Add decodeStructFromMap for map field name.

Author: Jimmy Spagnola

mapstructure.go

@@ -1593,6 +1593,8 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 		tagValue = strings.SplitN(tagValue, ",", 2)[0]
 		if tagValue != "" {
 			fieldName = tagValue
+		} else {
+			fieldName = d.config.MapFieldName(fieldName)
 		}
 
 		rawMapKey := reflect.ValueOf(fieldName)

mapstructure_test.go

@@ -3687,7 +3687,7 @@ type TestMapFieldName struct {
 	Username string
 }
 
-func TestDecoder_MapFieldName(t *testing.T) {
+func TestDecoder_MapFieldNameMapFromStruct(t *testing.T) {
 	var structKeys map[string]any
 
 	decoder, err := NewDecoder(&DecoderConfig{
@@ -3721,3 +3721,41 @@ func TestDecoder_MapFieldName(t *testing.T) {
 		t.Fatal("expected Username to exist")
 	}
 }
+
+func TestDecoder_MapFieldNameStructFromMap(t *testing.T) {
+	foo := TestMapFieldName{}
+
+	decoder, err := NewDecoder(&DecoderConfig{
+		Result: &foo,
+		MapFieldName: func(s string) string {
+			if s == "HostName" {
+				return "host_name"
+			}
+			if s == "Username" {
+				return "user_name"
+			}
+			return s
+		},
+	})
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	structKeys := map[string]any{
+		"host_name": "foo",
+		"user_name": "bar",
+	}
+
+	err = decoder.Decode(&structKeys)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if foo.HostName != "foo" {
+		t.Fatal("expected HostName to be foo")
+	}
+
+	if foo.Username != "bar" {
+		t.Fatal("expected Username to be bar")
+	}
+}