diff --git a/compiler/rustc_serialize/src/collection_impls.rs b/compiler/rustc_serialize/src/collection_impls.rs
deleted file mode 100644
index 8f8c504117cc1..0000000000000
--- a/compiler/rustc_serialize/src/collection_impls.rs
+++ /dev/null
@@ -1,279 +0,0 @@
-//! Implementations of serialization for structures found in liballoc
-
-use crate::{Decodable, Decoder, Encodable, Encoder};
-use smallvec::{Array, SmallVec};
-use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet, LinkedList, VecDeque};
-use std::hash::{BuildHasher, Hash};
-use std::rc::Rc;
-use std::sync::Arc;
-use thin_vec::ThinVec;
-
-impl<S: Encoder, A: Array<Item: Encodable<S>>> Encodable<S> for SmallVec<A> {
-    fn encode(&self, s: &mut S) {
-        let slice: &[A::Item] = self;
-        slice.encode(s);
-    }
-}
-
-impl<D: Decoder, A: Array<Item: Decodable<D>>> Decodable<D> for SmallVec<A> {
-    fn decode(d: &mut D) -> SmallVec<A> {
-        let len = d.read_usize();
-        (0..len).map(|_| Decodable::decode(d)).collect()
-    }
-}
-
-impl<S: Encoder, T: Encodable<S>> Encodable<S> for ThinVec<T> {
-    fn encode(&self, s: &mut S) {
-        self.as_slice().encode(s);
-    }
-}
-
-impl<D: Decoder, T: Decodable<D>> Decodable<D> for ThinVec<T> {
-    fn decode(d: &mut D) -> ThinVec<T> {
-        let len = d.read_usize();
-        (0..len).map(|_| Decodable::decode(d)).collect()
-    }
-}
-
-impl<S: Encoder, T: Encodable<S>> Encodable<S> for LinkedList<T> {
-    fn encode(&self, s: &mut S) {
-        s.emit_usize(self.len());
-        for e in self.iter() {
-            e.encode(s);
-        }
-    }
-}
-
-impl<D: Decoder, T: Decodable<D>> Decodable<D> for LinkedList<T> {
-    fn decode(d: &mut D) -> LinkedList<T> {
-        let len = d.read_usize();
-        (0..len).map(|_| Decodable::decode(d)).collect()
-    }
-}
-
-impl<S: Encoder, T: Encodable<S>> Encodable<S> for VecDeque<T> {
-    fn encode(&self, s: &mut S) {
-        s.emit_usize(self.len());
-        for e in self.iter() {
-            e.encode(s);
-        }
-    }
-}
-
-impl<D: Decoder, T: Decodable<D>> Decodable<D> for VecDeque<T> {
-    fn decode(d: &mut D) -> VecDeque<T> {
-        let len = d.read_usize();
-        (0..len).map(|_| Decodable::decode(d)).collect()
-    }
-}
-
-impl<S: Encoder, K, V> Encodable<S> for BTreeMap<K, V>
-where
-    K: Encodable<S> + PartialEq + Ord,
-    V: Encodable<S>,
-{
-    fn encode(&self, e: &mut S) {
-        e.emit_usize(self.len());
-        for (key, val) in self.iter() {
-            key.encode(e);
-            val.encode(e);
-        }
-    }
-}
-
-impl<D: Decoder, K, V> Decodable<D> for BTreeMap<K, V>
-where
-    K: Decodable<D> + PartialEq + Ord,
-    V: Decodable<D>,
-{
-    fn decode(d: &mut D) -> BTreeMap<K, V> {
-        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
-    }
-}
-
-impl<S: Encoder, T> Encodable<S> for BTreeSet<T>
-where
-    T: Encodable<S> + PartialEq + Ord,
-{
-    fn encode(&self, s: &mut S) {
-        s.emit_usize(self.len());
-        for e in self.iter() {
-            e.encode(s);
-        }
-    }
-}
-
-impl<D: Decoder, T> Decodable<D> for BTreeSet<T>
-where
-    T: Decodable<D> + PartialEq + Ord,
-{
-    fn decode(d: &mut D) -> BTreeSet<T> {
-        let len = d.read_usize();
-        let mut set = BTreeSet::new();
-        for _ in 0..len {
-            set.insert(Decodable::decode(d));
-        }
-        set
-    }
-}
-
-impl<E: Encoder, K, V, S> Encodable<E> for HashMap<K, V, S>
-where
-    K: Encodable<E> + Eq,
-    V: Encodable<E>,
-    S: BuildHasher,
-{
