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replace convert::Infallible with ! #49038

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Mar 22, 2018
Merged

replace convert::Infallible with ! #49038

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4647156
replace `convert::Infallible` with `!`
canndrew Mar 15, 2018
15bab45
Add From<!> for TryFromIntError
canndrew Mar 15, 2018
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21 changes: 1 addition & 20 deletions src/libcore/convert.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -48,25 +48,6 @@

#![stable(feature = "rust1", since = "1.0.0")]

use fmt;

/// A type used as the error type for implementations of fallible conversion
/// traits in cases where conversions cannot actually fail.
///
/// Because `Infallible` has no variants, a value of this type can never exist.
/// It is used only to satisfy trait signatures that expect an error type, and
/// signals to both the compiler and the user that the error case is impossible.
#[unstable(feature = "try_from", issue = "33417")]
#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
pub enum Infallible {}
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Looking through that list of derives, are we missing Copy and Clone for !? They're not in the rustdoc, at least: https://doc.rust-lang.org/nightly/std/primitive.never.html#implementations

(Not a blocker for this PR, of course.)

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They're implemented. No idea why they're not showing up in rustdoc.

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Ah, looks like this is a nearing-three-year-old bug with primitives: #25893


#[unstable(feature = "try_from", issue = "33417")]
impl fmt::Display for Infallible {
fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
match *self {
}
}
}
/// A cheap reference-to-reference conversion. Used to convert a value to a
/// reference value within generic code.
///
Expand Down Expand Up @@ -438,7 +419,7 @@ impl<T, U> TryInto<U> for T where U: TryFrom<T>
// with an uninhabited error type.
#[unstable(feature = "try_from", issue = "33417")]
impl<T, U> TryFrom<U> for T where T: From<U> {
type Error = Infallible;
type Error = !;

fn try_from(value: U) -> Result<Self, Self::Error> {
Ok(T::from(value))
Expand Down
18 changes: 5 additions & 13 deletions src/libcore/num/mod.rs
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Expand Up @@ -12,7 +12,7 @@

#![stable(feature = "rust1", since = "1.0.0")]

use convert::{Infallible, TryFrom};
use convert::TryFrom;
use fmt;
use intrinsics;
use ops;
Expand Down Expand Up @@ -3595,20 +3595,12 @@ impl fmt::Display for TryFromIntError {
}
}

#[unstable(feature = "try_from", issue = "33417")]
impl From<Infallible> for TryFromIntError {
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@scottmcm scottmcm Mar 15, 2018 •
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Maybe leave this as impl From<!> For TryFromIntError? This is here so that you can use u32: TryFrom<u16> in a method returning Result<_, TryFromIntError>, which I think is still wanted. (Not for those types literally, but for when they're c_whatever aliases.)

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@canndrew canndrew Mar 15, 2018 •
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I removed it thinking that we'd have impl<T> From<!> for T, but since we won't have that until we have specialization with intersection impls I guess I'd better add it back.

fn from(infallible: Infallible) -> TryFromIntError {
match infallible {
}
}
}

// no possible bounds violation
macro_rules! try_from_unbounded {
($source:ty, $($target:ty),*) => {$(
#[unstable(feature = "try_from", issue = "33417")]
impl TryFrom<$source> for $target {
type Error = Infallible;
type Error = !;

#[inline]
fn try_from(value: $source) -> Result<Self, Self::Error> {
Expand Down Expand Up @@ -3719,7 +3711,7 @@ try_from_lower_bounded!(isize, usize);
#[cfg(target_pointer_width = "16")]
mod ptr_try_from_impls {
use super::TryFromIntError;
use convert::{Infallible, TryFrom};
use convert::TryFrom;

try_from_upper_bounded!(usize, u8);
try_from_unbounded!(usize, u16, u32, u64, u128);
Expand All @@ -3745,7 +3737,7 @@ mod ptr_try_from_impls {
#[cfg(target_pointer_width = "32")]
mod ptr_try_from_impls {
use super::TryFromIntError;
use convert::{Infallible, TryFrom};
use convert::TryFrom;

try_from_upper_bounded!(usize, u8, u16);
try_from_unbounded!(usize, u32, u64, u128);
Expand All @@ -3771,7 +3763,7 @@ mod ptr_try_from_impls {
#[cfg(target_pointer_width = "64")]
mod ptr_try_from_impls {
use super::TryFromIntError;
use convert::{Infallible, TryFrom};
use convert::TryFrom;

try_from_upper_bounded!(usize, u8, u16, u32);
try_from_unbounded!(usize, u64, u128);
Expand Down
9 changes: 0 additions & 9 deletions src/libstd/error.rs
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Expand Up @@ -56,7 +56,6 @@ use any::TypeId;
use borrow::Cow;
use cell;
use char;
use convert;
use core::array;
use fmt::{self, Debug, Display};
use mem::transmute;
Expand Down Expand Up @@ -371,14 +370,6 @@ impl Error for char::ParseCharError {
}
}

#[unstable(feature = "try_from", issue = "33417")]
impl Error for convert::Infallible {
fn description(&self) -> &str {
match *self {
}
}
}

// copied from any.rs
impl Error + 'static {
/// Returns true if the boxed type is the same as `T`
Expand Down