Allow for custom level mapping between logrus and slog

hooks/slog/slog.go

@@ -9,9 +9,16 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+// LevelMapper maps a [github.com/sirupsen/logrus.Level] value to a
+// [slog.Leveler] value. To change the default level mapping, for instance
+// to allow mapping to custom or dynamic slog levels in your application, set
+// [SlogHook.LevelMapper] to your own implementation of this function.
+type LevelMapper func(logrus.Level) slog.Leveler
+
 // SlogHook sends logs to slog.
 type SlogHook struct {
-	logger *slog.Logger
+	logger      *slog.Logger
+	LevelMapper LevelMapper
 }
 
 var _ logrus.Hook = (*SlogHook)(nil)
@@ -31,7 +38,10 @@ func NewSlogHook(logger *slog.Logger) *SlogHook {
 	}
 }
 
-func (*SlogHook) toSlogLevel(level logrus.Level) slog.Level {
+func (h *SlogHook) toSlogLevel(level logrus.Level) slog.Leveler {
+	if h.LevelMapper != nil {
+		return h.LevelMapper(level)
+	}
 	switch level {
 	case logrus.PanicLevel, logrus.FatalLevel, logrus.ErrorLevel:
 		return slog.LevelError
@@ -62,7 +72,7 @@ func (h *SlogHook) Fire(entry *logrus.Entry) error {
 	}
 	h.logger.Log(
 		entry.Context,
-		h.toSlogLevel(entry.Level),
+		h.toSlogLevel(entry.Level).Level(),
 		entry.Message,
 		attrs...,
 	)

hooks/slog/slog_test.go

@@ -15,9 +15,10 @@ import (
 
 func TestSlogHook(t *testing.T) {
 	tests := []struct {
-		name string
-		fn   func(*logrus.Logger)
-		want []string
+		name   string
+		mapper LevelMapper
+		fn     func(*logrus.Logger)
+		want   []string
 	}{
 		{
 			name: "defaults",
@@ -39,6 +40,20 @@ func TestSlogHook(t *testing.T) {
 				"level=ERROR msg=error chicken=cluck",
 			},
 		},
+		{
+			name: "level mapper",
+			mapper: func(logrus.Level) slog.Leveler {
+				return slog.LevelInfo
+			},
+			fn: func(log *logrus.Logger) {
+				log.WithFields(logrus.Fields{
+					"chicken": "cluck",
+				}).Error("error")
+			},
+			want: []string{
+				"level=INFO msg=error chicken=cluck",
+			},
+		},
 	}
 
 	for _, tt := range tests {
@@ -55,7 +70,9 @@ func TestSlogHook(t *testing.T) {
 			}))
 			log := logrus.New()
 			log.Out = io.Discard
-			log.AddHook(NewSlogHook(slogLogger))
+			hook := NewSlogHook(slogLogger)
+			hook.LevelMapper = tt.mapper
+			log.AddHook(hook)
 			tt.fn(log)
 			got := strings.Split(strings.TrimSpace(buf.String()), "\n")
 			if len(got) != len(tt.want) {