feat: add TextFlag

* Added `TextFlag` which supports setting values for types that satisfies
  the `encoding.TextMarshaller` and `encoding.TextUnmarshaller` which is
  very handy when you want to set log levels or string-like types that
  satifies the interfaces.

Fixes: #2051
Signed-off-by: Tobias Dahlberg <lokskada@live.se>

Author: somebadcode

flag_text.go

@@ -0,0 +1,56 @@
+package cli
+
+import (
+	"encoding"
+	"strings"
+)
+
+type TextMarshalUnMarshaller interface {
+	encoding.TextMarshaler
+	encoding.TextUnmarshaler
+}
+
+// TextFlag enables you to set types that satisfies [TextMarshalUnMarshaller] using flags such as log levels.
+type TextFlag = FlagBase[TextMarshalUnMarshaller, StringConfig, TextValue]
+
+type TextValue struct {
+	Value  TextMarshalUnMarshaller
+	Config StringConfig
+}
+
+func (v TextValue) String() string {
+	text, err := v.Value.MarshalText()
+	if err != nil {
+		return ""
+	}
+
+	return string(text)
+}
+
+func (v TextValue) Set(s string) error {
+	if v.Config.TrimSpace {
+		return v.Value.UnmarshalText([]byte(strings.TrimSpace(s)))
+	}
+
+	return v.Value.UnmarshalText([]byte(s))
+}
+
+func (v TextValue) Get() any {
+	return v.Value
+}
+
+func (v TextValue) Create(t TextMarshalUnMarshaller, _ *TextMarshalUnMarshaller, c StringConfig) Value {
+	return &TextValue{
+		Value:  t,
+		Config: c,
+	}
+}
+
+func (v TextValue) ToString(t TextMarshalUnMarshaller) string {
+	text, err := t.MarshalText()
+	if err != nil {
+		return ""
+	}
+
+	return string(text)
+}

flag_text_test.go

@@ -0,0 +1,107 @@
+package cli
+
+import (
+	"errors"
+	"flag"
+	"io"
+	"log/slog"
+	"slices"
+	"testing"
+)
+
+func TestTextFlag(t *testing.T) {
+	tests := []struct {
+		name    string
+		flag    TextFlag
+		args    []string
+		want    string
+		wantErr bool
+	}{
+		{
+			name: "empty",
+			flag: TextFlag{
+				Name:  "log-level",
+				Value: &slog.LevelVar{},
+			},
+			want: "INFO",
+		},
+		{
+			name: "info",
+			flag: TextFlag{
+				Name:  "log-level",
+				Value: &slog.LevelVar{},
+				Validator: func(v TextMarshalUnMarshaller) error {
+					text, err := v.MarshalText()
+					if err != nil {
+						return err
+					}
+
+					if slices.Compare(text, []byte("INFO")) != 0 {
+						return errors.New("expected empty string")
+					}
+
+					return nil
+				},
+			},
+			args: []string{"--log-level", "info"},
+			want: "INFO",
+		},
+		{
+			name: "debug",
+			flag: TextFlag{
+				Name:  "log-level",
+				Value: &slog.LevelVar{},
+			},
+			args: []string{"--log-level", "debug"},
+			want: "DEBUG",
+		},
+		{
+			name: "debug_with_trim",
+			flag: TextFlag{
+				Name:   "log-level",
+				Value:  &slog.LevelVar{},
+				Config: StringConfig{TrimSpace: true},
+			},
+			args: []string{"--log-level", " debug   "},
+			want: "DEBUG",
+		},
+		{
+			name: "invalid",
+			flag: TextFlag{
+				Name:  "log-level",
+				Value: &slog.LevelVar{},
+			},
+			args:    []string{"--log-level", "invalid"},
+			wantErr: true,
+		},
+	}
+
+	t.Parallel()
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			set := flag.NewFlagSet(tt.name, flag.ContinueOnError)
+			if tt.wantErr {
+				set.SetOutput(io.Discard)
+			}
+
+			if err := tt.flag.Apply(set); err != nil {
+				t.Fatalf("Apply(%v) failed: %v", tt.args, err)
+			}
+
+			err := set.Parse(tt.args)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Parse() error = %v, wantErr %v", err, tt.wantErr)
+
+				return
+			} else if (err != nil) == tt.wantErr {
+				// Expected error.
+				return
+			}
+
+			if got := tt.flag.GetValue(); got != tt.want {
+				t.Errorf("Value = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}