io: add test for Pipe constructor allocations

Updates #67633

Change-Id: If3da9317ba36cb8a7868db94b45c402e1793e018
Reviewed-on: https://go-review.googlesource.com/c/go/+/588219
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/io/pipe_test.go

@@ -421,3 +421,21 @@ func sortBytesInGroups(b []byte, n int) []byte {
 	slices.SortFunc(groups, bytes.Compare)
 	return bytes.Join(groups, nil)
 }
+
+var (
+	rSink *PipeReader
+	wSink *PipeWriter
+)
+
+func TestPipeAllocations(t *testing.T) {
+	numAllocs := testing.AllocsPerRun(10, func() {
+		rSink, wSink = Pipe()
+	})
+
+	// go.dev/cl/473535 claimed Pipe() should only do 2 allocations,
+	// plus the 2 escaping to heap for simulating real world usages.
+	expectedAllocs := 4
+	if int(numAllocs) > expectedAllocs {
+		t.Fatalf("too many allocations for io.Pipe() call: %f", numAllocs)
+	}
+}