Cannot express never returning closure (FnOnce() -> !)

[diwic]

Here's a minimal example that shows the problem:

fn never_return<F: FnOnce() -> !>(f: F) -> ! {
    f();
}

which fails with:

error: expected type, found `!`
     fn never_return<F: FnOnce() -> !>(f: F) -> ! {

...AFAICS, there's no good reason this syntax is not allowed.

[pnkfelix]

The reason is the combination of :

• F(A, B) -> T is shorthand for F <(A, B), T>
• ! is itself not a type; it cannot be plugged into the A B nor T above.

If we were to readd it as a type (and note that some effort did go into removing it), that would probably require an RFC. (And also, I'd want a ?Marker on such type parameter analogous to ?Sized; something like ?Converge...)

[pnkfelix]

(Alternatively we could make a special case for the F(A, B) -> T shorthand where F(A, B) -> ! would expand in some entirely different way -- but again, that would require an RFC.)

[steveklabnik]

Agreed with @pnkfelix .