encoding/binary: add word size to the error message of the failed constraint

Test added.

Fixes #22860

Change-Id: I08304834a2b7b10b4ac729bf36761692eb4731da
Reviewed-on: https://go-review.googlesource.com/c/go/+/113075
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/encoding/binary/binary.go

@@ -451,7 +451,7 @@ func Write(w io.Writer, order ByteOrder, data any) error {
 	v := reflect.Indirect(reflect.ValueOf(data))
 	size := dataSize(v)
 	if size < 0 {
-		return errors.New("binary.Write: invalid type " + reflect.TypeOf(data).String())
+		return errors.New("binary.Write: some values are not fixed-sized in type " + reflect.TypeOf(data).String())
 	}
 	buf := make([]byte, size)
 	e := &encoder{order: order, buf: buf}

src/encoding/binary/binary_test.go

@@ -540,6 +540,30 @@ func testReadInvalidDestination(t *testing.T, order ByteOrder) {
 	}
 }
 
+func TestNoFixedSize(t *testing.T) {
+	type Person struct {
+		Age    int
+		Weight float64
+		Height float64
+	}
+
+	person := Person{
+		Age:    27,
+		Weight: 67.3,
+		Height: 177.8,
+	}
+
+	buf := new(bytes.Buffer)
+	err := Write(buf, LittleEndian, &person)
+	if err == nil {
+		t.Fatal("binary.Write: unexpected success as size of type *binary.Person is not fixed")
+	}
+	errs := "binary.Write: some values are not fixed-sized in type *binary.Person"
+	if err.Error() != errs {
+		t.Fatalf("got %q, want %q", err, errs)
+	}
+}
+
 type byteSliceReader struct {
 	remain []byte
 }