log/slog: Add WithAttrs and Logger.WithAttrs

This merge request introduces the WithAttrs function to the log/slog package, enabling efficient attribute-based logging of structured data.

Updates #66937

src/log/slog/logger.go

@@ -133,6 +133,16 @@ func (l *Logger) With(args ...any) *Logger {
 	return c
 }
 
+// WithAttrs is a more efficient version of [Logger.With] that accepts only Attrs.
+func (l *Logger) WithAttrs(attrs ...Attr) *Logger {
+	if len(attrs) == 0 {
+		return l
+	}
+	c := l.clone()
+	c.handler = l.handler.WithAttrs(attrs)
+	return c
+}
+
 // WithGroup returns a Logger that starts a group, if name is non-empty.
 // The keys of all attributes added to the Logger will be qualified by the given
 // name. (How that qualification happens depends on the [Handler.WithGroup]
@@ -161,6 +171,11 @@ func With(args ...any) *Logger {
 	return Default().With(args...)
 }
 
+// WithAttrs calls [Logger.WithAttrs] on the default logger.
+func WithAttrs(attrs ...Attr) *Logger {
+	return Default().WithAttrs(attrs...)
+}
+
 // Enabled reports whether l emits log records at the given context and level.
 func (l *Logger) Enabled(ctx context.Context, level Level) bool {
 	if ctx == nil {

src/log/slog/logger_test.go

@@ -167,10 +167,11 @@ func TestAttrs(t *testing.T) {
 
 	l1 := New(&captureHandler{}).With("a", 1)
 	l2 := New(l1.Handler()).With("b", 2)
-	l2.Info("m", "c", 3)
-	h := l2.Handler().(*captureHandler)
-	check(h.attrs, Int("a", 1), Int("b", 2))
-	check(attrsSlice(h.r), Int("c", 3))
+	l3 := New(l2.Handler()).WithAttrs(Int("c", 3))
+	l3.Info("m", "d", 4)
+	h := l3.Handler().(*captureHandler)
+	check(h.attrs, Int("a", 1), Int("b", 2), Int("c", 3))
+	check(attrsSlice(h.r), Int("d", 4))
 }
 
 func TestCallDepth(t *testing.T) {