[libc-commits] [libc] [libc] Make hdrgen support macro_header YAML field. (PR #123265)

[Nick Desaulniers via libc-commits]

nickdesaulniers wrote:

Ah, so this is more of "add checks to generated headers that if a given macro isn't defined as expected, we will `#error` out.

Coincidentally, I just reviewed a patch to bionic where I learned that they have dedicated header tests (seemingly only for POSIX IIUC): https://android.googlesource.com/platform/bionic/+/main/tests/headers/posix/

So I guess those are two different means of testing the generated headers.  The trade off being that with the approach in this PR, our headers are bigger; we pay that cost of the preprocessor checking, rechecking, rechecking, ... for every compile, regardless of whether or not we just mean to do a sanity check.

Privately, you seemed to indicate that your had more patches that build on top of this PR. Can you describe a little more how you plan to extend this feature?

Otherwise, I think I mildly prefer bionic's approach of just having unit tests.  We already have this concept in the tests under `libc/test/include/`.  I don't see why this could just be:
```c
#ifndef RTLD_LAZY
#error "WTF"
#endif
#ifndef ...
```
which is effectively what the bionic tests are doing.  That way, we keep our generated headers smaller (FWIW), and can remove macro_name/macro_value support from hdrgen (#124957). cc @enh-google 

https://github.com/llvm/llvm-project/pull/123265