diff --git a/src/librustc_metadata/creader.rs b/src/librustc_metadata/creader.rs
index d0237071a6058..d9038105da08b 100644
--- a/src/librustc_metadata/creader.rs
+++ b/src/librustc_metadata/creader.rs
@@ -51,7 +51,6 @@ pub struct Library {
 pub struct CrateLoader<'a> {
     pub sess: &'a Session,
     cstore: &'a CStore,
-    next_crate_num: CrateNum,
     local_crate_name: Symbol,
 }
 
@@ -102,7 +101,6 @@ impl<'a> CrateLoader<'a> {
         CrateLoader {
             sess,
             cstore,
-            next_crate_num: cstore.next_crate_num(),
             local_crate_name: Symbol::intern(local_crate_name),
         }
     }
@@ -198,8 +196,7 @@ impl<'a> CrateLoader<'a> {
         self.verify_no_symbol_conflicts(span, &crate_root);
 
         // Claim this crate number and cache it
-        let cnum = self.next_crate_num;
-        self.next_crate_num = CrateNum::from_u32(cnum.as_u32() + 1);
+        let cnum = self.cstore.alloc_new_crate_num();
 
         // Stash paths for top-most crate locally if necessary.
         let crate_paths = if root.is_none() {
diff --git a/src/librustc_metadata/cstore.rs b/src/librustc_metadata/cstore.rs
index abecee2277c4b..4872d560d27f0 100644
--- a/src/librustc_metadata/cstore.rs
+++ b/src/librustc_metadata/cstore.rs
@@ -96,16 +96,17 @@ pub struct CStore {
 impl CStore {
     pub fn new(metadata_loader: Box<MetadataLoader + Sync>) -> CStore {
         CStore {
-            metas: RwLock::new(IndexVec::new()),
+            metas: RwLock::new(IndexVec::from_elem_n(None, 1)),
             extern_mod_crate_map: Lock::new(FxHashMap()),
             metadata_loader,
         }
     }
 
-    /// You cannot use this function to allocate a CrateNum in a thread-safe manner.
-    /// It is currently only used in CrateLoader which is single-threaded code.
-    pub(super) fn next_crate_num(&self) -> CrateNum {
-        CrateNum::new(self.metas.borrow().len() + 1)
+    pub(super) fn alloc_new_crate_num(&self) -> CrateNum {
+        let mut metas = self.metas.borrow_mut();
+        let cnum = CrateNum::new(metas.len());
+        metas.push(None);
+        cnum
     }
 
     pub(super) fn get_crate_data(&self, cnum: CrateNum) -> Lrc<CrateMetadata> {
@@ -113,12 +114,9 @@ impl CStore {
     }
 
     pub(super) fn set_crate_data(&self, cnum: CrateNum, data: Lrc<CrateMetadata>) {
-        use rustc_data_structures::indexed_vec::Idx;
-        let mut met = self.metas.borrow_mut();
-        while met.len() <= cnum.index() {
-            met.push(None);
-        }
-        met[cnum] = Some(data);
+        let mut metas = self.metas.borrow_mut();
+        assert!(metas[cnum].is_none(), "Overwriting crate metadata entry");
+        metas[cnum] = Some(data);
     }
 
     pub(super) fn iter_crate_data<I>(&self, mut i: I)