[clang-tidy] Check request: portability-avoid-pragma-once

[denzor200]

The check prefer-pragma-once already suggested, also we can add the check for opposite purpose. The check might be useful for those who use a compiler which not support the pragma. The check must be disabled by default

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Denis Mikhailov (denzor200)

The check [prefer-pragma-once](https://github.com//issues/26800) already suggested, also we can add the check for opposite purpose. The check might be useful for those who use a compiler which not support the pragma. The check must be disabled by default

[LegalizeAdulthood]

those who use a compiler which not support the pragma

When I asked about suggesting a C++ guideline for #pragma once, the answer I got is that while it's not standard, every compiler supports it. What compiler doesn't support it?

[AaronBallman]

those who use a compiler which not support the pragma

When I asked about suggesting a C++ guideline for #pragma once, the answer I got is that while it's not standard, every compiler supports it. What compiler doesn't support it?

I think it's not so much about compilers not supporting it, but about #pragma once being a poor replacement for header guards in some circumstances. (For example, some network file systems make it impossible to know whether two different file paths correspond to the same file, so #pragma once will fail in those cases.) As a result, some coding styles recommend against the pragma.

[LegalizeAdulthood]

Those are indeed valid weirdnesses with pragma once, but the original request comments about compilers not supporting it. Since I wasn't aware of any compiler that didn't support it, I was curious if they had a specific compiler in mind.

[AaronBallman]

A few C compilers, at the least:

https://godbolt.org/z/aseToYKeq
https://godbolt.org/z/WMYfc831a

[LegalizeAdulthood]

Yeah, I am not surprised that small 8-bit micro C compilers don't support it. Most embedded systems development these days takes place with forks of clang or gcc.

[denzor200]

but the original request comments about compilers not supporting it

Yes, I was meaning embedded C compilers, hard to remember to show example. Also I heard GCC 3 doesn't support pragma once(is it still actual?).
Thanks for @AaronBallman for the good point regarding network file systems

[dl8sd11]

Hi, I drafted a PR for this check but I failed to write the test for it.
It would be nice if there is any suggestion for it #140388 (comment).