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Rollup merge of #83990 - the8472:take-trusted-len, r=dtolnay

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implement `TrustedRandomAccess` for `Take` iterator adapter

`TrustedRandomAccess` requires the iterator length to fit within `usize`. `take(n)` only constrains the upper bound of an iterator. So if the inner is `TrustedRandomAccess` (which already implies a finite length) then so can be `Take`.

```````@rustbot``````` label T-libs-impl
This commit is contained in:
Yuki Okushi 2021-04-24 12:17:01 +09:00 committed by GitHub
commit c00439f682
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1 changed files with 22 additions and 1 deletions
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23
library/core/src/iter/adapters/take.rs
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@ -1,5 +1,8 @@
use crate::cmp;
use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen};
use crate::iter::{
adapters::zip::try_get_unchecked, adapters::SourceIter, FusedIterator, InPlaceIterable,
TrustedLen, TrustedRandomAccess,
};
use crate::ops::{ControlFlow, Try};
/// An iterator that only iterates over the first `n` iterations of `iter`.
@ -111,6 +114,15 @@ fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
self.try_fold(init, ok(fold)).unwrap()
}
unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> <I as Iterator>::Item
where
Self: TrustedRandomAccess,
{
// SAFETY: the caller must uphold the contract for
// `Iterator::__iterator_get_unchecked`.
unsafe { try_get_unchecked(&mut self.iter, idx) }
}
}
#[unstable(issue = "none", feature = "inplace_iteration")]
@ -207,3 +219,12 @@ impl<I> FusedIterator for Take<I> where I: FusedIterator {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I: TrustedLen> TrustedLen for Take<I> {}
#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I> TrustedRandomAccess for Take<I>
where
I: TrustedRandomAccess,
{
const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
}