pw_protobuf: Allow copying of const callbacks

This fixes a bug where it wasn't possible to copy a const message
structure, the callback was intended to be "copyable as empty" but was
missing a 'const' in the right place.

Change-Id: I109d14ec16a5dcc7845654113419036ee6007117
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/92760
Pigweed-Auto-Submit: Scott James Remnant <keybuk@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Armando Montanez <amontanez@google.com>

pw_protobuf/codegen_message_test.cc

@@ -205,6 +205,40 @@ TEST(CodegenMessage, Inequality) {
  EXPECT_FALSE(one == two);
 }
 
+TEST(CodegenMessage, ConstCopyable) {
+ const Pigweed::Message one{
+ .magic_number = 0x49u,
+ .ziggy = -111,
+ .cycles = 0x40302010fecaaddeu,
+ .ratio = -1.42f,
+ .error_message = "not a typewriter",
+ .pigweed = {.status = Bool::FILE_NOT_FOUND},
+ .bin = Pigweed::Protobuf::Binary::ZERO,
+ .proto = {.bin = Proto::Binary::OFF,
+ .pigweed_pigweed_bin = Pigweed::Pigweed::Binary::ZERO,
+ .pigweed_protobuf_bin = Pigweed::Protobuf::Binary::ZERO,
+ .meta =
+ {
+ .file_name = "/etc/passwd",
+ .status = Pigweed::Protobuf::Compiler::Status::FUBAR,
+ .protobuf_bin = Pigweed::Protobuf::Binary::ONE,
+ .pigweed_bin = Pigweed::Pigweed::Binary::ONE,
+ }},
+ .data = {std::byte{0x10},
+ std::byte{0x20},
+ std::byte{0x30},
+ std::byte{0x40},
+ std::byte{0x50},
+ std::byte{0x60},
+ std::byte{0x70},
+ std::byte{0x80}},
+ .bungle = -111,
+ };
+ Pigweed::Message two = one;
+
+ EXPECT_TRUE(one == two);
+}
+
 PW_MODIFY_DIAGNOSTICS_POP();
 
 TEST(CodegenMessage, Read) {

pw_protobuf/public/pw_protobuf/internal/codegen.h

@@ -167,7 +167,7 @@ union Callback {
  constexpr Callback& operator=(Callback&& other) = default;
 
  // Copying a callback does not copy the functions.
- constexpr Callback(Callback&) : encode_() {}
+ constexpr Callback(const Callback&) : encode_() {}
  constexpr Callback& operator=(const Callback&) {
  encode_ = nullptr;
  return *this;