diff --git a/compiler/rustc_data_structures/src/fingerprint.rs b/compiler/rustc_data_structures/src/fingerprint.rs
index 01efcaf6f448d8f59778dce8a3e8989bd50fd441..8afe94ac8dba845679f30ed04bae4b2128317dde 100644
--- a/compiler/rustc_data_structures/src/fingerprint.rs
+++ b/compiler/rustc_data_structures/src/fingerprint.rs
@@ -4,7 +4,7 @@
     Decodable, Encodable,
 };
 use std::hash::{Hash, Hasher};
-use std::mem;
+use std::mem::{self, MaybeUninit};
 
 #[derive(Eq, PartialEq, Ord, PartialOrd, Debug, Clone, Copy)]
 pub struct Fingerprint(u64, u64);
@@ -61,7 +61,7 @@ pub fn encode_opaque(&self, encoder: &mut opaque::Encoder) -> EncodeResult {
     }
 
     pub fn decode_opaque(decoder: &mut opaque::Decoder<'_>) -> Result<Fingerprint, String> {
-        let mut bytes = [0; 16];
+        let mut bytes: [MaybeUninit<u8>; 16] = MaybeUninit::uninit_array();
 
         decoder.read_raw_bytes(&mut bytes)?;
 
diff --git a/compiler/rustc_macros/src/serialize.rs b/compiler/rustc_macros/src/serialize.rs
index dbeb3c755044f543bdb2a64eceea3bc5f4ae7e2b..72bd4804e98c0b81e4f5e20a6228db87f74e07a6 100644
--- a/compiler/rustc_macros/src/serialize.rs
+++ b/compiler/rustc_macros/src/serialize.rs
@@ -203,7 +203,7 @@ fn encodable_body(
                                 #field_name,
                                 #field_idx,
                                 |__encoder|
-                                ::rustc_serialize::Encodable::encode(#bind_ident, __encoder),
+                                ::rustc_serialize::Encodable::<#encoder_ty>::encode(#bind_ident, __encoder),
                             ) {
                                 ::std::result::Result::Ok(()) => (),
                                 ::std::result::Result::Err(__err)
@@ -237,7 +237,7 @@ fn encodable_body(
                                 __encoder,
                                 #field_idx,
                                 |__encoder|
-                                ::rustc_serialize::Encodable::encode(#bind_ident, __encoder),
+                                ::rustc_serialize::Encodable::<#encoder_ty>::encode(#bind_ident, __encoder),
                             ) {
                                 ::std::result::Result::Ok(()) => (),
                                 ::std::result::Result::Err(__err)
diff --git a/compiler/rustc_middle/src/ty/query/on_disk_cache.rs b/compiler/rustc_middle/src/ty/query/on_disk_cache.rs
index 8a1165bbd647a296bf8fc19ff2fdb387976e959c..9b40c9a7ed88ad42a7f2cf9942787af37e61e5b5 100644
--- a/compiler/rustc_middle/src/ty/query/on_disk_cache.rs
+++ b/compiler/rustc_middle/src/ty/query/on_disk_cache.rs
@@ -807,6 +807,15 @@ fn decode_alloc_id(&mut self) -> Result<interpret::AllocId, Self::Error> {
 
 crate::implement_ty_decoder!(CacheDecoder<'a, 'tcx>);
 
+// This ensures that the `Decodable<opaque::Decoder>::decode` specialization for `Vec<u8>` is used
+// when a `CacheDecoder` is passed to `Decodable::decode`. Unfortunately, we have to manually opt
+// into specializations this way, given how `CacheDecoder` and the decoding traits currently work.
+impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for Vec<u8> {
+    fn decode(d: &mut CacheDecoder<'a, 'tcx>) -> Result<Self, String> {
+        Decodable::decode(&mut d.opaque)
+    }
+}
+
 impl<'a, 'tcx> Decodable<CacheDecoder<'a, 'tcx>> for SyntaxContext {
     fn decode(decoder: &mut CacheDecoder<'a, 'tcx>) -> Result<Self, String> {
         let syntax_contexts = decoder.syntax_contexts;
@@ -1149,6 +1158,16 @@ fn emit_unit(&mut self) -> Result<(), Self::Error> {
     }
 }
 
