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Use const generics for array impls [part 1] #62435

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Jul 7, 2019
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Use const generics for array impls [part 1] #62435

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320 changes: 320 additions & 0 deletions src/libcore/array.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -81,6 +81,7 @@ impl From<Infallible> for TryFromSliceError {
}
}

#[cfg(bootstrap)]
macro_rules! __impl_slice_eq1 {
($Lhs: ty, $Rhs: ty) => {
__impl_slice_eq1! { $Lhs, $Rhs, Sized }
Expand All @@ -96,6 +97,7 @@ macro_rules! __impl_slice_eq1 {
}
}

#[cfg(bootstrap)]
macro_rules! __impl_slice_eq2 {
($Lhs: ty, $Rhs: ty) => {
__impl_slice_eq2! { $Lhs, $Rhs, Sized }
Expand All @@ -114,6 +116,7 @@ macro_rules! __impl_slice_eq2 {
}

// macro for implementing n-element array functions and operations
#[cfg(bootstrap)]
macro_rules! array_impls {
($($N:expr)+) => {
$(
Expand Down Expand Up @@ -264,6 +267,323 @@ macro_rules! array_impls {
}
}

#[cfg(not(bootstrap))]
mod impls_using_const_generics {
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These are separate because I don't want to rely on the bootstrap compiler's const generics.

use super::*;

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, const N: usize> AsRef<[T]> for [T; N]
where
[T; N]: LengthAtMost32,
{
#[inline]
fn as_ref(&self) -> &[T] {
&self[..]
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T, const N: usize> AsMut<[T]> for [T; N]
where
[T; N]: LengthAtMost32,
{
#[inline]
fn as_mut(&mut self) -> &mut [T] {
&mut self[..]
}
}

#[stable(feature = "array_borrow", since = "1.4.0")]
impl<T, const N: usize> Borrow<[T]> for [T; N]
where
[T; N]: LengthAtMost32,
{
fn borrow(&self) -> &[T] {
self
}
}

#[stable(feature = "array_borrow", since = "1.4.0")]
impl<T, const N: usize> BorrowMut<[T]> for [T; N]
where
[T; N]: LengthAtMost32,
{
fn borrow_mut(&mut self) -> &mut [T] {
self
}
}

#[stable(feature = "try_from", since = "1.34.0")]
impl<T, const N: usize> TryFrom<&[T]> for [T; N]
where
T: Copy,
[T; N]: LengthAtMost32,
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🤢

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@petrochenkov petrochenkov Jul 7, 2019 •
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Looks like rustfmt has a where_single_line option fixing the list-breaking behavior, but it still keeps where un-indented, which goes entirely again all sense of style.

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Style is objective, but since you’ve brought it up:

  1. I don’t find this objectionable at all. I can quickly skim it.
  2. Remember that all of the LengthAtMost32 bounds will disappear someday (ideally). The diff when that happens will be extremely easy to review.

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Self: LengthAtMost32,

{
type Error = TryFromSliceError;

fn try_from(slice: &[T]) -> Result<[T; N], TryFromSliceError> {
<&Self>::try_from(slice).map(|r| *r)
}
}

#[stable(feature = "try_from", since = "1.34.0")]
impl<'a, T, const N: usize> TryFrom<&'a [T]> for &'a [T; N]
where
[T; N]: LengthAtMost32,
{
type Error = TryFromSliceError;

fn try_from(slice: &[T]) -> Result<&[T; N], TryFromSliceError> {
if slice.len() == N {
let ptr = slice.as_ptr() as *const [T; N];
unsafe { Ok(&*ptr) }
} else {
Err(TryFromSliceError(()))
}
}
}

#[stable(feature = "try_from", since = "1.34.0")]
impl<'a, T, const N: usize> TryFrom<&'a mut [T]> for &'a mut [T; N]
where
[T; N]: LengthAtMost32,
{
type Error = TryFromSliceError;

fn try_from(slice: &mut [T]) -> Result<&mut [T; N], TryFromSliceError> {
if slice.len() == N {
let ptr = slice.as_mut_ptr() as *mut [T; N];
unsafe { Ok(&mut *ptr) }
} else {
Err(TryFromSliceError(()))
}
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Hash, const N: usize> Hash for [T; N]
where
[T; N]: LengthAtMost32,
{
fn hash<H: hash::Hasher>(&self, state: &mut H) {
Hash::hash(&self[..], state)
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: fmt::Debug, const N: usize> fmt::Debug for [T; N]
where
[T; N]: LengthAtMost32,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt::Debug::fmt(&&self[..], f)
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, const N: usize> IntoIterator for &'a [T; N]
where
[T; N]: LengthAtMost32,
{
type Item = &'a T;
type IntoIter = Iter<'a, T>;

fn into_iter(self) -> Iter<'a, T> {
self.iter()
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N]
where
[T; N]: LengthAtMost32,
{
type Item = &'a mut T;
type IntoIter = IterMut<'a, T>;

