Add missing UnmarshalTOML call (#996)

Fixes #994.

unmarshaler.go

@@ -1154,6 +1154,17 @@ func (d *decoder) handleKeyValuePart(key unstable.Iterator, value *unstable.Node
 	case reflect.Struct:
 		path, found := structFieldPath(v, string(key.Node().Data))
 		if !found {
+			// If no matching struct field is found but the target implements the
+			// unstable.Unmarshaler interface (and it is enabled), delegate the
+			// decoding of this value to the custom unmarshaler.
+			if d.unmarshalerInterface {
+				if v.CanAddr() && v.Addr().CanInterface() {
+					if outi, ok := v.Addr().Interface().(unstable.Unmarshaler); ok {
+						return reflect.Value{}, outi.UnmarshalTOML(value)
+					}
+				}
+			}
+			// Otherwise, keep previous behavior and skip until the next table.
 			d.skipUntilTable = true
 			break
 		}

unmarshaler_test.go

@@ -3998,3 +3998,45 @@ foo = "bar"`,
 		})
 	}
 }
+
+type doc994 struct{}
+
+func (d *doc994) UnmarshalTOML(value *unstable.Node) error {
+	return errors.New("expected-error")
+}
+
+func TestIssue994(t *testing.T) {
+	var _ unstable.Unmarshaler = (*doc994)(nil)
+	tomlBytes := []byte(`foo = "bar"`)
+	var d doc994
+	err := toml.NewDecoder(bytes.NewReader(tomlBytes)).
+		EnableUnmarshalerInterface().
+		Decode(&d)
+
+	assert.Error(t, err)
+
+	if err.Error() != "expected-error" {
+		t.Fatalf("expected error 'expected-error', got '%s'", err.Error())
+	}
+}
+
+type doc994ok struct {
+	S string
+}
+
+func (d *doc994ok) UnmarshalTOML(value *unstable.Node) error {
+	d.S = string(value.Data) + " from unmarshaler"
+	return nil
+}
+
+func TestIssue994_OK(t *testing.T) {
+	var _ unstable.Unmarshaler = (*doc994ok)(nil)
+	tomlBytes := []byte(`foo = "bar"`)
+	var d doc994ok
+	err := toml.NewDecoder(bytes.NewReader(tomlBytes)).
+		EnableUnmarshalerInterface().
+		Decode(&d)
+
+	assert.NoError(t, err)
+	assert.Equal(t, "bar from unmarshaler", d.S)
+}