Auto merge of #138835 - joboet:fold-via-try_fold, r=<try>

core: implement `fold` via `try_fold`

... and `rfold` via `try_rfold`. It's not possible to implement `try_fold` via `fold`, so this cannot lead to recursion. While unfortunately `try_fold` cannot be currently implemented for user types on stable, this paves a path to a future where implementing a good `try_fold` is enough to make nearly all `Iterator` traversal methods perform optimally.

r? libs-api

Author: bors

library/alloc/src/collections/vec_deque/into_iter.rs

@@ -114,16 +114,6 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
             .try_fold(init, &mut f)
     }
 
-    #[inline]
-    fn fold<B, F>(mut self, init: B, mut f: F) -> B
-    where
-        F: FnMut(B, Self::Item) -> B,
-    {
-        match self.try_fold(init, |b, item| Ok::<B, !>(f(b, item))) {
-            Ok(b) => b,
-        }
-    }
-
     #[inline]
     fn last(mut self) -> Option<Self::Item> {
         self.inner.pop_back()

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

@@ -1,5 +1,5 @@
 use crate::num::NonZero;
-use crate::ops::{ControlFlow, Try};
+use crate::ops::{ControlFlow, NeverShortCircuit, Try};
 
 /// An iterator able to yield elements from both ends.
 ///
@@ -298,16 +298,12 @@ pub trait DoubleEndedIterator: Iterator {
     #[doc(alias = "foldr")]
     #[inline]
     #[stable(feature = "iter_rfold", since = "1.27.0")]
-    fn rfold<B, F>(mut self, init: B, mut f: F) -> B
+    fn rfold<B, F>(mut self, init: B, f: F) -> B
     where
         Self: Sized,
         F: FnMut(B, Self::Item) -> B,
     {
-        let mut accum = init;
-        while let Some(x) = self.next_back() {
-            accum = f(accum, x);
-        }
-        accum
+        self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).0
     }
 
     /// Searches for an element of an iterator from the back that satisfies a predicate.

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

@@ -7,7 +7,7 @@ use super::super::{
 use crate::array;
 use crate::cmp::{self, Ordering};
 use crate::num::NonZero;
-use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
+use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, NeverShortCircuit, Residual, Try};
 
 fn _assert_is_dyn_compatible(_: &dyn Iterator<Item = ()>) {}
 
@@ -2550,16 +2550,12 @@ pub trait Iterator {
     #[doc(alias = "inject", alias = "foldl")]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn fold<B, F>(mut self, init: B, mut f: F) -> B
+    fn fold<B, F>(mut self, init: B, f: F) -> B
     where
         Self: Sized,
         F: FnMut(B, Self::Item) -> B,
     {
-        let mut accum = init;
-        while let Some(x) = self.next() {
-            accum = f(accum, x);
-        }
-        accum
+        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
     }
 
     /// Reduces the elements to a single one, by repeatedly applying a reducing