log/slog: implement encoding.TextAppender for Level and LevelVar

For #62384

api/next/62384.txt

@@ -3,3 +3,5 @@ pkg encoding, type BinaryAppender interface, AppendBinary([]uint8) ([]uint8, err
 pkg encoding, type TextAppender interface { AppendText } #62384
 pkg encoding, type TextAppender interface, AppendText([]uint8) ([]uint8, error) #62384
 pkg net/url, method (*URL) AppendBinary([]uint8) ([]uint8, error) #62384
+pkg log/slog, method (Level) AppendText([]uint8) ([]uint8, error) #62384
+pkg log/slog, method (*LevelVar) AppendText([]uint8) ([]uint8, error) #62384

doc/next/6-stdlib/99-minor/log/slog/62384.md

@@ -0,0 +1 @@
+[Level] and [LevelVar] now implement the [encoding.TextAppender] interface.

src/log/slog/level.go

@@ -98,10 +98,16 @@ func (l *Level) UnmarshalJSON(data []byte) error {
 	return l.parse(s)
 }
 
-// MarshalText implements [encoding.TextMarshaler]
+// AppendText implements [encoding.TextAppender]
 // by calling [Level.String].
+func (l Level) AppendText(b []byte) ([]byte, error) {
+	return append(b, l.String()...), nil
+}
+
+// MarshalText implements [encoding.TextMarshaler]
+// by calling [Level.AppendText].
 func (l Level) MarshalText() ([]byte, error) {
-	return []byte(l.String()), nil
+	return l.AppendText(nil)
 }
 
 // UnmarshalText implements [encoding.TextUnmarshaler].
@@ -172,10 +178,16 @@ func (v *LevelVar) String() string {
 	return fmt.Sprintf("LevelVar(%s)", v.Level())
 }
 
+// AppendText implements [encoding.TextAppender]
+// by calling [Level.AppendText].
+func (v *LevelVar) AppendText(b []byte) ([]byte, error) {
+	return v.Level().AppendText(b)
+}
+
 // MarshalText implements [encoding.TextMarshaler]
-// by calling [Level.MarshalText].
+// by calling [LevelVar.AppendText].
 func (v *LevelVar) MarshalText() ([]byte, error) {
-	return v.Level().MarshalText()
+	return v.AppendText(nil)
 }
 
 // UnmarshalText implements [encoding.TextUnmarshaler]

src/log/slog/level_test.go

@@ -89,6 +89,19 @@ func TestLevelMarshalText(t *testing.T) {
 	}
 }
 
+func TestLevelAppendText(t *testing.T) {
+	buf := make([]byte, 4, 16)
+	want := LevelWarn - 3
+	wantData := []byte("\x00\x00\x00\x00INFO+1")
+	data, err := want.AppendText(buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(data, wantData) {
+		t.Errorf("got %s, want %s", string(data), string(wantData))
+	}
+}
+
 func TestLevelParse(t *testing.T) {
 	for _, test := range []struct {
 		in   string
@@ -162,6 +175,23 @@ func TestLevelVarMarshalText(t *testing.T) {
 	}
 }
 
+func TestLevelVarAppendText(t *testing.T) {
+	var v LevelVar
+	v.Set(LevelWarn)
+	buf := make([]byte, 4, 16)
+	data, err := v.AppendText(buf)
+	if err != nil {
+		t.Fatal(err)
+	}
+	var v2 LevelVar
+	if err := v2.UnmarshalText(data[4:]); err != nil {
+		t.Fatal(err)
+	}
+	if g, w := v2.Level(), LevelWarn; g != w {
+		t.Errorf("got %s, want %s", g, w)
+	}
+}
+
 func TestLevelVarFlag(t *testing.T) {
 	fs := flag.NewFlagSet("test", flag.ContinueOnError)
 	v := &LevelVar{}