[cmake] Expose clang resource dir to installed CMake

[kimgr]

In IWYU's build, we guess Clang's resource dir based on the CMAKE_PREFIX_PATH, so we can collect the builtin/resource headers for our own build tree. Unfotunately, depending on packaging/platform, the path to the resource dir varies quite a bit.

Is there any chance to expose something like CLANG_RESOURCE_DIR in ClangConfig.cmake?

It's not fully clear to me what the layering looks like there, but it feels like it should be possible.

Thanks!

[kimgr]

(I'd be happy to start on a patch, but I need a thread to start pulling on. Is there an example patch doing something similar to give me an idea what would need to change? Thanks!)

[kimgr]

Eventually landed #103388 to support this. I'll ping back here once I have everything wired in IWYU and can explain how to do it properly.

[kimgr]

In IWYU, we ended up exposing two new CMake config variables:

• IWYU_RESOURCE_DIR (same semantics as CLANG_RESOURCE_DIR)
• IWYU_RESOURCE_RELATIVE_TO (literal values clang or iwyu)

This creates 4 configuration permutations:

• Let Clang figure it out
• Let Clang figure it out with a custom RESOURCE_DIR setting
• Replicate Clang's logic in IWYU with default RESOURCE_DIR
• Replicate Clang's logic in IWYU with custom RESOURCE_DIR

All of them are necessary to produce working builds under different package configurations and preferred standalone-ness of IWYU.

This is described in even more detail here:
https://github.com/include-what-you-use/include-what-you-use/blob/master/README.md#how-to-install

Not saying that's the only way to do it, but it might come in handy for some other tooling project. Closing this issue.