feat: Add assist to generate a type alias for a function

[Giga-Bowser]

closes #18343
Technically this doesn't support generating for <...> lifetimes, but I'm not very familiar with them, and they seem a bit outside of the scope of an assist like this. If somebody has a way to generate them feel free to add it.

Also, when generating aliases with self parameters, they are left unnamed even in the generate_fn_type_alias_named assist. I think this behavior makes the most sense but you could also name them something like this and add a placeholder to change the name if that's desired.

[Veykril]

There are more qualifiers to handle, const and async (keep ordering in mind, the qualifiers allow only a specific order)

[Giga-Bowser]

Function pointers cannot be const nor async. I suppose I could generate an alias returning Future for async functions, let me know if this is desirable. I'm not really sure of an equivalent for a const functions, but a non-const function pointer should accept a const function as an argument, so I think it's fine to just ignore a const qualifier.

[Veykril]

Technically this doesn't support generating for <...> lifetimes, but I'm not very familiar with them, and they seem a bit outside of the scope of an assist like this. If somebody has a way to generate them feel free to add it.

That's fine. One thing that would be nice to have though is that any generic parameters declared on the function are lifted to the type alias, that is:

fn f<T>(t: T) {} becomes type TFn<T> = fn(T).

Also, when generating aliases with self parameters, they are left unnamed even in the generate_fn_type_alias_named assist. I think this behavior makes the most sense but you could also name them something like this and add a placeholder to change the name if that's desired.

Yes thats fine that way I'd say

[Giga-Bowser]

One thing that would be nice to have though is that any generic parameters declared on the function are lifted to the type alias, that is:

fn f<T>(t: T) {} becomes type TFn<T> = fn(T).

Unless I misunderstand, this code does that. For example, the test generate_fn_alias_unnamed_generics ensures the assist turns

fn foo<A, B>(a: A, b: B) -> i32 { return 42; }

into

type FooFn<A, B> = fn(A, B) -> i32;

[Veykril]

Thanks!