Multiple input lifetimes when attempting elision in `fn` & unboxed closure type signatures gives imprecise `help`

[huonw]

type foo = fn(&u8, &u8) -> &u8;

fn main() {}

bad-error.rs:1:28: 1:31 error: missing lifetime specifier [E0106]
bad-error.rs:1 type foo = fn(&u8, &u8) -> &u8;
                                          ^~~
bad-error.rs:1:28: 1:31 help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `` or ``
bad-error.rs:1 type foo = fn(&u8, &u8) -> &u8;
                                          ^~~
error: aborting due to previous error

The empty `'d things would preferably be "first argument" and "second argument" (or something) respectively, rather than just empty strings.

A fix for this should consider/test unboxed closure function traits Fn(&u8, &u8) -> &u8 which does the same sort of lifetime elision.

[crhino]

This seems like a nice and easy issue to start off with and see the inner workings of the rust compiler. I'll take a look.