Clarify FFI-safety of Option<extern "C" fn>

[ramosbugs]

Relatively new to Rust, and reading through the Nomicon to make sure I build a new FFI module safely. I noticed one omission in https://github.com/rust-lang-nursery/nomicon/blob/master/src/other-reprs.md.

#13 added language to call out that Option<&T> is FFI-safe as long as *const T is FFI-safe. Based on rust-lang/rust#40913, it seems that the preferred way of expressing nullable function pointers (returned via FFI) is using Option<fn> (or I guess Option<extern "C" fn> since function pointers set by C code are presumably also C-ABI).

If my understanding is correct, shall we add this to the Nomicon? I'm happy to send a PR if that's desired.

Also, are Option<&T> and Option<extern "C" fn> an exhaustive list of repr(C) FFI-safe enums with fields, or are there other exceptions to document as well?

[ramosbugs]

Actually, it looks like this is covered by the nullable pointer optimization section, which specifically mentions function pointers (along with &T, &mut T, and boxes). I wonder if it's worth including a forward pointer to the more comprehensive section in the bullet added in #13 (cc: @geofft).

[Gankra]

Being more clear shouldn't hurt here.

[Gankra]

If possible the clarified language on other-reprs should link to the nullable pointer optimization. I'm thinking the wording should be something like:

If T is an FFI-safe non-nullable pointer type, Option<T> is guaranteed to have the same layout and ABI as T and is therefore also FFI-safe. As of this writing, this covers &, &mut, and function pointers.