+// This ensures that the `Encodable<opaque::Encoder>::encode` specialization for byte slices
+// is used when a `CacheEncoder` having an `opaque::Encoder` is passed to `Encodable::encode`.
+// Unfortunately, we have to manually opt into specializations this way, given how `CacheEncoder`
+// and the encoding traits currently work.
+impl<'a, 'tcx> Encodable<CacheEncoder<'a, 'tcx, opaque::Encoder>> for [u8] {
+    fn encode(&self, e: &mut CacheEncoder<'a, 'tcx, opaque::Encoder>) -> opaque::EncodeResult {
+        self.encode(e.encoder)
+    }
+}
+
 // An integer that will always encode to 8 bytes.
 struct IntEncodedWithFixedSize(u64);
 
diff --git a/compiler/rustc_serialize/src/collection_impls.rs b/compiler/rustc_serialize/src/collection_impls.rs
index 3d274cb01507b004bcde7191709a66c6c6bb4ed4..ae6d27e037b2dbda7a1f66ffa7712a4c0de77381 100644
--- a/compiler/rustc_serialize/src/collection_impls.rs
+++ b/compiler/rustc_serialize/src/collection_impls.rs
@@ -11,12 +11,8 @@
 
 impl<S: Encoder, A: Array<Item: Encodable<S>>> Encodable<S> for SmallVec<A> {
     fn encode(&self, s: &mut S) -> Result<(), S::Error> {
-        s.emit_seq(self.len(), |s| {
-            for (i, e) in self.iter().enumerate() {
-                s.emit_seq_elt(i, |s| e.encode(s))?;
-            }
-            Ok(())
-        })
+        let slice: &[A::Item] = self;
+        slice.encode(s)
     }
 }
 
@@ -292,46 +288,28 @@ fn decode(d: &mut D) -> Result<indexmap::IndexSet<T, S>, D::Error> {
 
 impl<E: Encoder, T: Encodable<E>> Encodable<E> for Rc<[T]> {
     fn encode(&self, s: &mut E) -> Result<(), E::Error> {
-        s.emit_seq(self.len(), |s| {
-            for (index, e) in self.iter().enumerate() {
-                s.emit_seq_elt(index, |s| e.encode(s))?;
-            }
-            Ok(())
-        })
+        let slice: &[T] = self;
+        slice.encode(s)
     }
 }
 
 impl<D: Decoder, T: Decodable<D>> Decodable<D> for Rc<[T]> {
     fn decode(d: &mut D) -> Result<Rc<[T]>, D::Error> {
-        d.read_seq(|d, len| {
-            let mut vec = Vec::with_capacity(len);
-            for index in 0..len {
-                vec.push(d.read_seq_elt(index, |d| Decodable::decode(d))?);
-            }
-            Ok(vec.into())
-        })
+        let vec: Vec<T> = Decodable::decode(d)?;
+        Ok(vec.into())
     }
 }
 
 impl<E: Encoder, T: Encodable<E>> Encodable<E> for Arc<[T]> {
     fn encode(&self, s: &mut E) -> Result<(), E::Error> {
-        s.emit_seq(self.len(), |s| {
-            for (index, e) in self.iter().enumerate() {
-                s.emit_seq_elt(index, |s| e.encode(s))?;
-            }
-            Ok(())
-        })
+        let slice: &[T] = self;
+        slice.encode(s)
     }
 }
 
 impl<D: Decoder, T: Decodable<D>> Decodable<D> for Arc<[T]> {
     fn decode(d: &mut D) -> Result<Arc<[T]>, D::Error> {
-        d.read_seq(|d, len| {
-            let mut vec = Vec::with_capacity(len);
-            for index in 0..len {
-                vec.push(d.read_seq_elt(index, |d| Decodable::decode(d))?);
-            }
-            Ok(vec.into())
-        })
+        let vec: Vec<T> = Decodable::decode(d)?;
+        Ok(vec.into())
     }
 }
diff --git a/compiler/rustc_serialize/src/lib.rs b/compiler/rustc_serialize/src/lib.rs
index ac1cdc6ad45f20338a8ad3ba8d55bed26304ab11..f58ed14d9971e09103241349f806c83484c8f162 100644
--- a/compiler/rustc_serialize/src/lib.rs
+++ b/compiler/rustc_serialize/src/lib.rs
@@ -14,6 +14,8 @@
 #![feature(nll)]
 #![feature(associated_type_bounds)]
 #![cfg_attr(bootstrap, feature(min_const_generics))]
+#![feature(min_specialization)]
+#![feature(vec_spare_capacity)]
 #![cfg_attr(test, feature(test))]
 #![allow(rustc::internal)]
 
diff --git a/compiler/rustc_serialize/src/opaque.rs b/compiler/rustc_serialize/src/opaque.rs
index 8b79c93e7605bf98b706175f06616b37d4eb0aff..673742df7f0dce75c3e99508075c1813e448becd 100644
--- a/compiler/rustc_serialize/src/opaque.rs
+++ b/compiler/rustc_serialize/src/opaque.rs
@@ -1,6 +1,8 @@
 use crate::leb128::{self, read_signed_leb128, write_signed_leb128};
 use crate::serialize;
 use std::borrow::Cow;
+use std::mem::MaybeUninit;
+use std::ptr;
 
