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Implement Random for array #136732

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Implement Random for array #136732

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f1580f3
Implement `Random` for array
sorairolake Feb 8, 2025
80d776e
Change to use `array::from_fn`
sorairolake Feb 9, 2025
5435be8
Add specialization `Random` for integer arrays
sorairolake Feb 9, 2025
b7d7fc6
Hide specialized `Random` for array behind private trait
sorairolake Feb 9, 2025
b33af51
Apply suggestions from code review
sorairolake Feb 28, 2025
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44 changes: 43 additions & 1 deletion library/core/src/array/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,7 +17,8 @@ use crate::ops::{
ChangeOutputType, ControlFlow, FromResidual, Index, IndexMut, NeverShortCircuit, Residual, Try,
};
use crate::ptr::{null, null_mut};
use crate::slice::{Iter, IterMut};
use crate::random::{Random, RandomSource};
use crate::slice::{self, Iter, IterMut};

mod ascii;
mod drain;
Expand Down Expand Up @@ -426,6 +427,47 @@ impl<T: Clone, const N: usize> Clone for [T; N] {
}
}

#[unstable(feature = "random", issue = "130703")]
impl<T: Random, const N: usize> Random for [T; N] {
default fn random(source: &mut (impl RandomSource + ?Sized)) -> Self {
from_fn(|_| T::random(source))
}
}

macro_rules! impl_random_for_integer_array {
($t:ty) => {
#[unstable(feature = "random", issue = "130703")]
impl<const N: usize> Random for [$t; N] {
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@the8472 the8472 Feb 9, 2025

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Specializations should be hidden behind a private trait. https://std-dev-guide.rust-lang.org/policy/specialization.html

Alternatively you can avoid specialization by implementing a buffering random source wrapper as mentioned in #136732 (comment)

fn random(source: &mut (impl RandomSource + ?Sized)) -> Self {
let mut buf = [const { MaybeUninit::<$t>::uninit() }; N];
// SAFETY: all elements in the buffer were initialized with
// random bytes.
unsafe {
let bytes = slice::from_raw_parts_mut(
&raw mut buf as *mut u8,
N * (<$t>::BITS as usize / 8),
);
source.fill_bytes(bytes);
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@joboet joboet Feb 28, 2025

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This is unsound: you create a &mut [u8] and pass it to user code – but nothing guarantees that the user code won't try to read the slice, which would be undefined behaviour. My recommendation would be to use MaybeUninit::zeroed to create buf.

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@dhardy dhardy Mar 14, 2025

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Ideally this needs a trait bound like zerocopy::FromBytes or bytemuck::Pod, but we don't have that in std, so... the macro should be renamed to unsafe_impl_... since safety depends on the macro caller?

mem::transmute_copy(&buf)
}
}
}
};
}

impl_random_for_integer_array!(u8);
impl_random_for_integer_array!(u16);
impl_random_for_integer_array!(u32);
impl_random_for_integer_array!(u64);
impl_random_for_integer_array!(u128);
impl_random_for_integer_array!(usize);
impl_random_for_integer_array!(i8);
impl_random_for_integer_array!(i16);
impl_random_for_integer_array!(i32);
impl_random_for_integer_array!(i64);
impl_random_for_integer_array!(i128);
impl_random_for_integer_array!(isize);

trait SpecArrayClone: Clone {
fn clone<const N: usize>(array: &[Self; N]) -> [Self; N];
}
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