unique: handle zero-size types

Fixes #69458

Change-Id: Ic7fda7f556522780b2819138dfc1277137398692
Reviewed-on: https://go-review.googlesource.com/c/go/+/613397
Reviewed-by: Carlos Amedee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Cuong Manh Le <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

src/unique/handle.go

@@ -10,9 +10,11 @@ import (
 	"internal/weak"
 	"runtime"
 	"sync"
-	_ "unsafe"
+	"unsafe"
 )
 
+var zero uintptr
+
 // Handle is a globally unique identity for some value of type T.
 //
 // Two handles compare equal exactly if the two values used to create the handles
@@ -32,6 +34,9 @@ func (h Handle[T]) Value() T {
 func Make[T comparable](value T) Handle[T] {
 	// Find the map for type T.
 	typ := abi.TypeFor[T]()
+	if typ.Size() == 0 {
+		return Handle[T]{(*T)(unsafe.Pointer(&zero))}
+	}
 	ma, ok := uniqueMaps.Load(typ)
 	if !ok {
 		// This is a good time to initialize cleanup, since we must go through

src/unique/handle_test.go

@@ -31,6 +31,7 @@ type testStruct struct {
 	z float64
 	b string
 }
+type testZeroSize struct{}
 
 func TestHandle(t *testing.T) {
 	testHandle(t, testString("foo"))
@@ -45,6 +46,7 @@ func TestHandle(t *testing.T) {
 	})
 	testHandle(t, testStruct{0.5, "184"})
 	testHandle(t, testEface("hello"))
+	testHandle(t, testZeroSize(struct{}{}))
 }
 
 func testHandle[T comparable](t *testing.T, value T) {
@@ -65,15 +67,19 @@ func testHandle[T comparable](t *testing.T, value T) {
 			t.Error("v0 != v1")
 		}
 
-		drainMaps(t)
+		drainMaps[T](t)
 		checkMapsFor(t, value)
 	})
 }
 
 // drainMaps ensures that the internal maps are drained.
-func drainMaps(t *testing.T) {
+func drainMaps[T comparable](t *testing.T) {
 	t.Helper()
 
+	if unsafe.Sizeof(*(new(T))) == 0 {
+		return // zero-size types are not inserted.
+	}
+
 	wait := make(chan struct{}, 1)
 
 	// Set up a one-time notification for the next time the cleanup runs.