diff --git a/internal/persistent/set.go b/internal/persistent/set.go
new file mode 100644
index 00000000000..348de5a71d2
--- /dev/null
+++ b/internal/persistent/set.go
@@ -0,0 +1,78 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package persistent
+
+import "golang.org/x/tools/internal/constraints"
+
+// Set is a collection of elements of type K.
+//
+// It uses immutable data structures internally, so that sets can be cloned in
+// constant time.
+//
+// The zero value is a valid empty set.
+type Set[K constraints.Ordered] struct {
+	impl *Map[K, struct{}]
+}
+
+// Clone creates a copy of the receiver.
+func (s *Set[K]) Clone() *Set[K] {
+	clone := new(Set[K])
+	if s.impl != nil {
+		clone.impl = s.impl.Clone()
+	}
+	return clone
+}
+
+// Destroy destroys the set.
+//
+// After Destroy, the Set should not be used again.
+func (s *Set[K]) Destroy() {
+	if s.impl != nil {
+		s.impl.Destroy()
+	}
+}
+
+// Contains reports whether s contains the given key.
+func (s *Set[K]) Contains(key K) bool {
+	if s.impl == nil {
+		return false
+	}
+	_, ok := s.impl.Get(key)
+	return ok
+}
+
+// Range calls f sequentially in ascending key order for all entries in the set.
+func (s *Set[K]) Range(f func(key K)) {
+	if s.impl != nil {
+		s.impl.Range(func(key K, _ struct{}) {
+			f(key)
+		})
+	}
+}
+
+// AddAll adds all elements from other to the receiver set.
+func (s *Set[K]) AddAll(other *Set[K]) {
+	if other.impl != nil {
+		if s.impl == nil {
+			s.impl = new(Map[K, struct{}])
+		}
+		s.impl.SetAll(other.impl)
+	}
+}
+
+// Add adds an element to the set.
+func (s *Set[K]) Add(key K) {
+	if s.impl == nil {
+		s.impl = new(Map[K, struct{}])
+	}
+	s.impl.Set(key, struct{}{}, nil)
+}
+
+// Remove removes an element from the set.
+func (s *Set[K]) Remove(key K) {
+	if s.impl != nil {
+		s.impl.Delete(key)
+	}
+}
diff --git a/internal/persistent/set_test.go b/internal/persistent/set_test.go
new file mode 100644
index 00000000000..59025140bce
--- /dev/null
+++ b/internal/persistent/set_test.go
@@ -0,0 +1,132 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package persistent_test
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"golang.org/x/tools/internal/constraints"
+	"golang.org/x/tools/internal/persistent"
+)
+
+func TestSet(t *testing.T) {
+	const (
+		add = iota
+		remove
+	)
+	type op struct {
+		op int
+		v  int
+	}
+
+	tests := []struct {
+		label string
+		ops   []op
+		want  []int
+	}{
+		{"empty", nil, nil},
+		{"singleton", []op{{add, 1}}, []int{1}},
+		{"add and remove", []op{
+			{add, 1},
+			{remove, 1},
+		}, nil},
+		{"interleaved and remove", []op{
+			{add, 1},
+			{add, 2},
+			{remove, 1},
+			{add, 3},
+		}, []int{2, 3}},
+	}
+
+	for _, test := range tests {
+		t.Run(test.label, func(t *testing.T) {
+			var s persistent.Set[int]
+			for _, op := range test.ops {
+				switch op.op {
+				case add:
+					s.Add(op.v)
+				case remove:
+					s.Remove(op.v)
+				}
+			}
+
+			if d := diff(&s, test.want); d != "" {
+				t.Errorf("unexpected diff:\n%s", d)
+			}
+		})
+	}
+}
+
+func TestSet_Clone(t *testing.T) {
+	s1 := new(persistent.Set[int])
+	s1.Add(1)
+	s1.Add(2)
+	s2 := s1.Clone()
+	s1.Add(3)
+	s2.Add(4)
+	if d := diff(s1, []int{1, 2, 3}); d != "" {
+		t.Errorf("s1: unexpected diff:\n%s", d)
+	}
+	if d := diff(s2, []int{1, 2, 4}); d != "" {
+		t.Errorf("s2: unexpected diff:\n%s", d)
+	}
+}
+
+func TestSet_AddAll(t *testing.T) {
+	s1 := new(persistent.Set[int])
+	s1.Add(1)
+	s1.Add(2)
+	s2 := new(persistent.Set[int])
+	s2.Add(2)
+	s2.Add(3)
+	s2.Add(4)
+	s3 := new(persistent.Set[int])
+
+	s := new(persistent.Set[int])
+	s.AddAll(s1)
+	s.AddAll(s2)
+	s.AddAll(s3)
+
+	if d := diff(s1, []int{1, 2}); d != "" {
+		t.Errorf("s1: unexpected diff:\n%s", d)
+	}
+	if d := diff(s2, []int{2, 3, 4}); d != "" {
+		t.Errorf("s2: unexpected diff:\n%s", d)
+	}
+	if d := diff(s3, nil); d != "" {
+		t.Errorf("s3: unexpected diff:\n%s", d)
+	}
+	if d := diff(s, []int{1, 2, 3, 4}); d != "" {
+		t.Errorf("s: unexpected diff:\n%s", d)
+	}
+}
+
+func diff[K constraints.Ordered](got *persistent.Set[K], want []K) string {
+	wantSet := make(map[K]struct{})
+	for _, w := range want {
+		wantSet[w] = struct{}{}
+	}
+	var diff []string
+	got.Range(func(key K) {
+		if _, ok := wantSet[key]; !ok {
+			diff = append(diff, fmt.Sprintf("+%v", key))
+		}
+	})
+	for key := range wantSet {
+		if !got.Contains(key) {
+			diff = append(diff, fmt.Sprintf("-%v", key))
+		}
+	}
+	if len(diff) > 0 {
+		d := new(strings.Builder)
+		for _, l := range diff {
+			fmt.Fprintln(d, l)
+		}
+		return d.String()
+	}
+	return ""
+}