I worry this could result in an accidental omission. How about return nil, json.OmitFieldErr instead?

What is the state for this, would a PR for this be accepted?

I really like the return nil, json.OmitFieldErr idea, though it should be either ErrOmitField to match conventions or strip the Err completely - maybe json.OmitFieldHint or something?

I just saw this comment is created from one of the ideas #11939, explicitly for return nil, nil and return nil, json.$someErr is one of the other proposals in that original issue.

@mitar what happens if MarshalJSON returns nil without omitempty.

type Nil struct{}

func (Nil) MarshalJSON() ([]byte, error) {
	return nil, nil
}

func main() {
	data, _ := json.Marshal(Nil{})
	fmt.Println(string(data)) // what's the expected output?
}

@mitar I implemented your idea here https://github.com/icholy/json if you want to play with it https://go.dev/play/p/7Cf6BgrNGpP

edit: I've also implemented the err_empty branch which requires the error return. Ex:

type MarshalNil struct{}

func (MarshalNil) MarshalJSON() ([]byte, error) {
	return []byte("null"), json.ErrEmpty
}

This type will marshal to null unless it's a struct field with omitempty set.

Regarding #50480 (comment)

Wouldn't nil, nil yield the invalid JSON of empty string?

This proposal is a duplicate of a previously discussed proposal, as noted above,
and there is no significant new information to justify reopening the discussion.
The issue has therefore been declined as a duplicate.
— rsc for the proposal review group

Sorry, where exactly has this already been discussed? So I found a mention in one long issue (on omitempty and structs), a much broader issue, where this was proposed but not acted upon, while this issue itself tries to be focused on exactly one proposal. So which discussion is this reopening? This is just trying to focus the discussion which have not happened. (And this issue has also 14 upvotes.)

@mitar #45669 is the one that's been put on the docket for this.

There are lots of different proposals to address the same underlying problem, but it's not clear yet which is the best approach to take. Hopefully discussion there will address it.

Hm, I saw that one, but I disagree that this one would be addressed with omitzero. But the opposite holds true: implementing this proposal would also provide a way to solve the issue which omitzero attempts to address. But omitzero (nor omitempty) do not influence JSON marshaling when struct implements MarshalJSON. And this issue is about that. If a struct has MarshalJSON, then you cannot omit it anymore. omitzero will not fix this.