-    fn encode(&self, e: &mut E) {
-        e.emit_usize(self.len());
-        for (key, val) in self.iter() {
-            key.encode(e);
-            val.encode(e);
-        }
-    }
-}
-
-impl<D: Decoder, K, V, S> Decodable<D> for HashMap<K, V, S>
-where
-    K: Decodable<D> + Hash + Eq,
-    V: Decodable<D>,
-    S: BuildHasher + Default,
-{
-    fn decode(d: &mut D) -> HashMap<K, V, S> {
-        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
-    }
-}
-
-impl<E: Encoder, T, S> Encodable<E> for HashSet<T, S>
-where
-    T: Encodable<E> + Eq,
-    S: BuildHasher,
-{
-    fn encode(&self, s: &mut E) {
-        s.emit_usize(self.len());
-        for e in self.iter() {
-            e.encode(s);
-        }
-    }
-}
-
-impl<D: Decoder, T, S> Decodable<D> for HashSet<T, S>
-where
-    T: Decodable<D> + Hash + Eq,
-    S: BuildHasher + Default,
-{
-    fn decode(d: &mut D) -> HashSet<T, S> {
-        let len = d.read_usize();
-        let state = Default::default();
-        let mut set = HashSet::with_capacity_and_hasher(len, state);
-        for _ in 0..len {
-            set.insert(Decodable::decode(d));
-        }
-        set
-    }
-}
-
-impl<E: Encoder, K, V, S> Encodable<E> for indexmap::IndexMap<K, V, S>
-where
-    K: Encodable<E> + Hash + Eq,
-    V: Encodable<E>,
-    S: BuildHasher,
-{
-    fn encode(&self, e: &mut E) {
-        e.emit_usize(self.len());
-        for (key, val) in self.iter() {
-            key.encode(e);
-            val.encode(e);
-        }
-    }
-}
-
-impl<D: Decoder, K, V, S> Decodable<D> for indexmap::IndexMap<K, V, S>
-where
-    K: Decodable<D> + Hash + Eq,
-    V: Decodable<D>,
-    S: BuildHasher + Default,
-{
-    fn decode(d: &mut D) -> indexmap::IndexMap<K, V, S> {
-        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
-    }
-}
-
-impl<E: Encoder, T, S> Encodable<E> for indexmap::IndexSet<T, S>
-where
-    T: Encodable<E> + Hash + Eq,
-    S: BuildHasher,
-{
-    fn encode(&self, s: &mut E) {
-        s.emit_usize(self.len());
-        for e in self.iter() {
-            e.encode(s);
-        }
-    }
-}
-
-impl<D: Decoder, T, S> Decodable<D> for indexmap::IndexSet<T, S>
-where
-    T: Decodable<D> + Hash + Eq,
-    S: BuildHasher + Default,
-{
-    fn decode(d: &mut D) -> indexmap::IndexSet<T, S> {
-        let len = d.read_usize();
-        let state = Default::default();
-        let mut set = indexmap::IndexSet::with_capacity_and_hasher(len, state);
-        for _ in 0..len {
-            set.insert(Decodable::decode(d));
-        }
-        set
-    }
-}
-
-impl<E: Encoder, T: Encodable<E>> Encodable<E> for Rc<[T]> {
-    fn encode(&self, s: &mut E) {
-        let slice: &[T] = self;
-        slice.encode(s);
-    }
-}
-
-impl<D: Decoder, T: Decodable<D>> Decodable<D> for Rc<[T]> {
-    fn decode(d: &mut D) -> Rc<[T]> {
-        let vec: Vec<T> = Decodable::decode(d);
-        vec.into()
-    }
-}
-
-impl<E: Encoder, T: Encodable<E>> Encodable<E> for Arc<[T]> {
-    fn encode(&self, s: &mut E) {
-        let slice: &[T] = self;
-        slice.encode(s);
-    }
-}
-
-impl<D: Decoder, T: Decodable<D>> Decodable<D> for Arc<[T]> {
-    fn decode(d: &mut D) -> Arc<[T]> {
-        let vec: Vec<T> = Decodable::decode(d);
-        vec.into()
-    }
-}
diff --git a/compiler/rustc_serialize/src/lib.rs b/compiler/rustc_serialize/src/lib.rs
index dd40b3cf0283d..5360aa9ea6a94 100644
--- a/compiler/rustc_serialize/src/lib.rs
+++ b/compiler/rustc_serialize/src/lib.rs
@@ -1,25 +1,19 @@
 //! Support code for encoding and decoding types.
 
