[clang-format] Add an option to configure C++ attributes

[cjdb]

When there are too many attributes, functions tend to get formatted like this:

[[attr1, attr2, attr3, attr4]] __keyword_attr constexpr void
f(args...);

It'd be nice if this were configurable, so that we could also have

// option 1
[[attr1, attr2, attr3, attr4]] __keyword_attr
constexpr void f(args...);

// option 2
[[attr1, attr2, attr3, attr4]]
__keyword_attr constexpr void f(args...);

[llvmbot]

@llvm/issue-subscribers-clang-format

[rymiel]

Related: #54265

[owenca]

https://reviews.llvm.org/D140956

[cjdb]

Thanks for implementing the initial functionality! I think it would be good if there were a mode so that short lines are preserved and long lines are broken (maybe an IfTooLong option?). For example, I don't think the [[nodiscard]] below should be on its own line.

[[nodiscard]] int zero()
{
  return 0;
}

[owenca]

It's hard to come up with a single measure for what is "short" and what is "long". Would you use the number of attributes, total length, or something else?

I included Leave as an option to not change line breaks after attributes and believe that it should work for your example.

[cjdb]

Agreed on the hardness issue. I was imagining with simply having the line break happen after one goes over the maximum column length (plus penalty).

I included Leave as an option to not change line breaks after attributes and believe that it should work for your example.

Oooh, thanks for pointing this out! For some reason I'd processed this a bit differently.

[owenca]

Np!