New Assist: From/To cfg_attr

[ModProg]

Wrapping an attribute into #[cfg_attr isn't a lot of work, but can be tedious nonetheless.

I propose an assist that will convert

#[some_attr(...)]    ->  #[cfg_attr(todo, some_attr(...))]
#[some_attr = ...]   ->  #[cfg_attr(todo, some_attr = ...)]
#[some_attr]         ->  #[cfg_attr(todo, some_attr)]

and back

#[cfg_attr(not(all(...)), some_attr(...))]   ->  #[some_attr(...)] 
#[cfg_attr(not(all(...)), some_attr = ...)]  ->  #[some_attr = ...]
#[cfg_attr(not(all(...)), some_attr)]        ->  #[some_attr]

Obviously, the back direction is lossy.

This would play quite well with e.g. #13935 allowing to convert a doc comment in two steps to a conditional doc attribute.

As I'm quite new to rust-analyzer I'm not sure if this would need to somehow understand weather cfg_attr is actually cfg_attr or some custom attribute macro, which could complicate things. But other than that, this should be pretty simple to implement.

[ModProg]

Is a simple comparison i.e. does the path match cfg_attr enough?
Or do I need to do a path resolve and if so, how would I check if it is the actual cfg_attr?

[ModProg]

Oh nevermind cfg_attr is a builtin one

[wyatt-herkamp]

@rustbot claim