diff --git a/src/librustc/session/mod.rs b/src/librustc/session/mod.rs
index 55da5b921af0..a92a2c916b26 100644
--- a/src/librustc/session/mod.rs
+++ b/src/librustc/session/mod.rs
@@ -118,8 +118,8 @@ pub struct Session {
     /// The metadata::creader module may inject an allocator/panic_runtime
     /// dependency if it didn't already find one, and this tracks what was
     /// injected.
-    pub injected_allocator: Cell<Option<CrateNum>>,
-    pub allocator_kind: Cell<Option<AllocatorKind>>,
+    pub injected_allocator: Once<Option<CrateNum>>,
+    pub allocator_kind: Once<Option<AllocatorKind>>,
     pub injected_panic_runtime: Cell<Option<CrateNum>>,
 
     /// Map from imported macro spans (which consist of
@@ -1105,8 +1105,8 @@ pub fn build_session_(
         const_eval_stack_frame_limit: 100,
         const_eval_step_limit: 1_000_000,
         next_node_id: OneThread::new(Cell::new(NodeId::new(1))),
-        injected_allocator: Cell::new(None),
-        allocator_kind: Cell::new(None),
+        injected_allocator: Once::new(),
+        allocator_kind: Once::new(),
         injected_panic_runtime: Cell::new(None),
         imported_macro_spans: OneThread::new(RefCell::new(HashMap::new())),
         incr_comp_session: OneThread::new(RefCell::new(IncrCompSession::NotInitialized)),
diff --git a/src/librustc_metadata/creader.rs b/src/librustc_metadata/creader.rs
index 5b54994b9cea..06baea53cd53 100644
--- a/src/librustc_metadata/creader.rs
+++ b/src/librustc_metadata/creader.rs
@@ -823,6 +823,8 @@ impl<'a> CrateLoader<'a> {
             needs_allocator = needs_allocator || data.needs_allocator(self.sess);
         });
         if !needs_allocator {
+            self.sess.injected_allocator.set(None);
+            self.sess.allocator_kind.set(None);
             return
         }
 
@@ -842,6 +844,8 @@ impl<'a> CrateLoader<'a> {
             }
         }
         if !need_lib_alloc && !need_exe_alloc {
+            self.sess.injected_allocator.set(None);
+            self.sess.allocator_kind.set(None);
             return
         }
 
@@ -879,6 +883,7 @@ impl<'a> CrateLoader<'a> {
         });
         if global_allocator.is_some() {
             self.sess.allocator_kind.set(Some(AllocatorKind::Global));
+            self.sess.injected_allocator.set(None);
             return
         }
 
@@ -922,6 +927,9 @@ impl<'a> CrateLoader<'a> {
             };
 
         let allocation_crate_data = exe_allocation_crate_data.or_else(|| {
+            // No allocator was injected
+            self.sess.injected_allocator.set(None);
+
             if attr::contains_name(&krate.attrs, "default_lib_allocator") {
                 // Prefer self as the allocator if there's a collision
                 return None;
diff --git a/src/librustc_trans/base.rs b/src/librustc_trans/base.rs
index c2d94a17f03d..f181275326c7 100644
--- a/src/librustc_trans/base.rs
+++ b/src/librustc_trans/base.rs
@@ -795,7 +795,7 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         codegen_units.len());
 
     // Translate an allocator shim, if any
-    let allocator_module = if let Some(kind) = tcx.sess.allocator_kind.get() {
+    let allocator_module = if let Some(kind) = *tcx.sess.allocator_kind.get() {
         unsafe {
             let llmod_id = "allocator";
             let (llcx, llmod) =