-/*
-Core encoding and decoding interfaces.
-*/
-
 #![doc(
     html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
     html_playground_url = "https://play.rust-lang.org/",
     test(attr(allow(unused_variables), deny(warnings)))
 )]
-#![feature(never_type)]
+#![feature(allocator_api)]
 #![feature(associated_type_bounds)]
-#![feature(min_specialization)]
+#![feature(const_option)]
 #![feature(core_intrinsics)]
-#![feature(maybe_uninit_slice)]
-#![feature(new_uninit)]
-#![feature(allocator_api)]
+#![feature(inline_const)]
+#![feature(min_specialization)]
+#![feature(never_type)]
 #![feature(ptr_sub_ptr)]
 #![feature(slice_first_last_chunk)]
-#![feature(inline_const)]
-#![feature(const_option)]
 #![cfg_attr(test, feature(test))]
 #![allow(rustc::internal)]
 #![deny(rustc::untranslatable_diagnostic)]
@@ -27,7 +21,6 @@ Core encoding and decoding interfaces.
 
 pub use self::serialize::{Decodable, Decoder, Encodable, Encoder};
 
-mod collection_impls;
 mod serialize;
 
 pub mod leb128;
diff --git a/compiler/rustc_serialize/src/serialize.rs b/compiler/rustc_serialize/src/serialize.rs
index 06166cabc1872..63bd3457eb97c 100644
--- a/compiler/rustc_serialize/src/serialize.rs
+++ b/compiler/rustc_serialize/src/serialize.rs
@@ -1,12 +1,15 @@
 //! Support code for encoding and decoding types.
 
-use std::alloc::Allocator;
+use smallvec::{Array, SmallVec};
 use std::borrow::Cow;
 use std::cell::{Cell, RefCell};
+use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet, VecDeque};
+use std::hash::{BuildHasher, Hash};
 use std::marker::PhantomData;
 use std::path;
 use std::rc::Rc;
 use std::sync::Arc;
+use thin_vec::ThinVec;
 
 /// A byte that [cannot occur in UTF8 sequences][utf8]. Used to mark the end of a string.
 /// This way we can skip validation and still be relatively sure that deserialization
@@ -273,9 +276,9 @@ impl<D: Decoder, T> Decodable<D> for PhantomData<T> {
     }
 }
 
-impl<D: Decoder, A: Allocator + Default, T: Decodable<D>> Decodable<D> for Box<[T], A> {
-    fn decode(d: &mut D) -> Box<[T], A> {
-        let v: Vec<T, A> = Decodable::decode(d);
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for Box<[T]> {
+    fn decode(d: &mut D) -> Box<[T]> {
+        let v: Vec<T> = Decodable::decode(d);
         v.into_boxed_slice()
     }
 }
@@ -303,33 +306,20 @@ impl<S: Encoder, T: Encodable<S>> Encodable<S> for [T] {
 
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for Vec<T> {
     fn encode(&self, s: &mut S) {
-        let slice: &[T] = self;
-        slice.encode(s);
+        self.as_slice().encode(s);
     }
 }
 
-impl<D: Decoder, T: Decodable<D>, A: Allocator + Default> Decodable<D> for Vec<T, A> {
-    default fn decode(d: &mut D) -> Vec<T, A> {
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for Vec<T> {
+    default fn decode(d: &mut D) -> Vec<T> {
         let len = d.read_usize();
-        let allocator = A::default();
-        // SAFETY: we set the capacity in advance, only write elements, and
-        // only set the length at the end once the writing has succeeded.
-        let mut vec = Vec::with_capacity_in(len, allocator);
-        unsafe {
-            let ptr: *mut T = vec.as_mut_ptr();
-            for i in 0..len {
-                std::ptr::write(ptr.add(i), Decodable::decode(d));
-            }
-            vec.set_len(len);
-        }
-        vec
+        (0..len).map(|_| Decodable::decode(d)).collect()
     }
 }
 
 impl<S: Encoder, T: Encodable<S>, const N: usize> Encodable<S> for [T; N] {
     fn encode(&self, s: &mut S) {
-        let slice: &[T] = self;
-        slice.encode(s);
+        self.as_slice().encode(s);
     }
 }
 
