unmarshal: support encoding.TextUnmarshaler (#375)

* unmarshal: support encoding.TextUnmarshaler

This PR adds support for decoding fields of primitive types that implement
encoding.TextUnmarshaler by calling the custom method.

Fields in anonymous structs are not supported at this point.

Co-authored-by: Lorenz Bauer <lmb@users.noreply.github.com>

marshal_test.go

@@ -7,6 +7,7 @@ import (
 	"io/ioutil"
 	"os"
 	"reflect"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -3226,3 +3227,74 @@ func TestDecoderStrictValid(t *testing.T) {
 		t.Fatal("unexpected error:", err)
 	}
 }
+
+type intWrapper struct {
+	Value int
+}
+
+func (w *intWrapper) UnmarshalText(text []byte) error {
+	var err error
+	if w.Value, err = strconv.Atoi(string(text)); err == nil {
+		return nil
+	}
+	if b, err := strconv.ParseBool(string(text)); err == nil {
+		if b {
+			w.Value = 1
+		}
+		return nil
+	}
+	if f, err := strconv.ParseFloat(string(text), 32); err == nil {
+		w.Value = int(f)
+		return nil
+	}
+	return fmt.Errorf("unsupported: %s", text)
+}
+
+func TestTextUnmarshal(t *testing.T) {
+	var doc struct {
+		UnixTime intWrapper
+		Version  *intWrapper
+
+		Bool  intWrapper
+		Int   intWrapper
+		Float intWrapper
+	}
+
+	input := `
+UnixTime = "12"
+Version = "42"
+Bool = true
+Int = 21
+Float = 2.0
+`
+
+	if err := Unmarshal([]byte(input), &doc); err != nil {
+		t.Fatalf("unexpected err: %s", err.Error())
+	}
+	if doc.UnixTime.Value != 12 {
+		t.Fatalf("expected UnixTime: 12 got: %d", doc.UnixTime.Value)
+	}
+	if doc.Version.Value != 42 {
+		t.Fatalf("expected Version: 42 got: %d", doc.Version.Value)
+	}
+	if doc.Bool.Value != 1 {
+		t.Fatalf("expected Bool: 1 got: %d", doc.Bool.Value)
+	}
+	if doc.Int.Value != 21 {
+		t.Fatalf("expected Int: 21 got: %d", doc.Int.Value)
+	}
+	if doc.Float.Value != 2 {
+		t.Fatalf("expected Float: 2 got: %d", doc.Float.Value)
+	}
+}
+
+func TestTextUnmarshalError(t *testing.T) {
+	var doc struct {
+		Failer intWrapper
+	}
+
+	input := `Failer = "hello"`
+	if err := Unmarshal([]byte(input), &doc); err == nil {
+		t.Fatalf("expected err, got none")
+	}
+}