diff --git a/compiler/rustc_borrowck/src/member_constraints.rs b/compiler/rustc_borrowck/src/member_constraints.rs
index 2e2578df011..f22d355e613 100644
--- a/compiler/rustc_borrowck/src/member_constraints.rs
+++ b/compiler/rustc_borrowck/src/member_constraints.rs
@@ -144,7 +144,7 @@ impl<R> MemberConstraintSet<'tcx, R>
 where
     R: Copy + Hash + Eq,
 {
-    crate fn all_indices(&self) -> impl Iterator<Item = NllMemberConstraintIndex> {
+    crate fn all_indices(&self) -> impl Iterator<Item = NllMemberConstraintIndex> + '_ {
         self.constraints.indices()
     }
 
diff --git a/compiler/rustc_borrowck/src/region_infer/mod.rs b/compiler/rustc_borrowck/src/region_infer/mod.rs
index 65d6e3a4ae5..734a5b4972b 100644
--- a/compiler/rustc_borrowck/src/region_infer/mod.rs
+++ b/compiler/rustc_borrowck/src/region_infer/mod.rs
@@ -497,7 +497,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     }
 
     /// Returns an iterator over all the region indices.
-    pub fn regions(&self) -> impl Iterator<Item = RegionVid> {
+    pub fn regions(&self) -> impl Iterator<Item = RegionVid> + '_ {
         self.definitions.indices()
     }
 
diff --git a/compiler/rustc_index/src/lib.rs b/compiler/rustc_index/src/lib.rs
index f18f322b222..a72a27e07bd 100644
--- a/compiler/rustc_index/src/lib.rs
+++ b/compiler/rustc_index/src/lib.rs
@@ -4,7 +4,6 @@
 #![feature(iter_zip)]
 #![feature(min_specialization)]
 #![feature(test)]
-#![feature(fn_traits)]
 
 pub mod bit_set;
 pub mod vec;
diff --git a/compiler/rustc_index/src/vec.rs b/compiler/rustc_index/src/vec.rs
index 8535a7c866d..56ea04539e5 100644
--- a/compiler/rustc_index/src/vec.rs
+++ b/compiler/rustc_index/src/vec.rs
@@ -3,9 +3,9 @@ use rustc_serialize::{Decodable, Decoder, Encodable, Encoder};
 use std::fmt;
 use std::fmt::Debug;
 use std::hash::Hash;
-use std::iter::{self, FromIterator};
+use std::iter::FromIterator;
 use std::marker::PhantomData;
-use std::ops::{Index, IndexMut, Range, RangeBounds};
+use std::ops::{Index, IndexMut, RangeBounds};
 use std::slice;
 use std::vec;
 
@@ -518,8 +518,6 @@ impl<I: Idx, T: fmt::Debug> fmt::Debug for IndexVec<I, T> {
     }
 }
 
-pub type Enumerated<I, J> = iter::Map<iter::Enumerate<J>, IntoIdx<I>>;
-
 impl<I: Idx, T> IndexVec<I, T> {
     #[inline]
     pub fn new() -> Self {
@@ -596,8 +594,10 @@ impl<I: Idx, T> IndexVec<I, T> {
     }
 
     #[inline]
-    pub fn into_iter_enumerated(self) -> Enumerated<I, vec::IntoIter<T>> {
-        self.raw.into_iter().enumerate().map(IntoIdx { _marker: PhantomData })
+    pub fn into_iter_enumerated(
+        self,
+    ) -> impl DoubleEndedIterator<Item = (I, T)> + ExactSizeIterator {
+        self.raw.into_iter().enumerate().map(|(n, t)| (I::new(n), t))
     }
 
     #[inline]
@@ -606,13 +606,15 @@ impl<I: Idx, T> IndexVec<I, T> {
     }
 
     #[inline]
-    pub fn iter_enumerated(&self) -> Enumerated<I, slice::Iter<'_, T>> {
-        self.raw.iter().enumerate().map(IntoIdx { _marker: PhantomData })
+    pub fn iter_enumerated(
+        &self,
+    ) -> impl DoubleEndedIterator<Item = (I, &T)> + ExactSizeIterator + '_ {
+        self.raw.iter().enumerate().map(|(n, t)| (I::new(n), t))
     }
 
