diff --git a/src/librustc_metadata/cstore.rs b/src/librustc_metadata/cstore.rs
index 202efb9276a8..22c80e8db3ef 100644
--- a/src/librustc_metadata/cstore.rs
+++ b/src/librustc_metadata/cstore.rs
@@ -93,11 +93,11 @@ pub struct CStore {
     metas: RefCell<IndexVec<CrateNum, Option<Lrc<CrateMetadata>>>>,
     /// Map from NodeId's of local extern crate statements to crate numbers
     extern_mod_crate_map: RefCell<NodeMap<CrateNum>>,
-    pub metadata_loader: Box<MetadataLoader>,
+    pub metadata_loader: Box<MetadataLoader + Sync>,
 }
 
 impl CStore {
-    pub fn new(metadata_loader: Box<MetadataLoader>) -> CStore {
+    pub fn new(metadata_loader: Box<MetadataLoader + Sync>) -> CStore {
         CStore {
             metas: RefCell::new(IndexVec::new()),
             extern_mod_crate_map: RefCell::new(FxHashMap()),
diff --git a/src/librustc_trans/lib.rs b/src/librustc_trans/lib.rs
index 39eb38658fee..29187c6c0a51 100644
--- a/src/librustc_trans/lib.rs
+++ b/src/librustc_trans/lib.rs
@@ -200,7 +200,7 @@ impl TransCrate for LlvmTransCrate {
         target_features(sess)
     }
 
-    fn metadata_loader(&self) -> Box<MetadataLoader> {
+    fn metadata_loader(&self) -> Box<MetadataLoader + Sync> {
         box metadata::LlvmMetadataLoader
     }
 
diff --git a/src/librustc_trans_utils/trans_crate.rs b/src/librustc_trans_utils/trans_crate.rs
index 0d4811c4b025..5cf9819288b5 100644
--- a/src/librustc_trans_utils/trans_crate.rs
+++ b/src/librustc_trans_utils/trans_crate.rs
@@ -58,7 +58,7 @@ pub trait TransCrate {
     fn print_version(&self) {}
     fn diagnostics(&self) -> &[(&'static str, &'static str)] { &[] }
 
-    fn metadata_loader(&self) -> Box<MetadataLoader>;
+    fn metadata_loader(&self) -> Box<MetadataLoader + Sync>;
     fn provide(&self, _providers: &mut Providers);
     fn provide_extern(&self, _providers: &mut Providers);
     fn trans_crate<'a, 'tcx>(
@@ -84,7 +84,7 @@ pub trait TransCrate {
 pub struct DummyTransCrate;
 
 impl TransCrate for DummyTransCrate {
-    fn metadata_loader(&self) -> Box<MetadataLoader> {
+    fn metadata_loader(&self) -> Box<MetadataLoader + Sync> {
         box DummyMetadataLoader(())
     }
 
@@ -195,7 +195,7 @@ impl TransCrate for MetadataOnlyTransCrate {
         }
     }
 
-    fn metadata_loader(&self) -> Box<MetadataLoader> {
+    fn metadata_loader(&self) -> Box<MetadataLoader + Sync> {
         box NoLlvmMetadataLoader
     }
 
diff --git a/src/test/run-make/hotplug_codegen_backend/the_backend.rs b/src/test/run-make/hotplug_codegen_backend/the_backend.rs
index 9e87268e6999..e266b0f5e834 100644
--- a/src/test/run-make/hotplug_codegen_backend/the_backend.rs
+++ b/src/test/run-make/hotplug_codegen_backend/the_backend.rs
@@ -28,7 +28,7 @@ use rustc_trans_utils::trans_crate::{TransCrate, MetadataOnlyTransCrate};
 struct TheBackend(Box<TransCrate>);
 
 impl TransCrate for TheBackend {
-    fn metadata_loader(&self) -> Box<MetadataLoader> {
+    fn metadata_loader(&self) -> Box<MetadataLoader + Sync> {
         self.0.metadata_loader()
     }