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#![cfg_attr(test, deny(warnings))]
#![deny(missing_docs)]
#![cfg_attr(not(feature = "std"), no_std)]
//! # Void
//!
//! The uninhabited void type for use in statically impossible cases.
//!
//! In its own crate so all the users in the ecosystem can share the same type.
//! This crate also comes ready with several extension traits for Result that add
//! extra functionality to `Result<T, Void>` and `Result<Void, E>`.
//!
#[cfg(not(feature = "std"))]
mod coreprovider {
extern crate core;
pub use core::{fmt, cmp};
}
#[cfg(feature = "std")]
mod coreprovider {
pub use std::{fmt, cmp, error};
}
use coreprovider::*;
/// The empty type for cases which can't occur.
#[derive(Copy)]
pub enum Void { }
impl Clone for Void {
fn clone(&self) -> Void {
unreachable(*self)
}
}
impl fmt::Debug for Void {
fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
unreachable(*self)
}
}
impl fmt::Display for Void {
fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
unreachable(*self)
}
}
impl<T> cmp::PartialEq<T> for Void {
fn eq(&self, _: &T) -> bool {
unreachable(*self)
}
}
impl<T> cmp::PartialOrd<T> for Void {
fn partial_cmp(&self, _: &T) -> Option<cmp::Ordering> {
unreachable(*self)
}
}
#[cfg(feature = "std")]
impl error::Error for Void {
fn description(&self) -> &str {
unreachable(*self)
}
fn cause(&self) -> Option<&error::Error> {
unreachable(*self)
}
}
/// A safe version of `intrinsincs::unreachable`.
///
/// If this typechecks, anything that causes this to run is unreachable code.
///
/// Calling this function in reachable code invokes undefined behavior, but
/// should not be possible unless `unsafe` was used elsewhere to construct
/// an instance of `Void` (which is already undefined behavior).
#[inline(always)]
pub fn unreachable(x: Void) -> ! {
match x {}
}
/// Extensions to `Result<T, Void>`
pub trait ResultVoidExt<T>: Sized {
/// Get the value out of a wrapper.
fn void_unwrap(self) -> T;
}
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)
}
}
}
/// Extensions to `Result<Void, E>`
pub trait ResultVoidErrExt<E>: Sized {
/// Get the error out of a wrapper.
fn void_unwrap_err(self) -> E;
}
impl<E> ResultVoidErrExt<E> for Result<Void, E> {
/// Get the error out of an always-err Result.
///
/// Never panics, since it is statically known to be Err.
#[inline]
fn void_unwrap_err(self) -> E {
match self {
Ok(v) => unreachable(v),
Err(e) => e
}
}
}
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