Rollup merge of #48157 - scottmcm:try-for-each, r=dtolnay
Add Iterator::try_for_each The fallible version of `for_each` aka the stateless version of `try_fold`. Inspired by @cuviper's comment in https://github.com/rust-lang/rust/pull/45379#issuecomment-338370020 as a more direct and obvious solution than `.map(f).collect::<Result<(), _>>()`. Like `for_each`, no need for an `r` version thanks to overrides in `Rev`. `iterator_try_fold` tracking issue: https://github.com/rust-lang/rust/issues/45594
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@ -1366,9 +1366,9 @@ pub trait Iterator {
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///
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/// In particular, try to have this call `try_fold()` on the internal parts
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/// from which this iterator is composed. If multiple calls are needed,
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/// the `?` operator be convenient for chaining the accumulator value along,
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/// but beware any invariants that need to be upheld before those early
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/// returns. This is a `&mut self` method, so iteration needs to be
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/// the `?` operator may be convenient for chaining the accumulator value
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/// along, but beware any invariants that need to be upheld before those
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/// early returns. This is a `&mut self` method, so iteration needs to be
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/// resumable after hitting an error here.
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///
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/// # Examples
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@ -1414,6 +1414,42 @@ pub trait Iterator {
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Try::from_ok(accum)
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}
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/// An iterator method that applies a fallible function to each item in the
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/// iterator, stopping at the first error and returning that error.
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///
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/// This can also be thought of as the fallible form of [`for_each()`]
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/// or as the stateless version of [`try_fold()`].
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///
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/// [`for_each()`]: #method.for_each
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/// [`try_fold()`]: #method.try_fold
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///
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/// # Examples
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///
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/// ```
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/// #![feature(iterator_try_fold)]
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/// use std::fs::rename;
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/// use std::io::{stdout, Write};
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/// use std::path::Path;
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///
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/// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
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///
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/// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
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/// assert!(res.is_ok());
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///
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/// let mut it = data.iter().cloned();
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/// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
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/// assert!(res.is_err());
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/// // It short-circuited, so the remaining items are still in the iterator:
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/// assert_eq!(it.next(), Some("stale_bread.json"));
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/// ```
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#[inline]
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#[unstable(feature = "iterator_try_fold", issue = "45594")]
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fn try_for_each<F, R>(&mut self, mut f: F) -> R where
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Self: Sized, F: FnMut(Self::Item) -> R, R: Try<Ok=()>
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{
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self.try_fold((), move |(), x| f(x))
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}
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/// An iterator method that applies a function, producing a single, final value.
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///
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/// `fold()` takes two arguments: an initial value, and a closure with two
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@ -1532,7 +1568,7 @@ pub trait Iterator {
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fn all<F>(&mut self, mut f: F) -> bool where
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Self: Sized, F: FnMut(Self::Item) -> bool
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{
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self.try_fold((), move |(), x| {
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self.try_for_each(move |x| {
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if f(x) { LoopState::Continue(()) }
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else { LoopState::Break(()) }
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}) == LoopState::Continue(())
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@ -1581,7 +1617,7 @@ pub trait Iterator {
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Self: Sized,
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F: FnMut(Self::Item) -> bool
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{
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self.try_fold((), move |(), x| {
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self.try_for_each(move |x| {
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if f(x) { LoopState::Break(()) }
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else { LoopState::Continue(()) }
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}) == LoopState::Break(())
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@ -1635,7 +1671,7 @@ pub trait Iterator {
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Self: Sized,
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P: FnMut(&Self::Item) -> bool,
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{
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self.try_fold((), move |(), x| {
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self.try_for_each(move |x| {
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if predicate(&x) { LoopState::Break(x) }
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else { LoopState::Continue(()) }
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}).break_value()
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