Decode: unstable/Unmarshal interface (#940)

Co-authored-by: Pavlos Karakalidis <pkarakal@pkarakal.com>
Co-authored-by: Thomas Pelletier <thomas@pelletier.codes>

unmarshaler.go

@@ -35,6 +35,9 @@ type Decoder struct {
 
 	// global settings
 	strict bool
+
+	// toggles unmarshaler interface
+	unmarshalerInterface bool
 }
 
 // NewDecoder creates a new Decoder that will read from r.
@@ -54,6 +57,24 @@ func (d *Decoder) DisallowUnknownFields() *Decoder {
 	return d
 }
 
+// EnableUnmarshalerInterface allows to enable unmarshaler interface.
+//
+// With this feature enabled, types implementing the unstable/Unmarshaler
+// interface can be decoded from any structure of the document. It allows types
+// that don't have a straightfoward TOML representation to provide their own
+// decoding logic.
+//
+// Currently, types can only decode from a single value. Tables and array tables
+// are not supported.
+//
+// *Unstable:* This method does not follow the compatibility guarantees of
+// semver. It can be changed or removed without a new major version being
+// issued.
+func (d *Decoder) EnableUnmarshalerInterface() *Decoder {
+	d.unmarshalerInterface = true
+	return d
+}
+
 // Decode the whole content of r into v.
 //
 // By default, values in the document that don't exist in the target Go value
@@ -108,6 +129,7 @@ func (d *Decoder) Decode(v interface{}) error {
 		strict: strict{
 			Enabled: d.strict,
 		},
+		unmarshalerInterface: d.unmarshalerInterface,
 	}
 
 	return dec.FromParser(v)
@@ -143,6 +165,9 @@ type decoder struct {
 	// Strict mode
 	strict strict
 
+	// Flag that enables/disables unmarshaler interface.
+	unmarshalerInterface bool
+
 	// Current context for the error.
 	errorContext *errorContext
 }
@@ -648,6 +673,14 @@ func (d *decoder) handleValue(value *unstable.Node, v reflect.Value) error {
 		v = initAndDereferencePointer(v)
 	}
 
+	if d.unmarshalerInterface {
+		if v.CanAddr() && v.Addr().CanInterface() {
+			if outi, ok := v.Addr().Interface().(unstable.Unmarshaler); ok {
+				return outi.UnmarshalTOML(value)
+			}
+		}
+	}
+
 	ok, err := d.tryTextUnmarshaler(value, v)
 	if ok || err != nil {
 		return err