Using decoder functions for unmanaged struct fields

envconfig.go

@@ -335,7 +335,11 @@ func processWith(ctx context.Context, i interface{}, l Lookuper, parentNoInit bo
 				return fmt.Errorf("%s: %w", tf.Name, err)
 			}
 
-			if found || usedDefault {
+			// When the env tag is not present on a struct field, the field is considered
+			// unmanaged and initialization should always occur through the decoder.
+			// Managed fields that have the env tag present will only use the decoder
+			// functions when a variable is set or there is a default.
+			if found || usedDefault || tag == "" {
 				if ok, err := processAsDecoder(val, ef); ok {
 					if err != nil {
 						return err

envconfig_test.go

@@ -98,6 +98,31 @@ func (c *CustomStdLibDecodingType) GobDecode(data []byte) error {
 	return nil
 }
 
+var (
+	_ encoding.TextUnmarshaler   = (*TextUnmarshalerType)(nil)
+)
+
+// TextUnmarshalerType demonstrates a type that contains a struct pointer with
+// no managed env tag. Nested types with no env tag should always defer to the
+// decoder for unmarshalling.
+type TextUnmarshalerType struct {
+	ExportedNameWrapper *NameWrapper
+}
+
+func (t *TextUnmarshalerType) UnmarshalText(text []byte) error {
+	if len(string(text)) > 0 {
+		t.ExportedNameWrapper = &NameWrapper{
+			name: string(text),
+		}
+	}
+
+	return nil
+}
+
+type NameWrapper struct {
+	name string
+}
+
 var (
 	_ Decoder                    = (*CustomTypeError)(nil)
 	_ encoding.BinaryUnmarshaler = (*CustomTypeError)(nil)
@@ -1401,6 +1426,20 @@ func TestProcessWith(t *testing.T) {
 			}),
 			errMsg: "broken",
 		},
+		{
+			name: "custom_decoder/unmanaged_tag_uses_decoder",
+			input: &struct {
+				Field TextUnmarshalerType
+			}{
+				Field: TextUnmarshalerType{},
+			},
+			exp: &struct {
+				Field TextUnmarshalerType
+			}{
+				Field: TextUnmarshalerType{},
+			},
+			lookuper: MapLookuper(map[string]string{}),
+		},
 		{
 			name: "custom_decoder/not_called_on_unset_envvar",
 			input: &struct {