@@ -497,15 +487,233 @@ impl<D: Decoder, T: Decodable<D>> Decodable<D> for Arc<T> {
     }
 }
 
-impl<S: Encoder, T: ?Sized + Encodable<S>, A: Allocator + Default> Encodable<S> for Box<T, A> {
+impl<S: Encoder, T: ?Sized + Encodable<S>> Encodable<S> for Box<T> {
     fn encode(&self, s: &mut S) {
         (**self).encode(s)
     }
 }
 
-impl<D: Decoder, A: Allocator + Default, T: Decodable<D>> Decodable<D> for Box<T, A> {
-    fn decode(d: &mut D) -> Box<T, A> {
-        let allocator = A::default();
-        Box::new_in(Decodable::decode(d), allocator)
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for Box<T> {
+    fn decode(d: &mut D) -> Box<T> {
+        Box::new(Decodable::decode(d))
+    }
+}
+
+impl<S: Encoder, A: Array<Item: Encodable<S>>> Encodable<S> for SmallVec<A> {
+    fn encode(&self, s: &mut S) {
+        self.as_slice().encode(s);
+    }
+}
+
+impl<D: Decoder, A: Array<Item: Decodable<D>>> Decodable<D> for SmallVec<A> {
+    fn decode(d: &mut D) -> SmallVec<A> {
+        let len = d.read_usize();
+        (0..len).map(|_| Decodable::decode(d)).collect()
+    }
+}
+
+impl<S: Encoder, T: Encodable<S>> Encodable<S> for ThinVec<T> {
+    fn encode(&self, s: &mut S) {
+        self.as_slice().encode(s);
+    }
+}
+
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for ThinVec<T> {
+    fn decode(d: &mut D) -> ThinVec<T> {
+        let len = d.read_usize();
+        (0..len).map(|_| Decodable::decode(d)).collect()
+    }
+}
+
+impl<S: Encoder, T: Encodable<S>> Encodable<S> for VecDeque<T> {
+    fn encode(&self, s: &mut S) {
+        s.emit_usize(self.len());
+        for e in self.iter() {
+            e.encode(s);
+        }
+    }
+}
+
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for VecDeque<T> {
+    fn decode(d: &mut D) -> VecDeque<T> {
+        let len = d.read_usize();
+        (0..len).map(|_| Decodable::decode(d)).collect()
+    }
+}
+
+impl<S: Encoder, K, V> Encodable<S> for BTreeMap<K, V>
+where
+    K: Encodable<S> + PartialEq + Ord,
+    V: Encodable<S>,
+{
+    fn encode(&self, e: &mut S) {
+        e.emit_usize(self.len());
+        for (key, val) in self.iter() {
+            key.encode(e);
+            val.encode(e);
+        }
+    }
+}
+
+impl<D: Decoder, K, V> Decodable<D> for BTreeMap<K, V>
+where
+    K: Decodable<D> + PartialEq + Ord,
+    V: Decodable<D>,
+{
+    fn decode(d: &mut D) -> BTreeMap<K, V> {
+        let len = d.read_usize();
+        (0..len).map(|_| (Decodable::decode(d), Decodable::decode(d))).collect()
+    }
+}
+
+impl<S: Encoder, T> Encodable<S> for BTreeSet<T>
+where
+    T: Encodable<S> + PartialEq + Ord,
+{
+    fn encode(&self, s: &mut S) {
+        s.emit_usize(self.len());
+        for e in self.iter() {
+            e.encode(s);
+        }
+    }
+}
+
+impl<D: Decoder, T> Decodable<D> for BTreeSet<T>
+where
+    T: Decodable<D> + PartialEq + Ord,
+{
+    fn decode(d: &mut D) -> BTreeSet<T> {
+        let len = d.read_usize();
+        (0..len).map(|_| Decodable::decode(d)).collect()
+    }
+}
+
+impl<E: Encoder, K, V, S> Encodable<E> for HashMap<K, V, S>
+where
+    K: Encodable<E> + Eq,
