feat: typed decode and encode errors with the key path

Assisted-by: GLM 5.3 Flash

encode_test.go

@@ -1160,3 +1160,73 @@ type Custom struct {
 }
 
 func (c Custom) MarshalTOML() (any, error) { return c.tag, nil }
+
+// encodeErrBad is a Marshaler whose MarshalTOML always fails.
+type encodeErrBad struct {
+	msg string
+}
+
+func (encodeErrBad) MarshalTOML() (any, error) { return nil, errors.New("bad timestamp") }
+
+func TestEncodeErrorCarriesPath(t *testing.T) {
+	type Inner struct {
+		Port encodeErrBad `toml:"port"`
+	}
+	type Cfg struct {
+		Server Inner `toml:"server"`
+	}
+	_, err := Marshal(Cfg{Server: Inner{Port: encodeErrBad{}}})
+	if err == nil {
+		t.Fatal("expected a marshal error")
+	}
+	ee, ok := errors.AsType[*EncodeError](err)
+	if !ok {
+		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
+	}
+	if ee.Path != "server.port" {
+		t.Fatalf("Path = %q, want %q", ee.Path, "server.port")
+	}
+	if ee.Err == nil || ee.Err.Error() != "bad timestamp" {
+		t.Fatalf("Err = %v", ee.Err)
+	}
+	if err.Error() != "interpres: server.port: bad timestamp" {
+		t.Fatalf("message = %q", err.Error())
+	}
+}
+
+func TestEncodeErrorTopLevelPathHasNoLeadingDot(t *testing.T) {
+	type Cfg struct {
+		Port encodeErrBad `toml:"port"`
+	}
+	_, err := Marshal(Cfg{})
+	ee, ok := errors.AsType[*EncodeError](err)
+	if !ok {
+		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
+	}
+	if ee.Path != "port" {
+		t.Fatalf("Path = %q, want %q", ee.Path, "port")
+	}
+	if err.Error() != "interpres: port: bad timestamp" {
+		t.Fatalf("message = %q", err.Error())
+	}
+}
+
+func TestEncodeErrorHeterogeneousArrayPath(t *testing.T) {
+	type Item struct {
+		N int `toml:"n"`
+	}
+	cfg := map[string]any{
+		"items": []any{Item{}, 3},
+	}
+	_, err := Marshal(cfg)
+	if err == nil {
+		t.Fatal("expected a heterogeneous array error")
+	}
+	ee, ok := errors.AsType[*EncodeError](err)
+	if !ok {
+		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
+	}
+	if ee.Path != "items[0]" {
+		t.Fatalf("Path = %q, want %q", ee.Path, "items[0]")
+	}
+}