diff --git a/compiler/rustc_serialize/src/collection_impls.rs b/compiler/rustc_serialize/src/collection_impls.rs
index 41faf4f70ac63..dee6dc010feef 100644
--- a/compiler/rustc_serialize/src/collection_impls.rs
+++ b/compiler/rustc_serialize/src/collection_impls.rs
@@ -81,15 +81,14 @@ where
     V: Decodable<D>,
 {
     fn decode(d: &mut D) -> BTreeMap<K, V> {
-        d.read_map(|d, len| {
-            let mut map = BTreeMap::new();
-            for _ in 0..len {
-                let key = Decodable::decode(d);
-                let val = Decodable::decode(d);
-                map.insert(key, val);
-            }
-            map
-        })
+        let len = d.read_usize();
+        let mut map = BTreeMap::new();
+        for _ in 0..len {
+            let key = Decodable::decode(d);
+            let val = Decodable::decode(d);
+            map.insert(key, val);
+        }
+        map
     }
 }
 
@@ -145,16 +144,15 @@ where
     S: BuildHasher + Default,
 {
     fn decode(d: &mut D) -> HashMap<K, V, S> {
-        d.read_map(|d, len| {
-            let state = Default::default();
-            let mut map = HashMap::with_capacity_and_hasher(len, state);
-            for _ in 0..len {
-                let key = Decodable::decode(d);
-                let val = Decodable::decode(d);
-                map.insert(key, val);
-            }
-            map
-        })
+        let len = d.read_usize();
+        let state = Default::default();
+        let mut map = HashMap::with_capacity_and_hasher(len, state);
+        for _ in 0..len {
+            let key = Decodable::decode(d);
+            let val = Decodable::decode(d);
+            map.insert(key, val);
+        }
+        map
     }
 }
 
@@ -223,16 +221,15 @@ where
     S: BuildHasher + Default,
 {
     fn decode(d: &mut D) -> indexmap::IndexMap<K, V, S> {
-        d.read_map(|d, len| {
-            let state = Default::default();
-            let mut map = indexmap::IndexMap::with_capacity_and_hasher(len, state);
-            for _ in 0..len {
-                let key = Decodable::decode(d);
-                let val = Decodable::decode(d);
-                map.insert(key, val);
-            }
-            map
-        })
+        let len = d.read_usize();
+        let state = Default::default();
+        let mut map = indexmap::IndexMap::with_capacity_and_hasher(len, state);
+        for _ in 0..len {
+            let key = Decodable::decode(d);
+            let val = Decodable::decode(d);
+            map.insert(key, val);
+        }
+        map
     }
 }
 
diff --git a/compiler/rustc_serialize/src/serialize.rs b/compiler/rustc_serialize/src/serialize.rs
index e36e3033fcc89..a012be2857e1e 100644
--- a/compiler/rustc_serialize/src/serialize.rs
+++ b/compiler/rustc_serialize/src/serialize.rs
@@ -200,14 +200,6 @@ pub trait Decoder {
     fn read_char(&mut self) -> char;
     fn read_str(&mut self) -> Cow<'_, str>;
     fn read_raw_bytes_into(&mut self, s: &mut [u8]);
-
-    fn read_map<T, F>(&mut self, f: F) -> T
-    where
-        F: FnOnce(&mut Self, usize) -> T,
-    {
-        let len = self.read_usize();
-        f(self, len)
-    }
 }
 
 /// Trait for types that can be serialized