Mention all missed but required fields in the error (#143)

aconfig.go

@@ -261,15 +261,22 @@ func (l *Loader) loadSources() error {
 }
 
 func (l *Loader) checkRequired() error {
+	missedFields := []string{}
 	for _, field := range l.fields {
 		if field.isSet {
 			continue
 		}
 		if field.isRequired || l.config.AllFieldRequired {
-			return fmt.Errorf("field %s is required but not set", field.name)
+			missedFields = append(missedFields, field.name)
 		}
 	}
-	return nil
+	if len(missedFields) == 0 {
+		return nil
+	}
+	if len(missedFields) == 1 {
+		return fmt.Errorf("field %s is required but not set", missedFields[0])
+	}
+	return fmt.Errorf("fields %s are required but not set", strings.Join(missedFields, ","))
 }
 
 func (l *Loader) loadDefaults() error {

aconfig_test.go

@@ -1087,6 +1087,38 @@ func TestBadRequiredTag(t *testing.T) {
 	f(&TestConfig{})
 }
 
+func TestMissingFieldWithRequiredTag(t *testing.T) {
+	cfg := struct {
+		Field1 string `required:"true"`
+	}{}
+	loader := LoaderFor(&cfg, Config{
+		SkipFlags: true,
+	})
+
+	err := loader.Load()
+
+	want := "load config: field Field1 is required but not set"
+	if err.Error() != want {
+		t.Fatalf("got %v, want %v", err, want)
+	}
+}
+func TestMissingFieldsWithRequiredTag(t *testing.T) {
+	cfg := struct {
+		Field1 string `required:"true"`
+		Field2 string `required:"true"`
+	}{}
+	loader := LoaderFor(&cfg, Config{
+		SkipFlags: true,
+	})
+
+	err := loader.Load()
+
+	want := "load config: fields Field1,Field2 are required but not set"
+	if err.Error() != want {
+		t.Fatalf("got %v, want %v", err, want)
+	}
+}
+
 func int32Ptr(a int32) *int32 {
 	return &a
 }