     #[inline]
-    pub fn indices(&self) -> iter::Map<Range<usize>, IntoIdx<I>> {
-        (0..self.len()).map(IntoIdx { _marker: PhantomData })
+    pub fn indices(&self) -> impl DoubleEndedIterator<Item = I> + ExactSizeIterator + 'static {
+        (0..self.len()).map(|n| I::new(n))
     }
 
     #[inline]
@@ -621,8 +623,10 @@ impl<I: Idx, T> IndexVec<I, T> {
     }
 
     #[inline]
-    pub fn iter_enumerated_mut(&mut self) -> Enumerated<I, slice::IterMut<'_, T>> {
-        self.raw.iter_mut().enumerate().map(IntoIdx { _marker: PhantomData })
+    pub fn iter_enumerated_mut(
+        &mut self,
+    ) -> impl DoubleEndedIterator<Item = (I, &mut T)> + ExactSizeIterator + '_ {
+        self.raw.iter_mut().enumerate().map(|(n, t)| (I::new(n), t))
     }
 
     #[inline]
@@ -638,7 +642,7 @@ impl<I: Idx, T> IndexVec<I, T> {
         &'a mut self,
         range: R,
     ) -> impl Iterator<Item = (I, T)> + 'a {
-        self.raw.drain(range).enumerate().map(IntoIdx { _marker: PhantomData })
+        self.raw.drain(range).enumerate().map(|(n, t)| (I::new(n), t))
     }
 
     #[inline]
@@ -832,36 +836,5 @@ impl<'a, I: Idx, T> IntoIterator for &'a mut IndexVec<I, T> {
     }
 }
 
-pub struct IntoIdx<I: Idx> {
-    _marker: PhantomData<fn(&I)>,
-}
-impl<I: Idx, T> FnOnce<((usize, T),)> for IntoIdx<I> {
-    type Output = (I, T);
-
-    extern "rust-call" fn call_once(self, ((n, t),): ((usize, T),)) -> Self::Output {
-        (I::new(n), t)
-    }
-}
-
-impl<I: Idx, T> FnMut<((usize, T),)> for IntoIdx<I> {
-    extern "rust-call" fn call_mut(&mut self, ((n, t),): ((usize, T),)) -> Self::Output {
-        (I::new(n), t)
-    }
-}
-
-impl<I: Idx> FnOnce<(usize,)> for IntoIdx<I> {
-    type Output = I;
-
-    extern "rust-call" fn call_once(self, (n,): (usize,)) -> Self::Output {
-        I::new(n)
-    }
-}
-
-impl<I: Idx> FnMut<(usize,)> for IntoIdx<I> {
-    extern "rust-call" fn call_mut(&mut self, (n,): (usize,)) -> Self::Output {
-        I::new(n)
-    }
-}
-
 #[cfg(test)]
 mod tests;
diff --git a/compiler/rustc_mir_dataflow/src/move_paths/mod.rs b/compiler/rustc_mir_dataflow/src/move_paths/mod.rs
index 699ec4bbff8..f2b34159382 100644
--- a/compiler/rustc_mir_dataflow/src/move_paths/mod.rs
+++ b/compiler/rustc_mir_dataflow/src/move_paths/mod.rs
@@ -1,6 +1,5 @@
-use core::slice::Iter;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_index::vec::{Enumerated, IndexVec};
+use rustc_index::vec::IndexVec;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{ParamEnv, Ty, TyCtxt};
 use rustc_span::Span;
@@ -337,7 +336,9 @@ impl MovePathLookup {
 
     /// An enumerated iterator of `local`s and their associated
     /// `MovePathIndex`es.
-    pub fn iter_locals_enumerated(&self) -> Enumerated<Local, Iter<'_, MovePathIndex>> {
+    pub fn iter_locals_enumerated(
+        &self,
+    ) -> impl DoubleEndedIterator<Item = (Local, &MovePathIndex)> + ExactSizeIterator {
         self.locals.iter_enumerated()
     }
 }