From dd7530dea941307d0f7fd054d6ae14d0e49b0650 Mon Sep 17 00:00:00 2001
From: Leonid Shevtsov <leonid@shevtsov.me>
Date: Tue, 10 Sep 2024 18:18:19 +0300
Subject: [PATCH] Fix panic on empty interface field

---
 devslog.go      |  8 ++++++--
 devslog_test.go | 24 ++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)

diff --git a/devslog.go b/devslog.go
index 9622792..e12e039 100644
--- a/devslog.go
+++ b/devslog.go
@@ -599,8 +599,12 @@ func (h *developHandler) elementType(t reflect.Type, v reflect.Value, l int, p i
 			}
 		}
 	case reflect.Interface:
-		v = reflect.ValueOf(v.Interface())
-		b = h.elementType(v.Type(), v, l, p)
+		if v.IsZero() {
+			b = nilString()
+		} else {
+			v = reflect.ValueOf(v.Interface())
+			b = h.elementType(v.Type(), v, l, p)
+		}
 	default:
 		b = atb("Unknown type: ")
 		b = append(b, atb(v.Kind())...)
diff --git a/devslog_test.go b/devslog_test.go
index f1a9e23..cc21a00 100644
--- a/devslog_test.go
+++ b/devslog_test.go
@@ -71,6 +71,7 @@ func Test_Types(t *testing.T) {
 	test_MapOfPointers(t, opts)
 	test_MapOfInterface(t, opts)
 	test_Struct(t, opts)
+	test_NilInterface(t, opts)
 	test_Group(t, opts)
 	test_LogValuer(t, opts)
 	test_LogValuerPanic(t, opts)
@@ -734,6 +735,29 @@ func test_Struct(t *testing.T, o *Options) {
 	}
 }
 
+func test_NilInterface(t *testing.T, o *Options) {
+	w := &MockWriter{}
+	logger := slog.New(NewHandler(w, o))
+
+	type StructWithInterface struct {
+		Data any
+	}
+
+	s := StructWithInterface{}
+
+	logger.Info("msg",
+		slog.Any("s", s),
+	)
+
+	expected := []byte(
+		"\x1b[2m\x1b[37m[]\x1b[0m \x1b[42m\x1b[30m INFO \x1b[0m \x1b[32mmsg\x1b[0m\n\x1b[33mS\x1b[0m \x1b[35ms\x1b[0m: \x1b[33md\x1b[0m\x1b[33me\x1b[0m\x1b[33mv\x1b[0m\x1b[33ms\x1b[0m\x1b[33ml\x1b[0m\x1b[33mo\x1b[0m\x1b[33mg\x1b[0m\x1b[33m.\x1b[0m\x1b[33mS\x1b[0m\x1b[33mt\x1b[0m\x1b[33mr\x1b[0m\x1b[33mu\x1b[0m\x1b[33mc\x1b[0m\x1b[33mt\x1b[0m\x1b[33mW\x1b[0m\x1b[33mi\x1b[0m\x1b[33mt\x1b[0m\x1b[33mh\x1b[0m\x1b[33mI\x1b[0m\x1b[33mn\x1b[0m\x1b[33mt\x1b[0m\x1b[33me\x1b[0m\x1b[33mr\x1b[0m\x1b[33mf\x1b[0m\x1b[33ma\x1b[0m\x1b[33mc\x1b[0m\x1b[33me\x1b[0m\n    \x1b[32mData\x1b[0m: \x1b[31m<\x1b[0m\x1b[33mnil\x1b[0m\x1b[31m>\x1b[0m\n\n",
+	)
+
+	if !bytes.Equal(w.WrittenData, expected) {
+		t.Errorf("\nExpected:\n%s\nGot:\n%s\nExpected:\n%[1]q\nGot:\n%[2]q", expected, w.WrittenData)
+	}
+}
+
 func test_Group(t *testing.T, o *Options) {
 	w := &MockWriter{}
 	logger := slog.New(NewHandler(w, o))