 // -----------------------------------------------------------------------------
 // Encoder
@@ -179,11 +181,19 @@ pub fn advance(&mut self, bytes: usize) {
     }
 
     #[inline]
-    pub fn read_raw_bytes(&mut self, s: &mut [u8]) -> Result<(), String> {
+    pub fn read_raw_bytes(&mut self, s: &mut [MaybeUninit<u8>]) -> Result<(), String> {
         let start = self.position;
         let end = start + s.len();
+        assert!(end <= self.data.len());
 
-        s.copy_from_slice(&self.data[start..end]);
+        // SAFETY: Both `src` and `dst` point to at least `s.len()` elements:
+        // `src` points to at least `s.len()` elements by above assert, and
+        // `dst` points to `s.len()` elements by derivation from `s`.
+        unsafe {
+            let src = self.data.as_ptr().add(start);
+            let dst = s.as_mut_ptr() as *mut u8;
+            ptr::copy_nonoverlapping(src, dst, s.len());
+        }
 
         self.position = end;
 
@@ -316,3 +326,36 @@ fn error(&mut self, err: &str) -> Self::Error {
         err.to_string()
     }
 }
+
+// Specializations for contiguous byte sequences follow. The default implementations for slices
+// encode and decode each element individually. This isn't necessary for `u8` slices when using
+// opaque encoders and decoders, because each `u8` is unchanged by encoding and decoding.
+// Therefore, we can use more efficient implementations that process the entire sequence at once.
+
+// Specialize encoding byte slices. This specialization also applies to encoding `Vec<u8>`s, etc.,
+// since the default implementations call `encode` on their slices internally.
+impl serialize::Encodable<Encoder> for [u8] {
+    fn encode(&self, e: &mut Encoder) -> EncodeResult {
+        serialize::Encoder::emit_usize(e, self.len())?;
+        e.emit_raw_bytes(self);
+        Ok(())
+    }
+}
+
+// Specialize decoding `Vec<u8>`. This specialization also applies to decoding `Box<[u8]>`s, etc.,
+// since the default implementations call `decode` to produce a `Vec<u8>` internally.
+impl<'a> serialize::Decodable<Decoder<'a>> for Vec<u8> {
+    fn decode(d: &mut Decoder<'a>) -> Result<Self, String> {
+        let len = serialize::Decoder::read_usize(d)?;
+
+        let mut v = Vec::with_capacity(len);
+        let buf = &mut v.spare_capacity_mut()[..len];
+        d.read_raw_bytes(buf)?;
+
+        unsafe {
+            v.set_len(len);
+        }
+
+        Ok(v)
+    }
+}
diff --git a/compiler/rustc_serialize/src/serialize.rs b/compiler/rustc_serialize/src/serialize.rs
index aa305f3c7fc3f7c69b5f485ecc7c04cd2c3b1943..47aad5b88c6229dc5532f820c575007354af80b1 100644
--- a/compiler/rustc_serialize/src/serialize.rs
+++ b/compiler/rustc_serialize/src/serialize.rs
@@ -527,7 +527,7 @@ fn decode(d: &mut D) -> Result<Rc<T>, D::Error> {
 }
 
 impl<S: Encoder, T: Encodable<S>> Encodable<S> for [T] {
-    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
+    default fn encode(&self, s: &mut S) -> Result<(), S::Error> {
         s.emit_seq(self.len(), |s| {
             for (i, e) in self.iter().enumerate() {
                 s.emit_seq_elt(i, |s| e.encode(s))?
@@ -545,7 +545,7 @@ fn encode(&self, s: &mut S) -> Result<(), S::Error> {
 }
 
 impl<D: Decoder, T: Decodable<D>> Decodable<D> for Vec<T> {
-    fn decode(d: &mut D) -> Result<Vec<T>, D::Error> {
+    default fn decode(d: &mut D) -> Result<Vec<T>, D::Error> {
         d.read_seq(|d, len| {
             let mut v = Vec::with_capacity(len);
             for i in 0..len {
@@ -591,13 +591,8 @@ impl<D: Decoder, T: Decodable<D> + ToOwned> Decodable<D> for Cow<'static, [T]>
     [T]: ToOwned<Owned = Vec<T>>,
 {
     fn decode(d: &mut D) -> Result<Cow<'static, [T]>, D::Error> {
-        d.read_seq(|d, len| {
-            let mut v = Vec::with_capacity(len);
-            for i in 0..len {
-                v.push(d.read_seq_elt(i, |d| Decodable::decode(d))?);
-            }
-            Ok(Cow::Owned(v))
-        })
+        let v: Vec<T> = Decodable::decode(d)?;
+        Ok(Cow::Owned(v))
     }
 }