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implement TrustedRandomAccess and TrustedLen for Skip

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The8472 2021-05-20 00:29:53 +02:00 committed by The 8472
parent a2a7caacf7
commit 451a3b1775
1 changed files with 51 additions and 1 deletions
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52
library/core/src/iter/adapters/skip.rs
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@ -1,6 +1,10 @@
use crate::intrinsics::unlikely;
use crate::iter::adapters::zip::try_get_unchecked;
use crate::iter::TrustedFused;
use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
use crate::iter::{
adapters::SourceIter, FusedIterator, InPlaceIterable, TrustedLen, TrustedRandomAccess,
TrustedRandomAccessNoCoerce,
};
use crate::num::NonZeroUsize;
use crate::ops::{ControlFlow, Try};
@ -152,6 +156,32 @@ where
NonZeroUsize::new(n).map_or(Ok(()), Err)
}
#[doc(hidden)]
unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item
where
Self: TrustedRandomAccessNoCoerce,
{
// SAFETY: the caller must uphold the contract for
// `Iterator::__iterator_get_unchecked`.
//
// Dropping the skipped prefix when index 0 is passed is safe
// since
// * the caller passing index 0 means that the inner iterator has more items than `self.n`
// * TRA contract requires that get_unchecked will only be called once
// (unless elements are copyable)
// * it does not conflict with in-place iteration since index 0 must be accessed
// before something is written into the storage used by the prefix
unsafe {
if Self::MAY_HAVE_SIDE_EFFECT && idx == 0 {
for skipped_idx in 0..self.n {
drop(try_get_unchecked(&mut self.iter, skipped_idx));
}
}
try_get_unchecked(&mut self.iter, idx + self.n)
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
@ -237,3 +267,23 @@ unsafe impl<I: InPlaceIterable> InPlaceIterable for Skip<I> {
const EXPAND_BY: Option<NonZeroUsize> = I::EXPAND_BY;
const MERGE_BY: Option<NonZeroUsize> = I::MERGE_BY;
}
#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I> TrustedRandomAccess for Skip<I> where I: TrustedRandomAccess {}
#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe impl<I> TrustedRandomAccessNoCoerce for Skip<I>
where
I: TrustedRandomAccessNoCoerce,
{
const MAY_HAVE_SIDE_EFFECT: bool = I::MAY_HAVE_SIDE_EFFECT;
}
// SAFETY: This adapter is shortening. TrustedLen requires the upper bound to be calculated correctly.
// These requirements can only be satisfied when the upper bound of the inner iterator's upper
// bound is never `None`. I: TrustedRandomAccess happens to provide this guarantee while
// I: TrustedLen would not.
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<I> TrustedLen for Skip<I> where I: Iterator + TrustedRandomAccess {}