Format options for attributes, ability to specify custom attributes.

[allisonvacanti]

Bugzilla Link	46623
Version	unspecified
OS	All
CC	@JonasGilg,@mydeveloperday

Extended Description

Clang-format currently does not have options to control attribute formatting.

Some projects prefer to put prefix attributes on a separate line, eg:

[[nodiscard]]
int foo();

instead of:

[[nodiscard]] int foo();

Additionally, it would be useful to be able to provide a list of regexes that identify custom attributes. Maintaining an exhaustive list of attributes in clang-format proper would be daunting, as many projects hide them behind macros for backward compatibility, or rely on platform-specific attributes. Allowing projects to extend the list of recognized attributes would be quite useful.

My personal interest in this stems from using clang-format on CUDA projects, where __host__ __device__ annotations are frequently used and commonly placed on the line above a function definition. Being able to achieve this common formatting style would ease adoption of clang-format by CUDA projects.

[mydeveloperday]

I think you are only talking about breaking after an attribute when that attribute is part of the return type correct?

should

class [[gnu::unused]] f {
   public:  f() 
   {}
}

become

class [[gnu::unused]] 
f {
  public:
  f() 
  {}
}

[allisonvacanti]

I'm interested in two situations primarily:

1. Prefix function attributes:

    __host__ __device__ [[nodiscard]]
    int foo();

2. Prefix variable attributes:

    __shared__ [[deprecated]]
    int bar;

I'm not concerned about:

• Infix attributes on anything:

    namespace [[deprecated]] kramble {
      int foo[[noreturn]]();
      struct [[deprecated]] baz;
    }

• Postfix attributes on anything:

int foo() [[nodiscard]];

[JonasGilg]

I would also prefer an option to move all identifiers and attributes that come before the return type to a separate line above. As C++ gains more and more keywords that inflate the function like:

struct my_class : public other_class {
  [[nodiscard]] return_t const& my_function(parameter_t const& p) const noexcept override;
}

The function declaration just gets too long. Additionally other languages also put the prefix attributes on the line above. I.e.: Java and C#.

[owenca]

https://reviews.llvm.org/D140956