Lint null function pointers

[oli-obk]

rust-lang/rust#40913 showed a real world case of assuming that fn() can be null.

patterns that should be linted are

std::mem::transmute::<Anything, SomeFnPtrType>(std::ptr::null())

(fn_ptr as *mut something).is_null()

[rillian]

I think also

(fn_ptr as *const something).is_null()

And perhaps suggest as Option<fn_ptr> instead.

I guess we can't lint a bare fn_ptr in a #[repr(C)] struct because if it's only created on the Rust side, C code could read it safely relying on it being non-null.

[Manishearth]

I don't see why you want to cast to a *const Option<fn>?

[rillian]

I don't properly understand the difference between *mut and *const with function pointers. (fn_ptr as *const something) compiles, so I figured the lint should take both into account.

[Manishearth]

Sure, my issue is with the Option being beind a pointer. You probably want to transmute the C void pointer to an Option<fn>, not *Option<fn>

[rillian]

Oh, right. Sorry! Yes that. :) I'll edit for clarity.

[oli-obk]

I would not transmute (cast is not possible anyway) to Option<fn()>, since that gives the wrong idea. You need to do Some(fn_ptr) if you want a nullable fn pointer.

Anyway, this is a clear bug-case, so we should not suggest anything, since that will totally depend on the use case and it will be incredibly hard to create a good suggestion.