add isEmptyStruct func

[kkHAIKE]

related to
#360

[arp242]

The isEmptyStruct() implementation doesn't just check if it's an empty struct, but also if it's an empty array, slice, or map, and the Array implementation is different from isEmpty:

type Struct struct {
	Arr  [3]string `toml:",omitempty"`  // Never empty as len always longer

	// But this behaves different, as it will check the zero value of Arr.
	Nest struct {
		Arr [3]string
	} `toml:",omitempty"`
}

In principle the second implementation is better, but right now it's documented as this for Encoder:

The toml struct tag can be used to provide the key name; if omitted the
struct field name will be used. If the "omitempty" option is present the
following value will be skipped:

  - arrays, slices, maps, and string with len of 0
  - struct with all zero values
  - bool false

I think we can change it without breaking compatibility; now omitempty on an array is basically useless just as it was for structs before the last release. But it should be identical throughout (and the documentation updated, and test added).

I think re-using isEmpty() for slices, maps, and arrays rather doing it in isEmptyStruct() is better, as it avoids implementations differing.

---

Also, I think the tests should be expanded, for example:

type Nest struct {
	Field struct {
		uncomp Uncomparable
	}
}

type Embed struct {
	Uncomparable
}

type EmbedNest struct {
	Field struct {
		Uncomparable
	}
}

type UncompSlice struct {
	[]Uncomparable
}

type UncompSliceNest struct {
	Field struct {
		uncomp []Uncomparable
	}
}

Maybe some more? That's all the cases I could think of right now.

[arp242]

I think changing the array behaviour would be done best in a different PR btw, and encoding/json has the same check:

case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
	return v.Len() == 0

So I'm not sure yet if we should change it.

[arp242]

Discussion about that on encoding/json: golang/go#29310

[kkHAIKE]

'omitempty' from encoding/json not work on struct type, so isEmpty only check array.len==0 is well.

Actually, original toml's behaviour 'reflect.Zero(rv.Type()).Interface() == rv.Interface()' is same as 'rv.IsZero()', because 'reflect.Zero' is make a zero value struct

I think 'isEmptyStruct' behaviour for Array type struct field is right, because 'reflect.Zero' can make a all zero value Array, not len==0 Array.

• struct with all zero values
  change to 'rv.IsZero()' simply ? it work with Uncomparable

[arp242]

Fixed via #366