fix(encode): reject a nil MarshalTOML result

Assisted-by: GLM 5.3

encode_test.go

@@ -383,6 +383,39 @@ func TestMarshalerErrorPropagates(t *testing.T) {
 	}
 }
 
+// nilMarshalerFunc is a Marshaler whose method returns nil with no error.
+type nilMarshalerFunc struct{}
+
+func (nilMarshalerFunc) MarshalTOML() (any, error) { return nil, nil }
+
+func TestMarshalRejectsNilMarshalerResult(t *testing.T) {
+	// nil has no TOML representation, so a MarshalTOML result of nil is an
+	// error, not a silently dropped field.
+	_, err := Marshal(struct {
+		F nilMarshalerFunc `toml:"f"`
+	}{})
+	if err == nil {
+		t.Fatal("expected an error for a nil MarshalTOML result")
+	}
+	ee, ok := errors.AsType[*EncodeError](err)
+	if !ok {
+		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
+	}
+	if ee.Path != "f" {
+		t.Fatalf("Path = %q, want %q", ee.Path, "f")
+	}
+
+	// Inside a value array the nil result used to reach reflection as a zero
+	// Value and panic.
+	_, err = Marshal(map[string]any{"arr": []any{1, nilMarshalerFunc{}}})
+	if err == nil {
+		t.Fatal("expected an error for a nil MarshalTOML result in an array")
+	}
+	if !strings.Contains(err.Error(), "MarshalTOML returned a nil value") {
+		t.Errorf("err = %v, want the nil-result message", err)
+	}
+}
+
 // Two fields that resolve to one TOML key must marshal as one key, resolved
 // the way the decoder resolves it, or the output would carry a duplicate key
 // and never re-parse.