diff --git a/Cargo.lock b/Cargo.lock
index b5932607128..b8d9502506e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4576,6 +4576,7 @@ 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..89b8c852649 100644
--- a/compiler/rustc_data_structures/src/sso/map.rs
+++ b/compiler/rustc_data_structures/src/sso/map.rs
@@ -1,24 +1,24 @@
-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;
 
-// For pointer-sized arguments arrays
-// are faster than set/map for up to 64
-// arguments.
-//
-// On the other hand such a big array
-// hurts cache performance, makes passing
-// sso structures around very expensive.
-//
-// Biggest performance benefit is gained
-// for reasonably small arrays that stay
-// small in vast majority of cases.
-//
-// '8' is chosen as a sane default, to be
-// reevaluated later.
+/// For pointer-sized arguments arrays
+/// are faster than set/map for up to 64
+/// arguments.
+///
+/// On the other hand such a big array
+/// hurts cache performance, makes passing
+/// sso structures around very expensive.
+///
+/// Biggest performance benefit is gained
+/// for reasonably small arrays that stay
+/// small in vast majority of cases.
+///
+/// '8' is chosen as a sane default, to be
+/// reevaluated later.
 const SSO_ARRAY_SIZE: usize = 8;
 
 /// Small-storage-optimized implementation of a map.
@@ -138,8 +138,8 @@ pub fn iter_mut(&mut self) -> impl Iterator<Item = (&'_ K, &'_ mut 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 @@ pub fn keys(&self) -> impl Iterator<Item = &'_ K> {
     /// 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 @@ pub fn values(&self) -> impl Iterator<Item = &'_ 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 @@ pub fn values_mut(&mut self) -> impl Iterator<Item = &'_ mut 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()),
         }
     }
 }
@@ -406,16 +406,16 @@ fn extend_reserve(&mut self, additional: usize) {
 }
 
 impl<K, V> IntoIterator for SsoHashMap<K, V> {
-    type IntoIter = EitherIter<
-        <ArrayVec<(K, V), 8> as IntoIterator>::IntoIter,
+    type IntoIter = Either<
+        <ArrayVec<(K, V), SSO_ARRAY_SIZE> as IntoIterator>::IntoIter,
         <FxHashMap<K, V> as IntoIterator>::IntoIter,
     >;
     type Item = <Self::IntoIter as Iterator>::Item;
 
     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()),
         }
     }
 }
@@ -435,9 +435,9 @@ 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,
+            <&'a ArrayVec<(K, V), SSO_ARRAY_SIZE> as IntoIterator>::IntoIter,
             fn(&'a (K, V)) -> (&'a K, &'a V),
         >,
         <&'a FxHashMap<K, V> as IntoIterator>::IntoIter,
@@ -446,16 +446,16 @@ 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,
+            <&'a mut ArrayVec<(K, V), SSO_ARRAY_SIZE> as IntoIterator>::IntoIter,
             fn(&'a mut (K, V)) -> (&'a K, &'a mut V),
         >,
         <&'a mut FxHashMap<K, V> as IntoIterator>::IntoIter,
@@ -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;