diff --git a/Cargo.lock b/Cargo.lock
index 6dd6fbe71b8..5f89f5e4a57 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4575,6 +4575,7 @@ dependencies = [
  "elsa",
  "ena",
  "indexmap",
+ "itertools",
  "jobserver",
  "libc",
  "measureme",
diff --git a/compiler/rustc_data_structures/Cargo.toml b/compiler/rustc_data_structures/Cargo.toml
index 24b6b5cfb1f..2102f09c56a 100644
--- a/compiler/rustc_data_structures/Cargo.toml
+++ b/compiler/rustc_data_structures/Cargo.toml
@@ -33,6 +33,7 @@ tempfile = "3.2"
 thin-vec = "0.2.12"
 tracing = "0.1"
 elsa = "=1.7.1"
+itertools = "0.10.1"
 
 [dependencies.parking_lot]
 version = "0.11"
diff --git a/compiler/rustc_data_structures/src/sso/either_iter.rs b/compiler/rustc_data_structures/src/sso/either_iter.rs
deleted file mode 100644
index bca6c0955b9..00000000000
--- a/compiler/rustc_data_structures/src/sso/either_iter.rs
+++ /dev/null
@@ -1,73 +0,0 @@
-use std::fmt;
-use std::iter::FusedIterator;
-
-/// Iterator which may contain instance of
-/// one of two specific implementations.
-///
-/// Note: For most methods providing custom
-///       implementation may marginally
-///       improve performance by avoiding
-///       doing Left/Right match on every step
-///       and doing it only once instead.
-#[derive(Clone)]
-pub enum EitherIter<L, R> {
-    Left(L),
-    Right(R),
-}
-
-impl<L, R> Iterator for EitherIter<L, R>
-where
-    L: Iterator,
-    R: Iterator<Item = L::Item>,
-{
-    type Item = L::Item;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        match self {
-            EitherIter::Left(l) => l.next(),
-            EitherIter::Right(r) => r.next(),
-        }
-    }
-
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        match self {
-            EitherIter::Left(l) => l.size_hint(),
-            EitherIter::Right(r) => r.size_hint(),
-        }
-    }
-}
-
-impl<L, R> ExactSizeIterator for EitherIter<L, R>
-where
-    L: ExactSizeIterator,
-    R: ExactSizeIterator,
-    EitherIter<L, R>: Iterator,
-{
-    fn len(&self) -> usize {
-        match self {
-            EitherIter::Left(l) => l.len(),
-            EitherIter::Right(r) => r.len(),
-        }
-    }
-}
-
-impl<L, R> FusedIterator for EitherIter<L, R>
-where
-    L: FusedIterator,
-    R: FusedIterator,
-    EitherIter<L, R>: Iterator,
-{
-}
-
-impl<L, R> fmt::Debug for EitherIter<L, R>
-where
-    L: fmt::Debug,
-    R: fmt::Debug,
-{
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            EitherIter::Left(l) => l.fmt(f),
-            EitherIter::Right(r) => r.fmt(f),
-        }
-    }
-}
diff --git a/compiler/rustc_data_structures/src/sso/map.rs b/compiler/rustc_data_structures/src/sso/map.rs
index 7cdac581977..f9e2dfd16d5 100644
--- a/compiler/rustc_data_structures/src/sso/map.rs
+++ b/compiler/rustc_data_structures/src/sso/map.rs
@@ -1,6 +1,6 @@
-use super::either_iter::EitherIter;
 use crate::fx::FxHashMap;
 use arrayvec::ArrayVec;
+use itertools::Either;
 use std::fmt;
 use std::hash::Hash;
 use std::ops::Index;
@@ -138,8 +138,8 @@ impl<K, V> SsoHashMap<K, V> {
     /// The iterator element type is `&'a K`.
     pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
         match self {
-            SsoHashMap::Array(array) => EitherIter::Left(array.iter().map(|(k, _v)| k)),
-            SsoHashMap::Map(map) => EitherIter::Right(map.keys()),
+            SsoHashMap::Array(array) => Either::Left(array.iter().map(|(k, _v)| k)),
+            SsoHashMap::Map(map) => Either::Right(map.keys()),
         }
     }
 
@@ -147,8 +147,8 @@ impl<K, V> SsoHashMap<K, V> {
     /// The iterator element type is `&'a V`.
     pub fn values(&self) -> impl Iterator<Item = &'_ V> {
         match self {
-            SsoHashMap::Array(array) => EitherIter::Left(array.iter().map(|(_k, v)| v)),
-            SsoHashMap::Map(map) => EitherIter::Right(map.values()),
+            SsoHashMap::Array(array) => Either::Left(array.iter().map(|(_k, v)| v)),
+            SsoHashMap::Map(map) => Either::Right(map.values()),
         }
     }
 
