Rollup merge of rust-lang#94041 - a-lafrance:try-collect, r=scottmcm

Add a `try_collect()` helper method to `Iterator`

Implement `Iterator::try_collect()` as a helper around `Iterator::collect()` as discussed [here](https://internals.rust-lang.org/t/idea-fallible-iterator-mapping-with-try-map/15715/5?u=a.lafrance).

First time contributor so definitely open to any feedback about my implementation! Specifically wondering if I should open a tracking issue for the unstable feature I introduced.

As the main participant in the internals discussion: r? `@scottmcm`

Author: matthiaskrgr

library/core/src/iter/traits/iterator.rs

@@ -1,6 +1,7 @@
 use crate::cmp::{self, Ordering};
 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
 
+use super::super::try_process;
 use super::super::TrustedRandomAccessNoCoerce;
 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
 use super::super::{FlatMap, Flatten};
@@ -1777,6 +1778,87 @@ pub trait Iterator {
         FromIterator::from_iter(self)
     }
 
+    /// Fallibly transforms an iterator into a collection, short circuiting if
+    /// a failure is encountered.
+    ///
+    /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
+    /// conversions during collection. Its main use case is simplifying conversions from
+    /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
+    /// types (e.g. [`Result`]).
+    ///
+    /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
+    /// only the inner type produced on `Try::Output` must implement it. Concretely,
+    /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
+    /// [`FromIterator`], even though [`ControlFlow`] doesn't.
+    ///
+    /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
+    /// may continue to be used, in which case it will continue iterating starting after the element that
+    /// triggered the failure. See the last example below for an example of how this works.
+    ///
+    /// # Examples
+    /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
+    /// ```
+    /// #![feature(iterator_try_collect)]
+    ///
+    /// let u = vec![Some(1), Some(2), Some(3)];
+    /// let v = u.into_iter().try_collect::<Vec<i32>>();
+    /// assert_eq!(v, Some(vec![1, 2, 3]));
+    /// ```
+    ///
+    /// Failing to collect in the same way:
+    /// ```
+    /// #![feature(iterator_try_collect)]
+    ///
+    /// let u = vec![Some(1), Some(2), None, Some(3)];
+    /// let v = u.into_iter().try_collect::<Vec<i32>>();
+    /// assert_eq!(v, None);
+    /// ```
+    ///
+    /// A similar example, but with `Result`:
+    /// ```
+    /// #![feature(iterator_try_collect)]
+    ///
+    /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
+    /// let v = u.into_iter().try_collect::<Vec<i32>>();
+    /// assert_eq!(v, Ok(vec![1, 2, 3]));
+    ///
+    /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
+    /// let v = u.into_iter().try_collect::<Vec<i32>>();
+    /// assert_eq!(v, Err(()));
+    /// ```
+    ///
+    /// Finally, even [`ControlFlow`] works, despite the fact that it
+    /// doesn't implement [`FromIterator`]. Note also that the iterator can
+    /// continue to be used, even if a failure is encountered:
+    ///
+    /// ```
+    /// #![feature(iterator_try_collect)]
+    ///
+    /// use core::ops::ControlFlow::{Break, Continue};
+    ///
+    /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
+    /// let mut it = u.into_iter();
+    ///
+    /// let v = it.try_collect::<Vec<_>>();
+    /// assert_eq!(v, Break(3));
+    ///
+    /// let v = it.try_collect::<Vec<_>>();
+    /// assert_eq!(v, Continue(vec![4, 5]));
+    /// ```
+    ///
+    /// [`collect`]: Iterator::collect
+    #[inline]
+    #[unstable(feature = "iterator_try_collect", issue = "94047")]
+    fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
+    where
+        Self: Sized,
+        <Self as Iterator>::Item: Try,
+        <<Self as Iterator>::Item as Try>::Residual: Residual<B>,
+        B: FromIterator<<Self::Item as Try>::Output>,
+    {
+        try_process(self, |i| i.collect())
+    }
+
     /// Consumes an iterator, creating two collections from it.
     ///
     /// The predicate passed to `partition()` can return `true`, or `false`.

library/core/tests/iter/traits/iterator.rs

@@ -497,6 +497,52 @@ fn test_collect() {
     assert!(a == b);
 }
 
+#[test]
+fn test_try_collect() {
+    use core::ops::ControlFlow::{Break, Continue};
+
+    let u = vec![Some(1), Some(2), Some(3)];
+    let v = u.into_iter().try_collect::<Vec<i32>>();
+    assert_eq!(v, Some(vec![1, 2, 3]));
+
+    let u = vec![Some(1), Some(2), None, Some(3)];
+    let mut it = u.into_iter();
+    let v = it.try_collect::<Vec<i32>>();
+    assert_eq!(v, None);
+    let v = it.try_collect::<Vec<i32>>();
+    assert_eq!(v, Some(vec![3]));
+
+    let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
+    let v = u.into_iter().try_collect::<Vec<i32>>();
+    assert_eq!(v, Ok(vec![1, 2, 3]));
+
+    let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
+    let v = u.into_iter().try_collect::<Vec<i32>>();
+    assert_eq!(v, Err(()));
+
+    let numbers = vec![1, 2, 3, 4, 5];
+    let all_positive = numbers
+        .iter()
+        .cloned()
+        .map(|n| if n > 0 { Some(n) } else { None })
+        .try_collect::<Vec<i32>>();
+    assert_eq!(all_positive, Some(numbers));
+
+    let numbers = vec![-2, -1, 0, 1, 2];
+    let all_positive =
+        numbers.into_iter().map(|n| if n > 0 { Some(n) } else { None }).try_collect::<Vec<i32>>();
+    assert_eq!(all_positive, None);
+
+    let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
+    let mut it = u.into_iter();
+
+    let v = it.try_collect::<Vec<_>>();
+    assert_eq!(v, Break(3));
+
+    let v = it.try_collect::<Vec<_>>();
+    assert_eq!(v, Continue(vec![4, 5]));
+}
+
 // just tests by whether or not this compiles
 fn _empty_impl_all_auto_traits<T>() {
     use std::panic::{RefUnwindSafe, UnwindSafe};

library/core/tests/lib.rs

@@ -67,6 +67,7 @@
 #![feature(iter_intersperse)]
 #![feature(iter_is_partitioned)]
 #![feature(iter_order_by)]
+#![feature(iterator_try_collect)]
 #![feature(iterator_try_reduce)]
 #![feature(const_mut_refs)]
 #![feature(const_pin)]