fn into_iter(self) -> IterMut<'a, T> {
self.iter_mut()
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b, A, B, const N: usize> PartialEq<[B; N]> for [A; N]
where
A: PartialEq<B>,
[A; N]: LengthAtMost32,
[B; N]: LengthAtMost32,
{
#[inline]
fn eq(&self, other: &[B; N]) -> bool {
self[..] == other[..]
}
#[inline]
fn ne(&self, other: &[B; N]) -> bool {
self[..] != other[..]
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b, A, B, const N: usize> PartialEq<[B]> for [A; N]
where
A: PartialEq<B>,
[A; N]: LengthAtMost32,
{
#[inline]
fn eq(&self, other: &[B]) -> bool {
self[..] == other[..]
}
#[inline]
fn ne(&self, other: &[B]) -> bool {
self[..] != other[..]
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b, A, B, const N: usize> PartialEq<[A; N]> for [B]
where
B: PartialEq<A>,
[A; N]: LengthAtMost32,
{
#[inline]
fn eq(&self, other: &[A; N]) -> bool {
self[..] == other[..]
}
#[inline]
fn ne(&self, other: &[A; N]) -> bool {
self[..] != other[..]
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b, A, B, const N: usize> PartialEq<&'b [B]> for [A; N]
where
A: PartialEq<B>,
[A; N]: LengthAtMost32,
{
#[inline]
fn eq(&self, other: &&'b [B]) -> bool {
self[..] == other[..]
}
#[inline]
fn ne(&self, other: &&'b [B]) -> bool {
self[..] != other[..]
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b, A, B, const N: usize> PartialEq<[A; N]> for &'b [B]
where
B: PartialEq<A>,
[A; N]: LengthAtMost32,
{
#[inline]
fn eq(&self, other: &[A; N]) -> bool {
self[..] == other[..]
}
#[inline]
fn ne(&self, other: &[A; N]) -> bool {
self[..] != other[..]
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b, A, B, const N: usize> PartialEq<&'b mut [B]> for [A; N]
where
A: PartialEq<B>,
[A; N]: LengthAtMost32,
{
#[inline]
fn eq(&self, other: &&'b mut [B]) -> bool {
self[..] == other[..]
}
#[inline]
fn ne(&self, other: &&'b mut [B]) -> bool {
self[..] != other[..]
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, 'b, A, B, const N: usize> PartialEq<[A; N]> for &'b mut [B]
where
B: PartialEq<A>,
[A; N]: LengthAtMost32,
{
#[inline]
fn eq(&self, other: &[A; N]) -> bool {
self[..] == other[..]
}
#[inline]
fn ne(&self, other: &[A; N]) -> bool {
self[..] != other[..]
}
}

// NOTE: some less important impls are omitted to reduce code bloat
// __impl_slice_eq2! { [A; $N], &'b [B; $N] }
// __impl_slice_eq2! { [A; $N], &'b mut [B; $N] }

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Eq, const N: usize> Eq for [T; N] where [T; N]: LengthAtMost32 {}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: PartialOrd, const N: usize> PartialOrd for [T; N]
where
[T; N]: LengthAtMost32,
{
#[inline]
fn partial_cmp(&self, other: &[T; N]) -> Option<Ordering> {
PartialOrd::partial_cmp(&&self[..], &&other[..])
}
#[inline]
fn lt(&self, other: &[T; N]) -> bool {
PartialOrd::lt(&&self[..], &&other[..])
}
#[inline]
fn le(&self, other: &[T; N]) -> bool {
PartialOrd::le(&&self[..], &&other[..])
}
#[inline]
fn ge(&self, other: &[T; N]) -> bool {
PartialOrd::ge(&&self[..], &&other[..])
}
#[inline]
fn gt(&self, other: &[T; N]) -> bool {
PartialOrd::gt(&&self[..], &&other[..])
}
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: Ord, const N: usize> Ord for [T; N]
where
[T; N]: LengthAtMost32,
{
#[inline]
fn cmp(&self, other: &[T; N]) -> Ordering {
Ord::cmp(&&self[..], &&other[..])
}
}
}

/// Implemented for lengths where trait impls are allowed on arrays in core/std
#[rustc_on_unimplemented(
message="arrays only have std trait implementations for lengths 0..=32",
)]
#[unstable(feature = "const_generic_impls_guard", issue = "0",
reason = "will never be stable, just a temporary step until const generics are stable")]
#[cfg(not(bootstrap))]
pub trait LengthAtMost32 {}
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@varkor varkor Jul 12, 2019 •
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Is it not possible to make this invisible outside std? It'd be better if users couldn't do:

use std::array::LengthAtMost32;

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The trait is unstable. Users can't do it unless they opt into instability.

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It's surprising that it's user-facing at all, even unstably. It's not a big problem though.

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I think it's good that it shows up in the impls, so that they don't look like they work for all N. And if it's going to be visible in the rustdoc there, I think it's better that the bound be clickable so people can see what it is.


#[cfg(not(bootstrap))]
macro_rules! array_impls {
($($N:literal)+) => {
$(
#[unstable(feature = "const_generic_impls_guard", issue = "0")]
impl<T> LengthAtMost32 for [T; $N] {}
)+
}
}

array_impls! {
0 1 2 3 4 5 6 7 8 9
10 11 12 13 14 15 16 17 18 19
Expand Down
1 change: 1 addition & 0 deletions src/libcore/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -74,6 +74,7 @@
#![feature(concat_idents)]
#![feature(const_fn)]
#![feature(const_fn_union)]
#![cfg_attr(not(bootstrap), feature(const_generics))]
#![feature(custom_inner_attributes)]
#![feature(doc_cfg)]
#![feature(doc_spotlight)]
Expand Down
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