diff --git a/src/librustc_middle/ty/list.rs b/src/librustc_middle/ty/list.rs
new file mode 100644
index 0000000000000..6427c547a8f29
--- /dev/null
+++ b/src/librustc_middle/ty/list.rs
@@ -0,0 +1,149 @@
+use crate::arena::Arena;
+
+use rustc_serialize::{Encodable, Encoder};
+
+use std::cmp::{self, Ordering};
+use std::fmt;
+use std::hash::{Hash, Hasher};
+use std::mem;
+use std::ops::Deref;
+use std::ptr;
+use std::slice;
+
+extern "C" {
+    /// A dummy type used to force `List` to be unsized while not requiring references to it be wide
+    /// pointers.
+    type OpaqueListContents;
+}
+
+/// A wrapper for slices with the additional invariant
+/// that the slice is interned and no other slice with
+/// the same contents can exist in the same context.
+/// This means we can use pointer for both
+/// equality comparisons and hashing.
+/// Note: `Slice` was already taken by the `Ty`.
+#[repr(C)]
+pub struct List<T> {
+    len: usize,
+    data: [T; 0],
+    opaque: OpaqueListContents,
+}
+
+unsafe impl<T: Sync> Sync for List<T> {}
+
+impl<T: Copy> List<T> {
+    #[inline]
+    pub(super) fn from_arena<'tcx>(arena: &'tcx Arena<'tcx>, slice: &[T]) -> &'tcx List<T> {
+        assert!(!mem::needs_drop::<T>());
+        assert!(mem::size_of::<T>() != 0);
+        assert!(!slice.is_empty());
+
+        // Align up the size of the len (usize) field
+        let align = mem::align_of::<T>();
+        let align_mask = align - 1;
+        let offset = mem::size_of::<usize>();
+        let offset = (offset + align_mask) & !align_mask;
+
+        let size = offset + slice.len() * mem::size_of::<T>();
+
+        let mem = arena
+            .dropless
+            .alloc_raw(size, cmp::max(mem::align_of::<T>(), mem::align_of::<usize>()));
+        unsafe {
+            let result = &mut *(mem.as_mut_ptr() as *mut List<T>);
+            // Write the length
+            result.len = slice.len();
+
+            // Write the elements
+            let arena_slice = slice::from_raw_parts_mut(result.data.as_mut_ptr(), result.len);
+            arena_slice.copy_from_slice(slice);
+
+            result
+        }
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for List<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+impl<T: Encodable> Encodable for List<T> {
+    #[inline]
+    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
+        (**self).encode(s)
+    }
+}
+
+impl<T> Ord for List<T>
+where
+    T: Ord,
+{
+    fn cmp(&self, other: &List<T>) -> Ordering {
+        if self == other { Ordering::Equal } else { <[T] as Ord>::cmp(&**self, &**other) }
+    }
+}
+
+impl<T> PartialOrd for List<T>
+where
+    T: PartialOrd,
+{
+    fn partial_cmp(&self, other: &List<T>) -> Option<Ordering> {
+        if self == other {
+            Some(Ordering::Equal)
+        } else {
+            <[T] as PartialOrd>::partial_cmp(&**self, &**other)
+        }
+    }
+}
+
+impl<T: PartialEq> PartialEq for List<T> {
+    #[inline]
+    fn eq(&self, other: &List<T>) -> bool {
+        ptr::eq(self, other)
+    }
+}
+impl<T: Eq> Eq for List<T> {}
+
+impl<T> Hash for List<T> {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        (self as *const List<T>).hash(s)
+    }
+}
+
+impl<T> Deref for List<T> {
+    type Target = [T];
+    #[inline(always)]
+    fn deref(&self) -> &[T] {
+        self.as_ref()
+    }
+}
+
+impl<T> AsRef<[T]> for List<T> {
+    #[inline(always)]
+    fn as_ref(&self) -> &[T] {
+        unsafe { slice::from_raw_parts(self.data.as_ptr(), self.len) }
+    }
+}
+
+impl<'a, T> IntoIterator for &'a List<T> {
+    type Item = &'a T;
+    type IntoIter = <&'a [T] as IntoIterator>::IntoIter;
+    #[inline(always)]
+    fn into_iter(self) -> Self::IntoIter {
+        self[..].iter()
+    }
+}
+
+impl<T> List<T> {
+    #[inline(always)]
+    pub fn empty<'a>() -> &'a List<T> {
+        #[repr(align(64), C)]
+        struct EmptySlice([u8; 64]);
+        static EMPTY_SLICE: EmptySlice = EmptySlice([0; 64]);
+        assert!(mem::align_of::<T>() <= 64);
+        unsafe { &*(&EMPTY_SLICE as *const _ as *const List<T>) }
+    }
+}
diff --git a/src/librustc_middle/ty/mod.rs b/src/librustc_middle/ty/mod.rs
index 8bb9934789a23..ae0951842995d 100644
--- a/src/librustc_middle/ty/mod.rs
+++ b/src/librustc_middle/ty/mod.rs
@@ -4,7 +4,6 @@ pub use self::BorrowKind::*;
 pub use self::IntVarValue::*;
 pub use self::Variance::*;
 
-use crate::arena::Arena;
 use crate::hir::exports::ExportMap;
 use crate::ich::StableHashingContext;
 use crate::infer::canonical::Canonical;
@@ -43,13 +42,11 @@ use rustc_span::Span;
 use rustc_target::abi::{Align, VariantIdx};
 
 use std::cell::RefCell;
-use std::cmp::{self, Ordering};
+use std::cmp::Ordering;
 use std::fmt;
 use std::hash::{Hash, Hasher};
-use std::ops::Deref;
 use std::ops::Range;
-use std::slice;
-use std::{mem, ptr};
+use std::ptr;
 
 pub use self::sty::BoundRegion::*;
 pub use self::sty::InferTy::*;
@@ -81,6 +78,8 @@ pub use self::context::{
 
 pub use self::instance::{Instance, InstanceDef};
 
