Add "clearomitted" directive

[klauspost]

Adds //msgp:clearomitted directive.

This will cause all omitempty and omitzero fields to be set to the zero value on Unmarshal and Decode, and the field wasn't present in the marshalled data.

This can be useful when de-serializing into reused objects, that can have values set for these, and we want to avoid clearing all fields, but also don't want existing fields to leak through.

Fields are tracked through a bit mask, and zeroed after the unmarshal loop, if no value has been written.

This does not affect marshaling.

DecodeMsg Example

// DecodeMsg implements msgp.Decodable
func (z *NamedStructCO) DecodeMsg(dc *msgp.Reader) (err error) {
	var field []byte
	_ = field
	var zb0001 uint32
	zb0001, err = dc.ReadMapHeader()
	if err != nil {
		err = msgp.WrapError(err)
		return
	}
	var zb0001Mask uint8 /* 2 bits */
	_ = zb0001Mask
	for zb0001 > 0 {
		zb0001--
		field, err = dc.ReadMapKeyPtr()
		if err != nil {
			err = msgp.WrapError(err)
			return
		}
		switch msgp.UnsafeString(field) {
		case "a":
			z.A, err = dc.ReadString()
			if err != nil {
				err = msgp.WrapError(err, "A")
				return
			}
			zb0001Mask |= 0x1
		case "b":
			z.B, err = dc.ReadString()
			if err != nil {
				err = msgp.WrapError(err, "B")
				return
			}
			zb0001Mask |= 0x2
		default:
			err = dc.Skip()
			if err != nil {
				err = msgp.WrapError(err)
				return
			}
		}
	}
	// Clear omitted fields.
	if zb0001Mask != 0x3 {
		if (zb0001Mask & 0x1) == 0 {
			z.A = ""
		}
		if (zb0001Mask & 0x2) == 0 {
			z.B = ""
		}
	}
	return
}

UnmarshalMsg Example

// UnmarshalMsg implements msgp.Unmarshaler
func (z *NamedStructCO) UnmarshalMsg(bts []byte) (o []byte, err error) {
	var field []byte
	_ = field
	var zb0001 uint32
	zb0001, bts, err = msgp.ReadMapHeaderBytes(bts)
	if err != nil {
		err = msgp.WrapError(err)
		return
	}
	var zb0001Mask uint8 /* 2 bits */
	_ = zb0001Mask
	for zb0001 > 0 {
		zb0001--
		field, bts, err = msgp.ReadMapKeyZC(bts)
		if err != nil {
			err = msgp.WrapError(err)
			return
		}
		switch msgp.UnsafeString(field) {
		case "a":
			z.A, bts, err = msgp.ReadStringBytes(bts)
			if err != nil {
				err = msgp.WrapError(err, "A")
				return
			}
			zb0001Mask |= 0x1
		case "b":
			z.B, bts, err = msgp.ReadStringBytes(bts)
			if err != nil {
				err = msgp.WrapError(err, "B")
				return
			}
			zb0001Mask |= 0x2
		default:
			bts, err = msgp.Skip(bts)
			if err != nil {
				err = msgp.WrapError(err)
				return
			}
		}
	}
	// Clear omitted fields.
	if zb0001Mask != 0x3 {
		if (zb0001Mask & 0x1) == 0 {
			z.A = ""
		}
		if (zb0001Mask & 0x2) == 0 {
			z.B = ""
		}
	}
	o = bts
	return
}

[klauspost]

Merging, but not publishing a release.