diff --git a/src/librustc/query/mod.rs b/src/librustc/query/mod.rs
index 2e2cb2db838..d6dbd49d173 100644
--- a/src/librustc/query/mod.rs
+++ b/src/librustc/query/mod.rs
@@ -724,12 +724,12 @@ rustc_queries! {
 
     TypeChecking {
         query implementations_of_trait(_: (CrateNum, DefId))
-            -> Lrc<Vec<DefId>> {
+            -> &'tcx [DefId] {
             no_force
             desc { "looking up implementations of a trait in a crate" }
         }
         query all_trait_implementations(_: CrateNum)
-            -> Lrc<Vec<DefId>> {
+            -> &'tcx [DefId] {
             desc { "looking up all (?) trait implementations" }
         }
     }
diff --git a/src/librustc_metadata/cstore_impl.rs b/src/librustc_metadata/cstore_impl.rs
index d94c07ce072..6b06a0712e1 100644
--- a/src/librustc_metadata/cstore_impl.rs
+++ b/src/librustc_metadata/cstore_impl.rs
@@ -208,18 +208,12 @@ provide! { <'tcx> tcx, def_id, other, cdata,
 
     extra_filename => { cdata.root.extra_filename.clone() }
 
-
     implementations_of_trait => {
-        let mut result = vec![];
-        let filter = Some(other);
-        cdata.get_implementations_for_trait(filter, &mut result);
-        Lrc::new(result)
+        cdata.get_implementations_for_trait(tcx, Some(other))
     }
 
     all_trait_implementations => {
-        let mut result = vec![];
-        cdata.get_implementations_for_trait(None, &mut result);
-        Lrc::new(result)
+        cdata.get_implementations_for_trait(tcx, None)
     }
 
     visibility => { cdata.get_visibility(def_id.index) }
diff --git a/src/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index a2ea0fd6f55..c4037dcfac0 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -1024,31 +1024,34 @@ impl<'a, 'tcx> CrateMetadata {
                                       .map(|index| self.local_def_id(index)))
     }
 
-    pub fn get_implementations_for_trait(&self,
-                                         filter: Option<DefId>,
-                                         result: &mut Vec<DefId>) {
+    pub fn get_implementations_for_trait(
+        &self,
+        tcx: TyCtxt<'_, 'tcx, '_>,
+        filter: Option<DefId>,
+    ) -> &'tcx [DefId] {
         if self.proc_macros.is_some() {
             // proc-macro crates export no trait impls.
-            return
+            return &[]
         }
 
         // Do a reverse lookup beforehand to avoid touching the crate_num
         // hash map in the loop below.
         let filter = match filter.map(|def_id| self.reverse_translate_def_id(def_id)) {
             Some(Some(def_id)) => Some((def_id.krate.as_u32(), def_id.index)),
-            Some(None) => return,
+            Some(None) => return &[],
             None => None,
         };
 
         if let Some(filter) = filter {
-            if let Some(impls) = self.trait_impls
-                                     .get(&filter) {
-                result.extend(impls.decode(self).map(|idx| self.local_def_id(idx)));
+            if let Some(impls) = self.trait_impls.get(&filter) {
+                tcx.arena.alloc_from_iter(impls.decode(self).map(|idx| self.local_def_id(idx)))
+            } else {
+                &[]
             }
         } else {
-            for impls in self.trait_impls.values() {
-                result.extend(impls.decode(self).map(|idx| self.local_def_id(idx)));
-            }
+            tcx.arena.alloc_from_iter(self.trait_impls.values().flat_map(|impls| {
+                impls.decode(self).map(|idx| self.local_def_id(idx))
+            }))
         }
     }