omitempty ignores types implementing TextMarshaler

[iSchluff]

Describe the bug
Marshalling netip.Addr with omitempty struct tag always returns empty, because the type doesn't have any exported fields.

To Reproduce
Minimal example:

func TestMarshal(t *testing.T) {
	tmp := struct {
		IP netip.Addr `toml:"ip,omitempty"`
	}{
		IP: netip.MustParseAddr("192.168.178.35"),
	}
	res, err := toml.Marshal(&tmp)
	if err != nil {
		t.Fatal(err)
	}
	if string(res) != "ip = '192.168.178.35'\n" {
		t.Fatalf("unexpected result: '%s'", res)
	}
}

Expected behavior
At the most basic I would expect a struct without any exported fields, but with a MarshalText implementation, to always be marshalled, just to be on the safe side.

Ideally there should be a way to instruct a custom zero behaviour, e.g. netip.Ip already exposes an IsZero() bool call, so possibly this could be used.

Versions

• go-toml: 2.2.2
• go: 1.22.4
• operating system: Linux

[pelletier]

Thanks for reporting this bug! Runnable example: https://go.dev/play/p/dUguDjMvVwe

I think you're right that at the very least, an type implementing MarshalText should always be marshaled, even with omitempty, as the goal is to mimic the behavior of encoding/json.

For skipping a value completely, I think we providing a toml.Marshaler interface to implement would be the right way to go.