#![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
        }
    }
}