Pointer and function pointer casting

[jacob-greenfield]

Continuing the conversation from Discord regarding #5906.

[jacob-greenfield]

To summarize the consensus:

1. Forcing the user to cast to a function pointer is considered bloating, just use a regular pointer instead
2. Whenever the lint is triggered, keep the existing suggestion (foo as *const ()), but also suggest calling the function in certain circumstances
3. Turbofish as usize should be linted

Some points that still need discussion:

1. When/whether to lint non-turbofish functions, or functions that take many arguments
2. When to suggest calling the function (and what to suggest for arguments)
3. How we should separate the lints

Also, I think we still need to discuss pointer casts in general (function and regular) to numeric types. E.g. *const T as i8, fn_ptr as i8. Maybe non-fn pointers are a separate topic but IMHO we should make the rules consistent and treat them the same as normal pointers when it comes to casting, and decide what to do about truncating/non-usize casts. (I also wonder if function pointers need to be reconsidered for some existing lints involving pointers, but that's probably off topic)

[kraktus]

I think this can be closed as implemented by #7705

[nk9]

@rustbot label +S-fixed