use pragma diagnostic to silence unused function in macro code

[hellow554]

Describe the bug
I use SDBUSCPP_REGISTER_STRUCT to generate code for my struct. The macro generated "more code than necessary" and therefore my -Werror strikes in.

To Reproduce

namespace {
struct PropEntry {
    std::string type;
    std::string           data;
};
}
SDBUSCPP_REGISTER_STRUCT(PropEntry, type, data);

Expected behavior
No warnings, just compile.

Real (buggy) behavior

src/plugins.cpp:150:1: error: unused function 'as_dictionary_if_struct' [-Werror,-Wunused-function]
  150 | SDBUSCPP_REGISTER_STRUCT(PropEntry, type, data);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/sdbus-c++/Types.h:463:21: note: expanded from macro 'SDBUSCPP_REGISTER_STRUCT'
  463 |         inline auto as_dictionary_if_struct(const STRUCT& object)
      |                     ^~~~~~~~~~~~~~~~~~~~~~~
src/plugins.cpp:150:1: error: unused function 'operator<<' [-Werror,-Wunused-function]
  150 | SDBUSCPP_REGISTER_STRUCT(PropEntry, type, data);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/sdbus-c++/Types.h:468:32: note: expanded from macro 'SDBUSCPP_REGISTER_STRUCT'
  468 |         inline sdbus::Message& operator<<(sdbus::Message& msg, const STRUCT& items)
      |                                ^~~~~~~~
src/plugins.cpp:150:1: error: unused function 'operator<<' [-Werror,-Wunused-function]
  150 | SDBUSCPP_REGISTER_STRUCT(PropEntry, type, data);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/include/sdbus-c++/Types.h:473:25: note: expanded from macro 'SDBUSCPP_REGISTER_STRUCT'
  473 |         inline Message& operator<<(Message& msg, const as_dictionary<STRUCT>& s)
      |                         ^~~~~~~~
3 errors generated.
ninja: build stopped: subcommand failed.

Additional context
One way to ignore these errors would be adding pragma inside the macro:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
// [..] macro code here
# pragma GCC diagnostic pop

I'm not sure which pragmas msvc would expect and I'm not sure if we want to go this path :/

Tell me what you think about this.

[sangelovic]

I tried to compile sdbus-c++ tests (where this macro is used several times) with both gcc v12.2 and gcc v13.3 and -Wunused-function turned on, but I was unable to reproduce the emission of that warning.

The gcc docs on this warning mentions unused static function. However, the macro uses inline functions without static keyword.

Anyhow, even when the warning was emitted, I would suggest that clients suppress the warning in their code. I don't think it is good idea to do it generally on sdbus-c++ level.