Custom UnmarshalTOML() decoded keys left in meta.Undecoded()

[mvo5]

First, thanks your amazing library!

It looks like when doing a custom UnmarshalTOML() for a struct this will leave the keys decoded there in meta.Undecoded().

Here is a minimal test case:

type CustomStruct struct {
	Foo string `json:"foo"`
}

func (cs *CustomStruct) UnmarshalTOML(data interface{}) error {
	d, _ := data.(map[string]interface{})
	cs.Foo = d["foo"].(string)
	return nil
}

func TestDecodeCustomStruct(t *testing.T) {
	var cs CustomStruct
	meta, err := Decode(`
		foo = "bar"
	`, &cs)
	if err != nil {
		t.Fatalf("Decode failed: %s", err)
	}

	if cs.Foo != "bar" {
		t.Errorf("\nhave:\n%v\nwant:\n%v\n", cs.Foo, "bar")
	}
	if len(meta.Undecoded()) > 0 {
		t.Errorf("\ncustom decode leaves unencoded fields: %v\n", meta.Undecoded())
	}
}

and the output:

$ go test -run TestDecodeCustomStruct
--- FAIL: TestDecodeCustomStruct (0.00s)
    decode_test.go:1174: 
        custom decode leaves unencoded fields: [foo]

Our use-case is very similar [0], we want to dynamically handle int or string sizes in the toml.

I think we can fix this particular issue via moving decoding to primitive types and not structs
but it would be nice to have a way in UnmarshalTOML() for complex types to signal to the
MetaData object that certain keys are decoded (or maybe this exists and I just couldn't
figure it out)?

[0] https://github.com/osbuild/images/blob/main/pkg/blueprint/filesystem_customizations.go#L27