pub fn new() -> Self {
         Cache {
-            predecessors: RwLock::new(None)
+            predecessors: None
         }
     }
 
-    pub fn invalidate(&self) {
+    pub fn invalidate(&mut self) {
         // FIXME: consider being more fine-grained
-        *self.predecessors.borrow_mut() = None;
+        self.predecessors = None;
     }
 
-    pub fn predecessors(
-        &self,
+    pub fn predecessors_ref(&self) -> &IndexVec<BasicBlock, Vec<BasicBlock>> {
+        assert!(self.predecessors.is_some());
+        self.predecessors.as_ref().unwrap()
+    }
+
+    pub fn predecessors_mut(
+        &mut self,
         body: &Body<'_>
-    ) -> MappedReadGuard<'_, IndexVec<BasicBlock, Vec<BasicBlock>>> {
-        if self.predecessors.borrow().is_none() {
-            *self.predecessors.borrow_mut() = Some(calculate_predecessors(body));
+    ) -> &mut IndexVec<BasicBlock, Vec<BasicBlock>> {
+        if self.predecessors.is_none() {
+            self.predecessors = Some(calculate_predecessors(body));
         }
 
-        ReadGuard::map(self.predecessors.borrow(), |p| p.as_ref().unwrap())
+        self.predecessors.as_mut().unwrap()
     }
 }
 
diff --git a/src/librustc/mir/mod.rs b/src/librustc/mir/mod.rs
index 300b0363e2d3..396953aa4120 100644
--- a/src/librustc/mir/mod.rs
+++ b/src/librustc/mir/mod.rs
@@ -25,7 +25,6 @@ use rustc_data_structures::graph::dominators::{dominators, Dominators};
 use rustc_data_structures::graph::{self, GraphPredecessors, GraphSuccessors};
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_data_structures::sync::Lrc;
-use rustc_data_structures::sync::MappedReadGuard;
 use rustc_macros::HashStable;
 use rustc_serialize::{Encodable, Decodable};
 use smallvec::SmallVec;
@@ -218,13 +217,19 @@ impl<'tcx> Body<'tcx> {
     }
 
     #[inline]
-    pub fn predecessors(&self) -> MappedReadGuard<'_, IndexVec<BasicBlock, Vec<BasicBlock>>> {
-        self.cache.predecessors(self)
+    pub fn predecessors_ref(&self) -> &IndexVec<BasicBlock, Vec<BasicBlock>> {
+        self.cache.predecessors_ref()
     }
 
     #[inline]
-    pub fn predecessors_for(&self, bb: BasicBlock) -> MappedReadGuard<'_, Vec<BasicBlock>> {
-        MappedReadGuard::map(self.predecessors(), |p| &p[bb])
+    pub fn predecessors_mut(&mut self) -> &mut IndexVec<BasicBlock, Vec<BasicBlock>> {
+        // TODO(nashenas88) figure out a way to get rid of this clone
+        self.cache.predecessors_mut(&self.clone())
+    }
+
+    #[inline]
+    pub fn predecessors_for(&self, bb: BasicBlock) -> &Vec<BasicBlock> {
+        &self.predecessors_ref()[bb]
     }
 
     #[inline]
diff --git a/src/librustc_mir/transform/add_call_guards.rs b/src/librustc_mir/transform/add_call_guards.rs
index bf3df1ae2fd8..bc54f48e578e 100644
--- a/src/librustc_mir/transform/add_call_guards.rs
+++ b/src/librustc_mir/transform/add_call_guards.rs
@@ -39,7 +39,7 @@ impl<'tcx> MirPass<'tcx> for AddCallGuards {
 impl AddCallGuards {
     pub fn add_call_guards(&self, body: &mut Body<'_>) {
         let pred_count: IndexVec<_, _> =
-            body.predecessors().iter().map(|ps| ps.len()).collect();
+            body.predecessors_mut().iter().map(|ps| ps.len()).collect();
 
         // We need a place to store the new blocks generated
         let mut new_blocks = Vec::new();
diff --git a/src/librustc_mir/util/liveness.rs b/src/librustc_mir/util/liveness.rs
index 63e4af0a56a7..4b67abca0f06 100644
--- a/src/librustc_mir/util/liveness.rs
+++ b/src/librustc_mir/util/liveness.rs
@@ -94,7 +94,7 @@ pub fn liveness_of_locals(
         dirty_queue.insert(bb);
     }
 
-    let predecessors = body.predecessors();
+    let predecessors = body.predecessors_ref();
 
     while let Some(bb) = dirty_queue.pop() {
         // bits = use ∪ (bits - def)