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Auto merge of #39399 - clarcharr:iter_rfind, r=alexcrichton
Add Iterator::rfind. I found it weird that `Iterator` has `rpostition` but not `rfind`. This adds that method.
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src/libcore/iter/traits.rs

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@@ -414,6 +414,64 @@ pub trait DoubleEndedIterator: Iterator {
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/// ```
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#[stable(feature = "rust1", since = "1.0.0")]
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fn next_back(&mut self) -> Option<Self::Item>;
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/// Searches for an element of an iterator from the right that satisfies a predicate.
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///
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/// `rfind()` takes a closure that returns `true` or `false`. It applies
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/// this closure to each element of the iterator, starting at the end, and if any
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/// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
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/// `false`, it returns [`None`].
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///
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/// `rfind()` is short-circuiting; in other words, it will stop processing
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/// as soon as the closure returns `true`.
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///
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/// Because `rfind()` takes a reference, and many iterators iterate over
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/// references, this leads to a possibly confusing situation where the
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/// argument is a double reference. You can see this effect in the
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/// examples below, with `&&x`.
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///
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/// [`Some(element)`]: ../../std/option/enum.Option.html#variant.Some
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/// [`None`]: ../../std/option/enum.Option.html#variant.None
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///
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/// # Examples
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///
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/// Basic usage:
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///
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/// ```
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/// #![feature(iter_rfind)]
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///
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/// let a = [1, 2, 3];
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///
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/// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
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///
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/// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
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/// ```
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///
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/// Stopping at the first `true`:
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///
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/// ```
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/// #![feature(iter_rfind)]
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///
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/// let a = [1, 2, 3];
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///
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/// let mut iter = a.iter();
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///
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/// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
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///
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/// // we can still use `iter`, as there are more elements.
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/// assert_eq!(iter.next_back(), Some(&1));
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/// ```
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#[inline]
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#[unstable(feature = "iter_rfind", issue = "39480")]
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fn rfind<P>(&mut self, mut predicate: P) -> Option<Self::Item> where
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Self: Sized,
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P: FnMut(&Self::Item) -> bool
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{
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for x in self.by_ref().rev() {
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if predicate(&x) { return Some(x) }
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}
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None
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}
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}
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#[stable(feature = "rust1", since = "1.0.0")]

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