encoding/json: optimizing memory allocation of json.RawMessage

[hafus]

What version of Go are you using (go version)?

$ go version
All versions

Does this issue reproduce with the latest release?

Yes, It does

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
All OS/CPU arch

What did you do?

This is the first time I open an issue in Golang repo and I don't know if it's under the right category. I would like to propose an optimization to json.RawMessage struct to get rid of an extract memory allocation on call of UnmarshalJSON func of json.RawMessage struct as stated below:

Current

func (m *RawMessage) UnmarshalJSON(data []byte) error {
	if m == nil {
		return errors.New("json.RawMessage: UnmarshalJSON on nil pointer")
	}
	*m = append((*m)[0:0], data...)
	return nil
}

Proposal

func (m *RawMessage) UnmarshalJSON(data []byte) error {
	if m == nil {
		return errors.New("json.RawMessage: UnmarshalJSON on nil pointer")
	}

	*m = data
	return nil
}

To verify the above optimization, I run below benchmark.

type msg struct {
	RawData json.RawMessage
}

var raw = []byte(`{"RawData":{"name":"test", "age":22}}`)

func BenchmarkRawMsg(b *testing.B) {

	for i := 0; i < b.N; i++ {
		var m msg
		json.Unmarshal(raw, &m)
	}

}

What did you expect to see?

BenchmarkRawMsg-16 1476358 797.3 ns/op 256 B/op 6 allocs/op

What did you see instead?

BenchmarkRawMsg-16 1467776 842.1 ns/op 288 B/op 7 allocs/op

[mateusz834]

https://pkg.go.dev/encoding/json#Unmarshaler

UnmarshalJSON must copy the JSON data if it wishes to retain the data after returning.

[seankhliao]

see above, working as intended