Add support for C++20 modules

[ChuanqiXu9]

C++20 Named Modules introduce new dependent information to the project, so the legacy dependency discovery method doesn't work properly for C++20 Named Modules. clang-scan-deps has just landed the support for discovering dependencies for named modules. The manual can be found here: https://clang.llvm.org/docs/StandardCPlusPlusModules.html#discover-dependencies. It would be pretty cool if ccache could support named modules.

[bharsaklemukesh975]

Can I work on this?

[ChuanqiXu9]

Can I work on this?

I don't know ccache a lot. But you can contact me if you need information from modules : )

[bharsaklemukesh975]

Hello @ChuanqiXu9 , Could you please provide some examples of how to use modules? I will start working on it. Looking forward to learning and implementing modules.

[ChuanqiXu9]

Hello @ChuanqiXu9 , Could you please provide some examples of how to use modules? I will start working on it. Looking forward to learning and implementing modules.

Hello. https://clang.llvm.org/docs/StandardCPlusPlusModules.html is the document about how to use C++20 modules in clang. And the blog https://www.kitware.com/import-cmake-c20-modules/ tells how to use cmake to compile C++20 modules. It may be a good start to use cmake to compile the hello world example.

And for the ccache, I think the key point is the p1689 format produced from clang-scan-deps. You can find the document in https://clang.llvm.org/docs/StandardCPlusPlusModules.html#discover-dependencies. And you can find the formal paper in https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1689r5.html.

For the more complex example, I think https://github.com/alibaba/async_simple/tree/CXX20Modules is the most complex example for modules as far as I know. You can find there a lot of module interfaces there. So I think it would be a great success if the ccache works for that one. Note that for the above example in async_simple, you would better to use the same standard library otherwise there may be problems.

[kelteseth]

We now track ccache modules support progress at https://arewemodulesyet.org/tools/ . Feel free to create a PR if the status changes at https://github.com/kelteseth/arewemodulesyet/ 😊