chunks_exact variant with const generic argument #60735
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Labels
A-const-generics
Area: const generics (parameters and arguments)
A-iterators
Area: Iterators
C-feature-request
Category: A feature request, i.e: not implemented / a PR.
T-libs-api
Relevant to the library API team, which will review and decide on the PR/issue.
Comments
jonas-schievink
added
A-const-generics
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This also applies to the |
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We should actually be able to implement this and test it out unstably now. |
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Im willing to give this a shot in the weekend and next week! |
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@rustbot assign @DutchGhost |
This was referenced Mar 28, 2020
Closed
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So, a simple PoC fails to compile horribly Backtrace
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Notice that a freestanding function does work: #![feature(const_generics)]
fn const_chunks_exact<T, const N: usize>(slice: &[T]) -> ConstChunksExact<'_, T, { N }> {
assert!(N != 0);
let rem = slice.len() % N;
let len = slice.len() - rem;
let (fst, snd) = slice.split_at(len);
ConstChunksExact::<T, { N }> { v: fst, rem: snd }
}
struct ConstChunksExact<'a, T: 'a, const N: usize> {
v: &'a [T],
rem: &'a [T],
}
impl<'a, T: 'a, const N: usize> Iterator for ConstChunksExact<'a, T, { N }> {
type Item = &'a [T; N];
fn next(&mut self) -> Option<Self::Item> {
if self.v.len() < N {
None
} else {
let (fst, snd) = self.v.split_at(N);
self.v = snd;
let ptr = fst.as_ptr() as *const [T; N];
// SAFETY: ok because we just checked that the length fits
unsafe { Some(&*ptr) }
}
}
}
fn main() {
let mut slice = const_chunks_exact::<_, 3>(&[1, 2, 3, 4, 5, 6i32]);
for [a, b, c] in slice {
dbg!(a, b, c);
}
} |
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Keep going on, those bugs/limitations are worth fixing anyway :) |
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to_array() could be a new function related to take(), it collects the first N items of an iterator into an array of length given as const generic argument (it panics if the iterator isn't long enough). Usable like this (using iterate() from itertools): |
EDIT: This is already PR'd: #69985 |
We need a good built-in static_assert that works with constant generics too :) |
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In the stdlib I'd like an iterator like chunks_exact that yields fixed-sized arrays, using a const usize argument (mostly useful for short arrays).
Those arrays could allow better optimizations and also better pattern matching.
This issue has been assigned to @DutchGhost via this comment.
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