Suppress "unknown attribute 'no_unique_address' ignored" with MSVC ABI

[bebuch]

MSVC introduced [[msvc::no_unique_address]] to workaround ABI break with [[no_unique_address]]. For portability you have to use both.

clang-cl gives me the warning:

unknown attribute 'no_unique_address' ignored [-Wunknown-attributes]

I don't want to suppress warnings for all unknown attributes. But I want to suppress this one, at least if [[msvc::no_unique_address]] is defined as well.

I propose to add a compiler flag to suppress warnings for specific attributes.

I also propose to silently ignore [[no_unique_address]] if [[msvc::no_unique_address]] is defined as well.

[AaronBallman]

I propose to add a compiler flag to suppress warnings for specific attributes.

This gets a bit unwieldy because the semantics of -Wno-ignored-attribute=something is not really well-defined. The current best approach is to use the diagnostic pragmas to silence the diagnostics locally; it's not ideal, but it's usually pretty clear.

I also propose to silently ignore [[no_unique_address]] if [[msvc::no_unique_address]] is defined as well.

No; we learned long ago that diagnosing ignored attributes is very important. And for [[no_unique_address]], it's especially important as ignoring the attribute changes the layout of the object. Consider: https://godbolt.org/z/GjdYqGTKG

Note: these two attributes are synonyms in terms of how they impact layout, but they're not the same thing. [[no_unique_address]] is not supported by MSVC, so in compatibility mode, we don't support it (ignore it). [[msvc::no_unique_address]] is supported by MSVC, so in compatibility mode, we honor it.

[bebuch]

In my opinion it would be very useful to be able to suppress specific attribute warnings with -Wno-ignored-attribute=something or -Wno-ignored-attribute=vendor::. I understand that arguments on warning flags are something that doesn't exist yet. But here it would be very helpful. Attributes can do a lot of different things. To handle them all as one thing is not optimal.

Pragmas and preprocessor defines make the code hard to read. I think it would be better if you could handle this in the build system.

[a-tarasyuk]

There is a quite similar case related to namespaces #120925