+pub use self::list::List;
+
 pub use self::trait_def::TraitDef;
 
 pub use self::query::queries;
@@ -112,6 +111,7 @@ pub mod walk;
 mod context;
 mod diagnostics;
 mod instance;
+mod list;
 mod structural_impls;
 mod sty;
 
@@ -663,148 +663,9 @@ pub type Ty<'tcx> = &'tcx TyS<'tcx>;
 
 impl<'tcx> rustc_serialize::UseSpecializedEncodable for Ty<'tcx> {}
 impl<'tcx> rustc_serialize::UseSpecializedDecodable for Ty<'tcx> {}
-
-pub type CanonicalTy<'tcx> = Canonical<'tcx, Ty<'tcx>>;
-
-extern "C" {
-    /// A dummy type used to force `List` to be unsized while not requiring references to it be wide
-    /// pointers.
-    type OpaqueListContents;
-}
-
-/// A wrapper for slices with the additional invariant
-/// that the slice is interned and no other slice with
-/// the same contents can exist in the same context.
-/// This means we can use pointer for both
-/// equality comparisons and hashing.
-/// Note: `Slice` was already taken by the `Ty`.
-#[repr(C)]
-pub struct List<T> {
-    len: usize,
-    data: [T; 0],
-    opaque: OpaqueListContents,
-}
-
-unsafe impl<T: Sync> Sync for List<T> {}
-
-impl<T: Copy> List<T> {
-    #[inline]
-    fn from_arena<'tcx>(arena: &'tcx Arena<'tcx>, slice: &[T]) -> &'tcx List<T> {
-        assert!(!mem::needs_drop::<T>());
-        assert!(mem::size_of::<T>() != 0);
-        assert!(!slice.is_empty());
-
-        // Align up the size of the len (usize) field
-        let align = mem::align_of::<T>();
-        let align_mask = align - 1;
-        let offset = mem::size_of::<usize>();
-        let offset = (offset + align_mask) & !align_mask;
-
-        let size = offset + slice.len() * mem::size_of::<T>();
-
-        let mem = arena
-            .dropless
-            .alloc_raw(size, cmp::max(mem::align_of::<T>(), mem::align_of::<usize>()));
-        unsafe {
-            let result = &mut *(mem.as_mut_ptr() as *mut List<T>);
-            // Write the length
-            result.len = slice.len();
-
-            // Write the elements
-            let arena_slice = slice::from_raw_parts_mut(result.data.as_mut_ptr(), result.len);
-            arena_slice.copy_from_slice(slice);
-
-            result
-        }
-    }
-}
-
-impl<T: fmt::Debug> fmt::Debug for List<T> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        (**self).fmt(f)
-    }
-}
-
-impl<T: Encodable> Encodable for List<T> {
-    #[inline]
-    fn encode<S: Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
-        (**self).encode(s)
-    }
-}
-
-impl<T> Ord for List<T>
-where
-    T: Ord,
-{
-    fn cmp(&self, other: &List<T>) -> Ordering {
-        if self == other { Ordering::Equal } else { <[T] as Ord>::cmp(&**self, &**other) }
-    }
-}
-
-impl<T> PartialOrd for List<T>
-where
-    T: PartialOrd,
-{
-    fn partial_cmp(&self, other: &List<T>) -> Option<Ordering> {
-        if self == other {
-            Some(Ordering::Equal)
-        } else {
-            <[T] as PartialOrd>::partial_cmp(&**self, &**other)
-        }
-    }
-}
-
-impl<T: PartialEq> PartialEq for List<T> {
-    #[inline]
-    fn eq(&self, other: &List<T>) -> bool {
-        ptr::eq(self, other)
-    }
-}
-impl<T: Eq> Eq for List<T> {}
-
-impl<T> Hash for List<T> {
-    #[inline]
-    fn hash<H: Hasher>(&self, s: &mut H) {
-        (self as *const List<T>).hash(s)
-    }
-}
-
-impl<T> Deref for List<T> {
-    type Target = [T];
-    #[inline(always)]
-    fn deref(&self) -> &[T] {
-        self.as_ref()
-    }
-}
-
-impl<T> AsRef<[T]> for List<T> {
-    #[inline(always)]
-    fn as_ref(&self) -> &[T] {
-        unsafe { slice::from_raw_parts(self.data.as_ptr(), self.len) }
-    }
-}
-
-impl<'a, T> IntoIterator for &'a List<T> {
-    type Item = &'a T;
-    type IntoIter = <&'a [T] as IntoIterator>::IntoIter;
-    #[inline(always)]
-    fn into_iter(self) -> Self::IntoIter {
-        self[..].iter()
-    }
-}
-
 impl<'tcx> rustc_serialize::UseSpecializedDecodable for &'tcx List<Ty<'tcx>> {}
 
-impl<T> List<T> {
-    #[inline(always)]
-    pub fn empty<'a>() -> &'a List<T> {
-        #[repr(align(64), C)]
-        struct EmptySlice([u8; 64]);
-        static EMPTY_SLICE: EmptySlice = EmptySlice([0; 64]);
-        assert!(mem::align_of::<T>() <= 64);
-        unsafe { &*(&EMPTY_SLICE as *const _ as *const List<T>) }
-    }
-}
+pub type CanonicalTy<'tcx> = Canonical<'tcx, Ty<'tcx>>;
 
 #[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, HashStable)]
 pub struct UpvarPath {