+    V: Encodable<E>,
+    S: BuildHasher,
+{
+    fn encode(&self, e: &mut E) {
+        e.emit_usize(self.len());
+        for (key, val) in self.iter() {
+            key.encode(e);
+            val.encode(e);
+        }
+    }
+}
+
+impl<D: Decoder, K, V, S> Decodable<D> for HashMap<K, V, S>
+where
+    K: Decodable<D> + Hash + Eq,
+    V: Decodable<D>,
+    S: BuildHasher + Default,
+{
+    fn decode(d: &mut D) -> HashMap<K, V, S> {
+        let len = d.read_usize();
+        (0..len).map(|_| (Decodable::decode(d), Decodable::decode(d))).collect()
+    }
+}
+
+impl<E: Encoder, T, S> Encodable<E> for HashSet<T, S>
+where
+    T: Encodable<E> + Eq,
+    S: BuildHasher,
+{
+    fn encode(&self, s: &mut E) {
+        s.emit_usize(self.len());
+        for e in self.iter() {
+            e.encode(s);
+        }
+    }
+}
+
+impl<D: Decoder, T, S> Decodable<D> for HashSet<T, S>
+where
+    T: Decodable<D> + Hash + Eq,
+    S: BuildHasher + Default,
+{
+    fn decode(d: &mut D) -> HashSet<T, S> {
+        let len = d.read_usize();
+        (0..len).map(|_| Decodable::decode(d)).collect()
+    }
+}
+
+impl<E: Encoder, K, V, S> Encodable<E> for indexmap::IndexMap<K, V, S>
+where
+    K: Encodable<E> + Hash + Eq,
+    V: Encodable<E>,
+    S: BuildHasher,
+{
+    fn encode(&self, e: &mut E) {
+        e.emit_usize(self.len());
+        for (key, val) in self.iter() {
+            key.encode(e);
+            val.encode(e);
+        }
+    }
+}
+
+impl<D: Decoder, K, V, S> Decodable<D> for indexmap::IndexMap<K, V, S>
+where
+    K: Decodable<D> + Hash + Eq,
+    V: Decodable<D>,
+    S: BuildHasher + Default,
+{
+    fn decode(d: &mut D) -> indexmap::IndexMap<K, V, S> {
+        let len = d.read_usize();
+        (0..len).map(|_| (Decodable::decode(d), Decodable::decode(d))).collect()
+    }
+}
+
+impl<E: Encoder, T, S> Encodable<E> for indexmap::IndexSet<T, S>
+where
+    T: Encodable<E> + Hash + Eq,
+    S: BuildHasher,
+{
+    fn encode(&self, s: &mut E) {
+        s.emit_usize(self.len());
+        for e in self.iter() {
+            e.encode(s);
+        }
+    }
+}
+
+impl<D: Decoder, T, S> Decodable<D> for indexmap::IndexSet<T, S>
+where
+    T: Decodable<D> + Hash + Eq,
+    S: BuildHasher + Default,
+{
+    fn decode(d: &mut D) -> indexmap::IndexSet<T, S> {
+        let len = d.read_usize();
+        (0..len).map(|_| Decodable::decode(d)).collect()
+    }
+}
+
+impl<E: Encoder, T: Encodable<E>> Encodable<E> for Rc<[T]> {
+    fn encode(&self, s: &mut E) {
+        let slice: &[T] = self;
+        slice.encode(s);
+    }
+}
+
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for Rc<[T]> {
+    fn decode(d: &mut D) -> Rc<[T]> {
+        let vec: Vec<T> = Decodable::decode(d);
+        vec.into()
+    }
+}
+
+impl<E: Encoder, T: Encodable<E>> Encodable<E> for Arc<[T]> {
+    fn encode(&self, s: &mut E) {
+        let slice: &[T] = self;
+        slice.encode(s);
+    }
+}
+
+impl<D: Decoder, T: Decodable<D>> Decodable<D> for Arc<[T]> {
+    fn decode(d: &mut D) -> Arc<[T]> {
+        let vec: Vec<T> = Decodable::decode(d);
+        vec.into()
     }
 }