Function pointers don't seem to be mentioned in the book

[niconii]

The fact that function pointers implement the closure traits Fn/FnMut/FnOnce also doesn't seem to be mentioned in either the book or the reference.

[steveklabnik]

We do mention the type, but not that they implement Fn: http://doc.rust-lang.org/book/primitive-types.html#functions

[reem]

Note that fn items are not actually function pointers, they share the same mechanism as closures of being anonymous types with impls of Fn*; they are only coercible to fn pointers (fn(X) -> Y).