diff --git a/compiler/rustc_serialize/src/collection_impls.rs b/compiler/rustc_serialize/src/collection_impls.rs
index e53a6dc47b4..439641400d5 100644
--- a/compiler/rustc_serialize/src/collection_impls.rs
+++ b/compiler/rustc_serialize/src/collection_impls.rs
@@ -81,7 +81,7 @@ where
         d.read_map(|d, len| {
             let mut map = BTreeMap::new();
             for _ in 0..len {
-                let key = d.read_map_elt_key(|d| Decodable::decode(d));
+                let key = Decodable::decode(d);
                 let val = d.read_map_elt_val(|d| Decodable::decode(d));
                 map.insert(key, val);
             }
@@ -147,7 +147,7 @@ where
             let state = Default::default();
             let mut map = HashMap::with_capacity_and_hasher(len, state);
             for _ in 0..len {
-                let key = d.read_map_elt_key(|d| Decodable::decode(d));
+                let key = Decodable::decode(d);
                 let val = d.read_map_elt_val(|d| Decodable::decode(d));
                 map.insert(key, val);
             }
@@ -226,7 +226,7 @@ where
             let state = Default::default();
             let mut map = indexmap::IndexMap::with_capacity_and_hasher(len, state);
             for _ in 0..len {
-                let key = d.read_map_elt_key(|d| Decodable::decode(d));
+                let key = Decodable::decode(d);
                 let val = d.read_map_elt_val(|d| Decodable::decode(d));
                 map.insert(key, val);
             }
diff --git a/compiler/rustc_serialize/src/serialize.rs b/compiler/rustc_serialize/src/serialize.rs
index 16d9270b609..53ab6eda6f1 100644
--- a/compiler/rustc_serialize/src/serialize.rs
+++ b/compiler/rustc_serialize/src/serialize.rs
@@ -226,14 +226,6 @@ pub trait Decoder {
         f(self, len)
     }
 
-    #[inline]
-    fn read_map_elt_key<T, F>(&mut self, f: F) -> T
-    where
-        F: FnOnce(&mut Self) -> T,
-    {
-        f(self)
-    }
-
     #[inline]
     fn read_map_elt_val<T, F>(&mut self, f: F) -> T
     where