Function types have no primitive page in docs

[45ujksy5l03jcvf]

Similar to #15318 and #15654

[steveklabnik]

@alexcrichton @huonw how do I make this happen? is this another 'dummy module' thing?

[alexcrichton]

It would likely require modifying code somewhere around here and adding a dummy (private) module or declaring another one as #![doc(primitive = "fn")]

[steveklabnik]

Triage: no change

[QuietMisdreavus]

Adding a page for function pointers as a whole (under some general name fn or the like) is a small enough change, but unless i've missed something, there are no impl blocks anywhere that could be picked up for that page. It's the curse of variadic generics, but unlike arrays and tuples, there are no stock impls for function pointer types up to a certain arity. Is a nearly-empty page (save for the doc(primitive) block that creates the page in the first place) acceptable?

cc @rust-lang/docs: What do we want to display for documentation on function pointers as a whole? I have a short blurb that mainly just links to the Fn* traits, and a note that function pointers are Copy, so far, but do we have anything else to add?

[petrochenkov]

@QuietMisdreavus

unlike arrays and tuples, there are no stock impls for function pointer types up to a certain arity.

rust/src/libcore/ptr.rs

Line 884 in eba9d7f

// Impls for function pointers