unmarshal: use UnmarshalText for any type (#642)

Not only structs can implement TextUnmarshaler.

Fixes #564

unmarshaler.go

@@ -560,10 +560,6 @@ func (d *decoder) handleTablePart(key ast.Iterator, v reflect.Value) (reflect.Va
 }
 
 func (d *decoder) tryTextUnmarshaler(node *ast.Node, v reflect.Value) (bool, error) {
-	if v.Kind() != reflect.Struct {
-		return false, nil
-	}
-
 	// Special case for time, because we allow to unmarshal to it from
 	// different kind of AST nodes.
 	if v.Type() == timeType {

unmarshaler_test.go

@@ -1,6 +1,7 @@
 package toml_test
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -1822,6 +1823,30 @@ foo = "bar"`
 	require.Error(t, err)
 }
 
+type uuid [16]byte
+
+func (u *uuid) UnmarshalText(text []byte) (err error) {
+	// Note: the original reported issue had a more complex implementation
+	// of this function. But the important part is to verify that a
+	// non-struct type implementing UnmarshalText works with the unmarshal
+	// process.
+	placeholder := bytes.Repeat([]byte{0xAA}, 16)
+	copy(u[:], placeholder)
+	return nil
+}
+
+func TestIssue564(t *testing.T) {
+	type Config struct {
+		ID uuid
+	}
+
+	var config Config
+
+	err := toml.Unmarshal([]byte(`id = "0818a52b97b94768941ba1172c76cf6c"`), &config)
+	require.NoError(t, err)
+	require.Equal(t, uuid{0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA, 0xAA}, config.ID)
+}
+
 //nolint:funlen
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {