Auto merge of rust-lang#119599 - marthadev:position, r=<try>

Rewrite Iterator::position default impl

Storing the accumulating value outside the fold in an attempt to improve code generation has shown speedups on various handwritten benchmarks, see discussion at rust-lang#119551.

Author: bors

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

@@ -3094,16 +3094,23 @@ pub trait Iterator {
         P: FnMut(Self::Item) -> bool,
     {
         #[inline]
-        fn check<T>(
-            mut predicate: impl FnMut(T) -> bool,
-        ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
+        fn check<'a, T>(
+            mut predicate: impl FnMut(T) -> bool + 'a,
+            acc: &'a mut usize,
+        ) -> impl FnMut((), T) -> ControlFlow<usize, ()> + 'a {
             #[rustc_inherit_overflow_checks]
-            move |i, x| {
-                if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
+            move |_, x| {
+                if predicate(x) {
+                    ControlFlow::Break(*acc)
+                } else {
+                    *acc += 1;
+                    ControlFlow::Continue(())
+                }
             }
         }
 
-        self.try_fold(0, check(predicate)).break_value()
+        let mut acc = 0;
+        self.try_fold((), check(predicate, &mut acc)).break_value()
     }
 
     /// Searches for an element in an iterator from the right, returning its