[$] Stabilizing Rust's never type

A function's return type is supposed to indicate the kind of data that itproduces. Rust's "never" type, which isdenoted by an exclamation mark ("!"), is the type the language uses to marka function that never returns and other places where a value can never occur. Fora long time, the never type was used internally by the compiler, but wasconsidered an unstable feature. OnAugust24, after more than two years of work,Rust-compiler-contributor "waffle" finally managed to stabilize the type. Ittook so long, in part, because it involved a small breakingchange to previous Rust editions, which the compiler maintainers needed toensure did not impact much real code.