Support C++20 modules

[abergmeier]

Description of the problem / feature request / question:

Starting to look into C++ Modules. While it will earliest officially arrive in 2020, would be good to have support for that soonish.
Heard there is some Module support inside of Google. Can/Will that be opensourced?

[hlopko]

Hi Andreas,
everything regarding modules (only for clang) is open sourced already. The only missing piece is a CROSSTOOL that makes use of them and provides all necessary features. But I think it would be quite a challenge to write such CROSSTOOL without understanding the implementation. If I don't find a good movie on my flight to bazel conference I might take a look and hack something together. No promises though, since this would be a free time exercise, not something that is a priority for bazel right now.

[abergmeier]

Interesting.
@mhlopko Can you perhaps elaborate a bit, what parts of CROSSTOOL would interact with modules? Personally I thought it would be more in the realm of cc-providers and additional actions for module validity checks.

[hlopko]

Yeah you are right, the implementation does deal with providers and actions, but all that logic is already in bazel and ready to be used. What is missing is a bunch of features (and maybe action_configs, I'm not sure) in the CROSSTOOL to pass the right flags for the right actions. Names of the features are hardcoded in bazel, so they need to match.

[hlopko]

So in other words, it's all there, just not very well documented.

[abergmeier]

just not very well documented

Like all else about CC toolchains you mean ;)

Thanks. I am trying to get a custom hermetic Clang toolchain running anyways. Perhaps will then try this out a bit.

[hlopko]

Exactly :) I hope I'll see you next week on the crosstool breakout session during bazel conference? :) Btw, just wrote this yesterday, I really hope I'm not one day late: https://github.com/bazelbuild/bazel/wiki/Yet-Another-CROSSTOOL-Writing-Tutorial

Anyway, don't hesitate to ask if you get stuck on the way. I'm also grateful for any input/comments/suggestions.

[abergmeier]

Since I do have problems with the include paths for libc (https://stackoverflow.com/questions/47078520/clang-toolchain-fails-for-system-libc-files), how does libc_top (inputsForLibc) work?

Especially since I cannot seem to be able to glob system files.