fix(encoding): render <nil> for typed-nil Stringer pointers instead of panicking

encoding.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"log/slog"
 	"path/filepath"
+	"reflect"
 	"runtime"
 	"time"
 )
@@ -147,6 +148,19 @@ func (e encoder) writeValue(buf *buffer, value slog.Value) {
 			e.writeColoredString(buf, v.Error(), e.opts.Theme.AttrValueError())
 			return
 		case fmt.Stringer:
+			// A typed-nil pointer still satisfies the Stringer
+			// interface (e.g. (*time.Time)(nil) picks up the
+			// auto-generated pointer-receiver wrapper for
+			// time.Time.String), so calling v.String() would
+			// dispatch the method on a nil receiver and panic
+			// with 'value method ... called using nil * pointer'
+			// (#22). Render the literal <nil> instead so a
+			// missing optional field is just a missing attribute,
+			// not a crashed logger.
+			if rv := reflect.ValueOf(v); rv.Kind() == reflect.Pointer && rv.IsNil() {
+				e.writeColoredString(buf, "<nil>", attrValue)
+				return
+			}
 			e.writeColoredString(buf, v.String(), attrValue)
 			return
 		}

handler_test.go

@@ -434,3 +434,27 @@ func TestThemes(t *testing.T) {
 		})
 	}
 }
+
+// TestHandler_NilStringerPointer is a regression for #22. Logging a
+// typed-nil pointer whose underlying value type implements fmt.Stringer
+// (e.g. (*time.Time)(nil)) used to dispatch the String method on the
+// nil receiver and panic with 'value method time.Time.String called
+// using nil *Time pointer'. The handler must render "<nil>" instead.
+func TestHandler_NilStringerPointer(t *testing.T) {
+	buf := bytes.Buffer{}
+	h := NewHandler(&buf, &HandlerOptions{NoColor: true})
+	rec := slog.NewRecord(time.Time{}, slog.LevelInfo, "foobar", 0)
+
+	var expiration *time.Time
+	rec.AddAttrs(slog.Any("expiration", expiration))
+
+	defer func() {
+		if r := recover(); r != nil {
+			t.Fatalf("handler panicked on nil *time.Time: %v", r)
+		}
+	}()
+	AssertNoError(t, h.Handle(context.Background(), rec))
+
+	expected := "INF foobar expiration=<nil>\n"
+	AssertEqual(t, expected, buf.String())
+}