Tag omitzero respects IsZero()

[lzap]

This patch adds support for IsZero method semantics introduced in Go 1.24 for JSON. When a type has this method implemented as func (T) IsZero() bool and it returns true and TOML Go struct omitzero exists, the library will omit it completely.

This introduces a change, Go built-in type time.Time already has this method and previously when an empty time was marshaled it created TOML time = 0001-01-01T00:00:00Z while now this value is skipped. But this will be only done for those fields which are tagged with omitzero which is not an issue, no?

Solves: #443

Note this feature does not need Go 1.24, it only follows what Go team is doing for JSON but this will work with any Go version supported by toml.

[arp242]

previously when an empty time was marshaled it created TOML time = 0001-01-01T00:00:00Z

This should not be the case; unlike e.g. encoding/json it should skip empty structs with the omitempty tag; for example:

func main() {
	type s struct {
		I int       `toml:"i"`
		T time.Time `toml:"t,omitempty"`
	}

	{
		t, err := toml.Marshal(s{1, time.Now()})
		if err != nil {
			panic(err)
		}
		fmt.Printf("%q\n", string(t))
	}
	{
		t, err := toml.Marshal(s{2, time.Time{}})
		if err != nil {
			panic(err)
		}
		fmt.Printf("%q\n", string(t))
	}
}

Outputs:

"i = 1\nt = 2025-06-04T22:45:18.68785576+01:00\n"
"i = 2\n"

---

The entire situation is a bit unfortunate, as omitempty/omitzero tags have somewhat different meanings than in stdlib (this library goes back to before Go 1.0). I'm not sure what the best solution is 🤔

[lzap]

Oh so is my understanding that my patch is completely useless and the library already does this? Wow, this is not the first time this happened to me. I can close then.

[arp242]

Not entirely; ideally I would like this to behave identical to stdlib. I'm just not entirely sure what the best path for that is 🤔 I'd like to avoid a v2 if at all possible.

Just saying that you shouldn't need this patch for your particular issue.

[lzap]

Okay let me know if you want me to rewrite the patch.