log/slog: add DiscardHandler

This adds a package-level variable, slog.Discard, which is a
slog.Handler which performs no output. This serves a similar purpose to
io.Discard.

Fixes golang#62005

api/next/62005.txt

@@ -0,0 +1 @@
+pkg log/slog, var DiscardHandler Handler #62005

src/log/slog/discard.go

@@ -0,0 +1,14 @@
+package slog
+
+import "context"
+
+// DiscardHandler discards all log output.
+// DiscardHandler.Enabled returns false for all Levels.
+var DiscardHandler Handler = discardHandler{}
+
+type discardHandler struct{}
+
+func (dh discardHandler) Enabled(context.Context, Level) bool  { return false }
+func (dh discardHandler) Handle(context.Context, Record) error { return nil }
+func (dh discardHandler) WithAttrs(attrs []Attr) Handler       { return dh }
+func (dh discardHandler) WithGroup(name string) Handler        { return dh }

src/log/slog/discard_test.go

@@ -0,0 +1,28 @@
+package slog
+
+import (
+	"context"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestDiscardHandler(t *testing.T) {
+	ctx := context.Background()
+	stdout, stderr := os.Stdout, os.Stderr
+	os.Stdout, os.Stderr = nil, nil // panic on write
+	t.Cleanup(func() {
+		os.Stdout, os.Stderr = stdout, stderr
+	})
+
+	// Just ensure nothing panics during normal usage
+	l := New(DiscardHandler)
+	l.Info("msg", "a", 1, "b", 2)
+	l.Debug("bg", Int("a", 1), "b", 2)
+	l.Warn("w", Duration("dur", 3*time.Second))
+	l.Error("bad", "a", 1)
+	l.Log(ctx, LevelWarn+1, "w", Int("a", 1), String("b", "two"))
+	l.LogAttrs(ctx, LevelInfo+1, "a b c", Int("a", 1), String("b", "two"))
+	l.Info("info", "a", []Attr{Int("i", 1)})
+	l.Info("info", "a", GroupValue(Int("i", 1)))
+}

src/log/slog/example_discard_test.go

@@ -0,0 +1,23 @@
+package slog_test
+
+import (
+	"log/slog"
+	"log/slog/internal/slogtest"
+	"os"
+)
+
+func ExampleDiscardHandler() {
+	// A slog.TextHandler will output logs
+	logger1 := slog.New(slog.NewTextHandler(
+		os.Stdout,
+		&slog.HandlerOptions{ReplaceAttr: slogtest.RemoveTime},
+	))
+	logger1.Info("message 1")
+
+	// A slog.DiscardHandler will discard all messages
+	logger2 := slog.New(slog.DiscardHandler)
+	logger2.Info("message 2")
+
+	// Output:
+	// level=INFO msg="message 1"
+}

src/log/slog/logger_test.go

@@ -231,7 +231,7 @@ func TestCallDepth(t *testing.T) {
 
 func TestAlloc(t *testing.T) {
 	ctx := context.Background()
-	dl := New(discardHandler{})
+	dl := New(discardTestHandler{})
 	defer SetDefault(Default()) // restore
 	SetDefault(dl)
 
@@ -258,7 +258,7 @@ func TestAlloc(t *testing.T) {
 		})
 	})
 	t.Run("2 pairs disabled inline", func(t *testing.T) {
-		l := New(discardHandler{disabled: true})
+		l := New(DiscardHandler)
 		s := "abc"
 		i := 2000
 		wantAllocs(t, 2, func() {
@@ -269,7 +269,7 @@ func TestAlloc(t *testing.T) {
 		})
 	})
 	t.Run("2 pairs disabled", func(t *testing.T) {
-		l := New(discardHandler{disabled: true})
+		l := New(DiscardHandler)
 		s := "abc"
 		i := 2000
 		wantAllocs(t, 0, func() {
@@ -305,7 +305,7 @@ func TestAlloc(t *testing.T) {
 		})
 	})
 	t.Run("attrs3 disabled", func(t *testing.T) {
-		logger := New(discardHandler{disabled: true})
+		logger := New(DiscardHandler)
 		wantAllocs(t, 0, func() {
 			logger.LogAttrs(ctx, LevelInfo, "hello", Int("a", 1), String("b", "two"), Duration("c", time.Second))
 		})
@@ -568,18 +568,17 @@ func (c *captureHandler) clear() {
 	c.r = Record{}
 }
 
-type discardHandler struct {
-	disabled bool
-	attrs    []Attr
+type discardTestHandler struct {
+	attrs []Attr
 }
 
-func (d discardHandler) Enabled(context.Context, Level) bool { return !d.disabled }
-func (discardHandler) Handle(context.Context, Record) error  { return nil }
-func (d discardHandler) WithAttrs(as []Attr) Handler {
+func (d discardTestHandler) Enabled(context.Context, Level) bool { return true }
+func (discardTestHandler) Handle(context.Context, Record) error  { return nil }
+func (d discardTestHandler) WithAttrs(as []Attr) Handler {
 	d.attrs = concat(d.attrs, as)
 	return d
 }
-func (h discardHandler) WithGroup(name string) Handler {
+func (h discardTestHandler) WithGroup(name string) Handler {
 	return h
 }