@@ -156,8 +156,8 @@ impl<K, V> SsoHashMap<K, V> {
     /// The iterator element type is `&'a mut V`.
     pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut V> {
         match self {
-            SsoHashMap::Array(array) => EitherIter::Left(array.iter_mut().map(|(_k, v)| v)),
-            SsoHashMap::Map(map) => EitherIter::Right(map.values_mut()),
+            SsoHashMap::Array(array) => Either::Left(array.iter_mut().map(|(_k, v)| v)),
+            SsoHashMap::Map(map) => Either::Right(map.values_mut()),
         }
     }
 
@@ -165,8 +165,8 @@ impl<K, V> SsoHashMap<K, V> {
     /// allocated memory for reuse.
     pub fn drain(&mut self) -> impl Iterator<Item = (K, V)> + '_ {
         match self {
-            SsoHashMap::Array(array) => EitherIter::Left(array.drain(..)),
-            SsoHashMap::Map(map) => EitherIter::Right(map.drain()),
+            SsoHashMap::Array(array) => Either::Left(array.drain(..)),
+            SsoHashMap::Map(map) => Either::Right(map.drain()),
         }
     }
 }
@@ -406,7 +406,7 @@ where
 }
 
 impl<K, V> IntoIterator for SsoHashMap<K, V> {
-    type IntoIter = EitherIter<
+    type IntoIter = Either<
         <ArrayVec<(K, V), 8> as IntoIterator>::IntoIter,
         <FxHashMap<K, V> as IntoIterator>::IntoIter,
     >;
@@ -414,8 +414,8 @@ impl<K, V> IntoIterator for SsoHashMap<K, V> {
 
     fn into_iter(self) -> Self::IntoIter {
         match self {
-            SsoHashMap::Array(array) => EitherIter::Left(array.into_iter()),
-            SsoHashMap::Map(map) => EitherIter::Right(map.into_iter()),
+            SsoHashMap::Array(array) => Either::Left(array.into_iter()),
+            SsoHashMap::Map(map) => Either::Right(map.into_iter()),
         }
     }
 }
@@ -435,7 +435,7 @@ fn adapt_array_mut_it<K, V>(pair: &mut (K, V)) -> (&K, &mut V) {
 }
 
 impl<'a, K, V> IntoIterator for &'a SsoHashMap<K, V> {
-    type IntoIter = EitherIter<
+    type IntoIter = Either<
         std::iter::Map<
             <&'a ArrayVec<(K, V), 8> as IntoIterator>::IntoIter,
             fn(&'a (K, V)) -> (&'a K, &'a V),
@@ -446,14 +446,14 @@ impl<'a, K, V> IntoIterator for &'a SsoHashMap<K, V> {
 
     fn into_iter(self) -> Self::IntoIter {
         match self {
-            SsoHashMap::Array(array) => EitherIter::Left(array.into_iter().map(adapt_array_ref_it)),
-            SsoHashMap::Map(map) => EitherIter::Right(map.iter()),
+            SsoHashMap::Array(array) => Either::Left(array.into_iter().map(adapt_array_ref_it)),
+            SsoHashMap::Map(map) => Either::Right(map.iter()),
         }
     }
 }
 
 impl<'a, K, V> IntoIterator for &'a mut SsoHashMap<K, V> {
-    type IntoIter = EitherIter<
+    type IntoIter = Either<
         std::iter::Map<
             <&'a mut ArrayVec<(K, V), 8> as IntoIterator>::IntoIter,
             fn(&'a mut (K, V)) -> (&'a K, &'a mut V),
@@ -464,8 +464,8 @@ impl<'a, K, V> IntoIterator for &'a mut SsoHashMap<K, V> {
 
     fn into_iter(self) -> Self::IntoIter {
         match self {
-            SsoHashMap::Array(array) => EitherIter::Left(array.into_iter().map(adapt_array_mut_it)),
-            SsoHashMap::Map(map) => EitherIter::Right(map.iter_mut()),
+            SsoHashMap::Array(array) => Either::Left(array.into_iter().map(adapt_array_mut_it)),
+            SsoHashMap::Map(map) => Either::Right(map.iter_mut()),
         }
     }
 }
diff --git a/compiler/rustc_data_structures/src/sso/mod.rs b/compiler/rustc_data_structures/src/sso/mod.rs
index dd21bc8e696..ef634b9adce 100644
--- a/compiler/rustc_data_structures/src/sso/mod.rs
+++ b/compiler/rustc_data_structures/src/sso/mod.rs
@@ -1,4 +1,3 @@
-mod either_iter;
 mod map;
 mod set;