C++20 module improvements

[sharadhr]

There are a few things I'd like to improve about vulkan.cppm, stemming from my own experience, and bug reports around.

• Using any struct that declares/exports some sort of comparison or operator<=> requires that the consumer #include <compare>, which is very unintuitive; I'd class this as a bug. Some good resources:
  • DevComm thread: Template exports requiring importing of dependent packages (C++20 modules). The suggestion made was to export import <compare>; but header units are generally considered a bad idea.
  • Reddit comment from a MSVC compiler developer with further detail, making a similar suggestion, but this time suggests using import std; in the producing module without exporting any additional symbol. Which brings me to...
• Using import std; breaks badly with import vulkan-hpp; because the latter still relies on #include <some_std_header> for all its dependencies.
  • The general recommendation is to not mix standard headers and the standard library module.
  • I propose using import std; guarded behind the appropriate feature macro, __cpp_lib_modules.
  • It's worth noting that the std library module has been back-ported to C++20.
    • MSVC STL: Supporting import std; in C++20 microsoft/STL#3945
    • LLVM libc++: https://reviews.llvm.org/D158358
    • GNU libstdc++: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106852 (in-progress)

I'd like to spur some discussion on this, and if it's worthwhile, start a relevant PR.

[g-martin772]

Making the vulkan_hpp module work with import std; would be great!

[sharadhr]

@asuessenbach and @theHamsta, do you have any input on this? Thanks!

[asuessenbach]

Please sorry for the delay.
I think, you're right, using import std seems to be the right move. Would be great if you could start a PR on it.

[stripe2933]

Hello, I created a draft commit for enable import std conditionally using VULKAN_HPP_USE_STD_MODULE CMake option. It works with my Clang 17 + custom libc++ build, but I have no idea that it works on other environment (also it may breaks the usage for non-module usage). Are there anything I should do before the PR? Here's the commit in the forked repository.

[sharadhr]

Thanks @stripe2933! I've personally been putting off this because CMake 3.30 (probably releasing in a couple of months; 3.29 is already at rc4) will have full support for import std without our having to do much: https://gitlab.kitware.com/cmake/cmake/-/merge_requests/9337

[stripe2933]

Oh, I was one step behind. That sounds great. It will fix the module usage with MSVC.