Rust Empty Type
(Jin Qing’s Column, Sep., 2024)
Crate void defines an empty type enum Void {}.
Empty type is an enum with no variants. Empty type can not be instantiated.
See: https://doc.rust-lang.org/nomicon/exotic-sizes.html
Void is used in statically impossible cases (type-level unreachability).
For instance, a return type of Result<T, Void> indicates that it always returns Ok.
Trait void::ResultVoidErrExt can be used to unwrap the result, statically indicating the unreachability.
impl<T> ResultVoidExt<T> for Result<T, Void> {/// Get the value out of an always-ok Result.////// Never panics, since it is statically known to be Ok.#[inline]fn void_unwrap(self) -> T {match self {Ok(val) => val,Err(e) => unreachable(e)}}
}
rust-libp2p\swarm\src\dummy.rs uses Void to indicate that FromBehaviour event is impossible.
impl crate::handler::ConnectionHandler for ConnectionHandler {type FromBehaviour = Void;...fn on_behaviour_event(&mut self, event: Self::FromBehaviour) {void::unreachable(event)}
...
}