feat: add DecodeSquash functional option helper for Unmarshal

viper.go

@@ -68,6 +68,14 @@ func DecodeHook(hook mapstructure.DecodeHookFunc) DecoderConfigOption {
 	}
 }
 
+// DecodeSquash returns a DecoderConfigOption that configures whether
+// embedded/anonymous fields are squashed into the parent structure during decoding.
+func DecodeSquash(squash bool) DecoderConfigOption {
+	return func(c *mapstructure.DecoderConfig) {
+		c.Squash = squash
+	}
+}
+
 // Viper is a prioritized configuration registry. It
 // maintains a set of configuration sources, fetches
 // values to populate those, and provides them according

viper_test.go

@@ -964,6 +964,23 @@ func TestUnmarshalWithDecoderOptions(t *testing.T) {
 	}, &C)
 }
 
+func TestUnmarshalEmbeddedUnexportedStruct(t *testing.T) {
+	v := New()
+	v.Set("field", "value")
+
+	type unexportedConfig struct {
+		Field string
+	}
+
+	type config struct {
+		unexportedConfig
+	}
+
+	var cfg config
+	require.NoError(t, v.Unmarshal(&cfg, DecodeSquash(true)))
+	assert.Equal(t, "value", cfg.Field)
+}
+
 func TestUnmarshalWithAutomaticEnv(t *testing.T) {
 	t.Setenv("PORT", "1313")
 	t.Setenv("NAME", "Steve")