[PATCH] D105439: [clang] protects users from relying on libc++ detail headers

Tue Jul 13 12:01:53 PDT 2021

cjdb added a comment.

In D105439#2874805 <https://reviews.llvm.org/D105439#2874805>, @cjdb wrote:

> In D105439#2874733 <https://reviews.llvm.org/D105439#2874733>, @lebedev.ri wrote:
>
>> In D105439#2874706 <https://reviews.llvm.org/D105439#2874706>, @ldionne wrote:
>>
>>> I'm not entirely sure I understand the purpose of this patch. So the idea is that let's say a tool suggests including `<__algorithm/find.h>` to get the definition of `std::find` as a IWYU fix-it sort of suggestion, the user would naively do that, and then the compiler (with this patch) would error out saying "woops, you can't include that libc++ detail header". Is that the idea?
>>>
>>> If that's it, then I would much rather fix the tools that incorrectly suggest including those implementation detail headers in the first place. Users will be less confused and we won't have to special-case a special directory name, which I can imagine could cause issues. I think it's great to try and give the proper diagnostic to users, but I think the correct place to do that is in the tool that suggests it in the first place. Thoughts?
>>
>> Regardless of whether this is about enhacing IWYU QoL or completely preventing inclusion of separate sub-headers,
>> it seems like that should be done in libc++ itself, it already has the means for that.
>
> I've already pointed out that it can't be done in libc++. The proposed header alternative doesn't translate to Clang modules, and private modules tank build times.

After a bit more experimenting (I was benchmarking to provide evidence), it seems as though private headers impact the first run of `llvm-lit` on libc++ tests, but not subsequent ones. Since this patch is user-oriented, I'm now in agreement that it //can// be punted back to the libc++ space. Thanks @lebedev.ri and apologies for my initial misleading assertion!

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