[libcxx-commits] [libcxx] [libc++] Allow the use of extensions in the implementation (PR #79532)

[Sam McCall via libcxx-commits]

sam-mccall wrote:

We're seeing major compile-time regressions (50%+) from this change in normal-looking code that uses libc++, using clang from head.

My best guess is that we're now adding lots of diagnostics-overridden regions to most TUs, and querying them (when clang checks: should we emit diagnostic X) is inefficient. I will dig a bit into this.

If this is not related to our use of modules (I need to check) then I think we should revert this and hopefully find a cheaper way. WDYT?
If it is modules-related, then it seems more complicated and will depend on the details.

https://github.com/llvm/llvm-project/pull/79532