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Implement more Iterator methods on core::iter::Repeat #85338

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May 18, 2021
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Implement more Iterator methods on core::iter::Repeat #85338

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35 changes: 35 additions & 0 deletions library/core/src/iter/sources/repeat.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -72,10 +72,32 @@ impl<A: Clone> Iterator for Repeat<A> {
fn next(&mut self) -> Option<A> {
Some(self.element.clone())
}

#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
(usize::MAX, None)
}

#[inline]
fn advance_by(&mut self, n: usize) -> Result<(), usize> {
// Advancing an infinite iterator of a single element is a no-op.
let _ = n;
Ok(())
}

#[inline]
fn nth(&mut self, n: usize) -> Option<A> {
let _ = n;
Comment on lines +82 to +90
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@joshtriplett @lopopolo

These two functions, as well as their counterparts in DoubleEndedIterator are incorrect. It changes behavior because there can be an arbitrary Drop impl for A, which means the side effects are removed by this PR.

Some(self.element.clone())
}

fn last(self) -> Option<A> {
loop {}
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Wouldn't self.element be the last one?
Just optimized from running infinite time to a no-op :D

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Repeat is an infinite iterator so it has no last element.

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But it has last element according to next_back implementation.

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@hbina hbina Jul 23, 2021 •
edited
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Can we please have this behavior instead? I am using it to represent something like [[*] in tr where it expands however much is required. To make this easier I need last() to actually return something to act as a fallback in case the sequences in set1 is longer than set2.

Is there something to be gain from this being an infinite loop?

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This code does not change the behavior of repeat when calling last from Repeat as it existed prior to this PR. The default implementation it replaces calls next until None is returned, which never happens for Repeat.

}

fn count(self) -> usize {
loop {}
}
}

#[stable(feature = "rust1", since = "1.0.0")]
Expand All @@ -84,6 +106,19 @@ impl<A: Clone> DoubleEndedIterator for Repeat<A> {
fn next_back(&mut self) -> Option<A> {
Some(self.element.clone())
}

#[inline]
fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
// Advancing an infinite iterator of a single element is a no-op.
let _ = n;
Ok(())
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<A> {
let _ = n;
Some(self.element.clone())
}
}

#[stable(feature = "fused", since = "1.26.0")]
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