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diff --git a/compiler/rustc_data_structures/src/sorted_map/index_map.rs b/compiler/rustc_data_structures/src/sorted_map/index_map.rs
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+//! A variant of `SortedMap` that preserves insertion order.
+
+use std::hash::{Hash, Hasher};
+use std::iter::FromIterator;
+
+use crate::stable_hasher::{HashStable, StableHasher};
+use rustc_index::vec::{Idx, IndexVec};
+
+/// An indexed multi-map that preserves insertion order while permitting both *O*(log *n*) lookup of
+/// an item by key and *O*(1) lookup by index.
+///
+/// This data structure is a hybrid of an [`IndexVec`] and a [`SortedMap`]. Like `IndexVec`,
+/// `SortedIndexMultiMap` assigns a typed index to each item while preserving insertion order.
+/// Like `SortedMap`, `SortedIndexMultiMap` has efficient lookup of items by key. However, this
+/// is accomplished by sorting an array of item indices instead of the items themselves.
+///
+/// Unlike `SortedMap`, this data structure can hold multiple equivalent items at once, so the
+/// `get_by_key` method and its variants return an iterator instead of an `Option`. Equivalent
+/// items will be yielded in insertion order.
+///
+/// Unlike a general-purpose map like `BTreeSet` or `HashSet`, `SortedMap` and
+/// `SortedIndexMultiMap` require *O*(*n*) time to insert a single item. This is because we may need
+/// to insert into the middle of the sorted array. Users should avoid mutating this data structure
+/// in-place.
+///
+/// [`SortedMap`]: super::SortedMap
+#[derive(Clone, Debug)]
+pub struct SortedIndexMultiMap<I: Idx, K, V> {
+    /// The elements of the map in insertion order.
+    items: IndexVec<I, (K, V)>,
+
+    /// Indices of the items in the set, sorted by the item's key.
+    idx_sorted_by_item_key: Vec<I>,
+}
+
+impl<I: Idx, K: Ord, V> SortedIndexMultiMap<I, K, V> {
+    #[inline]
+    pub fn new() -> Self {
+        SortedIndexMultiMap { items: IndexVec::new(), idx_sorted_by_item_key: Vec::new() }
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.items.len()
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.items.is_empty()
+    }
+
+    /// Returns an iterator over the items in the map in insertion order.
+    #[inline]
+    pub fn into_iter(self) -> impl DoubleEndedIterator<Item = (K, V)> {
+        self.items.into_iter()
+    }
+
+    /// Returns an iterator over the items in the map in insertion order along with their indices.
+    #[inline]
+    pub fn into_iter_enumerated(self) -> impl DoubleEndedIterator<Item = (I, (K, V))> {
+        self.items.into_iter_enumerated()
+    }
+
+    /// Returns an iterator over the items in the map in insertion order.
+    #[inline]
+    pub fn iter(&self) -> impl '_ + DoubleEndedIterator<Item = (&K, &V)> {
+        self.items.iter().map(|(ref k, ref v)| (k, v))
+    }
+
+    /// Returns an iterator over the items in the map in insertion order along with their indices.
+    #[inline]
+    pub fn iter_enumerated(&self) -> impl '_ + DoubleEndedIterator<Item = (I, (&K, &V))> {
+        self.items.iter_enumerated().map(|(i, (ref k, ref v))| (i, (k, v)))
+    }
+
+    /// Returns the item in the map with the given index.
+    #[inline]
+    pub fn get(&self, idx: I) -> Option<&(K, V)> {
+        self.items.get(idx)
+    }
+
+    /// Returns an iterator over the items in the map that are equal to `key`.
+    ///
+    /// If there are multiple items that are equivalent to `key`, they will be yielded in
+    /// insertion order.
+    #[inline]
+    pub fn get_by_key(&self, key: K) -> impl Iterator<Item = &V> + '_ {
+        self.get_by_key_enumerated(key).map(|(_, v)| v)
+    }
+
+    /// Returns an iterator over the items in the map that are equal to `key` along with their
+    /// indices.
+    ///
+    /// If there are multiple items that are equivalent to `key`, they will be yielded in
+    /// insertion order.
+    #[inline]
+    pub fn get_by_key_enumerated(&self, key: K) -> impl Iterator<Item = (I, &V)> + '_ {
+        let lower_bound = self.idx_sorted_by_item_key.partition_point(|&i| self.items[i].0 < key);
+        self.idx_sorted_by_item_key[lower_bound..].iter().map_while(move |&i| {
+            let (k, v) = &self.items[i];
+            (k == &key).then_some((i, v))
+        })
+    }
+}
+
+impl<I: Idx, K: Eq, V: Eq> Eq for SortedIndexMultiMap<I, K, V> {}
+impl<I: Idx, K: PartialEq, V: PartialEq> PartialEq for SortedIndexMultiMap<I, K, V> {
+    fn eq(&self, other: &Self) -> bool {
+        // No need to compare the sorted index. If the items are the same, the index will be too.
+        self.items == other.items
+    }
+}
+
+impl<I: Idx, K, V> Hash for SortedIndexMultiMap<I, K, V>
+where
+    K: Hash,
+    V: Hash,
+{
+    fn hash<H: Hasher>(&self, hasher: &mut H) {
+        self.items.hash(hasher)
+    }
+}
+impl<I: Idx, K, V, C> HashStable<C> for SortedIndexMultiMap<I, K, V>
+where
+    K: HashStable<C>,
+    V: HashStable<C>,
+{
+    fn hash_stable(&self, ctx: &mut C, hasher: &mut StableHasher) {
+        self.items.hash_stable(ctx, hasher)
+    }
+}
+
+impl<I: Idx, K: Ord, V> FromIterator<(K, V)> for SortedIndexMultiMap<I, K, V> {
+    fn from_iter<J>(iter: J) -> Self
+    where
+        J: IntoIterator<Item = (K, V)>,
+    {
+        let items = IndexVec::from_iter(iter);
+        let mut idx_sorted_by_item_key: Vec<_> = items.indices().collect();
+
+        // `sort_by_key` is stable, so insertion order is preserved for duplicate items.
+        idx_sorted_by_item_key.sort_by_key(|&idx| &items[idx].0);
+
+        SortedIndexMultiMap { items, idx_sorted_by_item_key }
+    }
+}
+
+impl<I: Idx, K, V> std::ops::Index<I> for SortedIndexMultiMap<I, K, V> {
+    type Output = V;
+
+    fn index(&self, idx: I) -> &Self::Output {
+        &self.items[idx].1
+    }
+}