Add ErrorUnset option to DecoderConfig and Unset array to Metddata

mapstructure.go

@@ -200,6 +200,11 @@ type DecoderConfig struct {
 	// (extra keys).
 	ErrorUnused bool
 
+	// If ErrorUnset is true, then it is an error for there to exist
+	// fields in the result that were not set in the decoding process
+	// (extra fields).
+	ErrorUnset bool
+
 	// ZeroFields, if set to true, will zero fields before writing them.
 	// For example, a map will be emptied before decoded values are put in
 	// it. If this is false, a map will be merged.
@@ -264,6 +269,11 @@ type Metadata struct {
 	// Unused is a slice of keys that were found in the raw value but
 	// weren't decoded since there was no matching field in the result interface
 	Unused []string
+
+	// Unset is a slice of field names that were found in the result interface
+	// but weren't set in the decoding process since there was no matching value
+	// in the input
+	Unset []string
 }
 
 // Decode takes an input structure and uses reflection to translate it to
@@ -355,6 +365,10 @@ func NewDecoder(config *DecoderConfig) (*Decoder, error) {
 		if config.Metadata.Unused == nil {
 			config.Metadata.Unused = make([]string, 0)
 		}
+
+		if config.Metadata.Unset == nil {
+			config.Metadata.Unset = make([]string, 0)
+		}
 	}
 
 	if config.TagName == "" {
@@ -1225,6 +1239,7 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 		dataValKeysUnused[dataValKey.Interface()] = struct{}{}
 	}
 
+	targetValKeysUnused := make(map[interface{}]struct{})
 	errors := make([]string, 0)
 
 	// This slice will keep track of all the structs we'll be decoding.
@@ -1329,7 +1344,8 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 
 			if !rawMapVal.IsValid() {
 				// There was no matching key in the map for the value in
-				// the struct. Just ignore.
+				// the struct. Remember it for potential errors and metadata.
+				targetValKeysUnused[fieldName] = struct{}{}
 				continue
 			}
 		}
@@ -1389,6 +1405,17 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 		errors = appendErrors(errors, err)
 	}
 
+	if d.config.ErrorUnset && len(targetValKeysUnused) > 0 {
+		keys := make([]string, 0, len(targetValKeysUnused))
+		for rawKey := range targetValKeysUnused {
+			keys = append(keys, rawKey.(string))
+		}
+		sort.Strings(keys)
+
+		err := fmt.Errorf("'%s' has unset fields: %s", name, strings.Join(keys, ", "))
+		errors = appendErrors(errors, err)
+	}
+
 	if len(errors) > 0 {
 		return &Error{errors}
 	}
@@ -1403,6 +1430,14 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 
 			d.config.Metadata.Unused = append(d.config.Metadata.Unused, key)
 		}
+		for rawKey := range targetValKeysUnused {
+			key := rawKey.(string)
+			if name != "" {
+				key = name + "." + key
+			}
+
+			d.config.Metadata.Unset = append(d.config.Metadata.Unset, key)
+		}
 	}
 
 	return nil

mapstructure_test.go

@@ -1225,6 +1225,30 @@ func TestDecoder_ErrorUnused_NotSetable(t *testing.T) {
 		t.Fatal("expected error")
 	}
 }
+func TestDecoder_ErrorUnset(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"vstring": "hello",
+		"foo":     "bar",
+	}
+
+	var result Basic
+	config := &DecoderConfig{
+		ErrorUnset: true,
+		Result:     &result,
+	}
+
+	decoder, err := NewDecoder(config)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	err = decoder.Decode(input)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+}
 
 func TestMap(t *testing.T) {
 	t.Parallel()
@@ -2192,6 +2216,14 @@ func TestMetadata(t *testing.T) {
 	if !reflect.DeepEqual(md.Unused, expectedUnused) {
 		t.Fatalf("bad unused: %#v", md.Unused)
 	}
+
+	expectedUnset := []string{
+		"Vbar.Vbool", "Vbar.Vdata", "Vbar.Vextra", "Vbar.Vfloat", "Vbar.Vint",
+		"Vbar.VjsonFloat", "Vbar.VjsonInt", "Vbar.VjsonNumber"}
+	sort.Strings(md.Unset)
+	if !reflect.DeepEqual(md.Unset, expectedUnset) {
+		t.Fatalf("bad unset: %#v", md.Unset)
+	}
 }
 
 func TestMetadata_Embedded(t *testing.T) {