Currently, when the ErrorUnset option is enabled and the root structure is missing some keys, the error messages include empty quotes, which can be confusing.

There was some related work in #113 to improve error messages for unexpected keys (with ErrorUnused), but the current behavior is inconsistent:

• For missing keys, the root name is displayed as empty quotes.
• For unexpected keys, the root object type name is shown.
• This also becomes awkward with anonymous struct types.

Proposal:

Introduce an additional option to manually specify a root object name.

I’m not entirely sure whether the logic introduced in #113 should remain. If we don’t consider error messages part of the API contract, it might make sense to simplify or remove it (though I’d defer to @jmacd and others on this point). If we want to keep the existing logic, then perhaps we should extend it to missing keys as well for consistency - though that would also change the error messages.

Example:

input := map[string]any{
	"surname": "green",
	"relation": map[string]any{
		"surname": "black",
	},
}

var result struct {
	Name     string `mapstructure:"name"`
	Relation struct {
		Name string `mapstructure:"name"`
	} `mapstructure:"relation"`
}

decoder, _ := NewDecoder(&DecoderConfig{
	ErrorUnset:  true,
	ErrorUnused: true,
	Result:      &result,
})

fmt.Println(decoder.Decode(input))

Actual output:

decoding failed due to the following error(s):

'relation' has invalid keys: surname
'relation' has unset fields: name
'struct { Name string "mapstructure:\"name\""; Relation struct { Name string "mapstructure:\"name\"" } "mapstructure:\"relation\"" }' has invalid keys: surname
'' has unset fields: name

Suggested option:

decoder, _ := NewDecoder(&DecoderConfig{
	ErrorUnset:  true,
	ErrorUnused: true,
	Result:      &result,
	RootName:    "root", // here
})

Desired output:

decoding failed due to the following error(s):

'root.relation' has invalid keys: surname
'root.relation' has unset fields: name
'root' has invalid keys: surname
'root' has unset fields: name