[Perf Experiment] Introduce NeverShortCircuit to see if it helps fold-via-try_fold

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

@@ -1,4 +1,4 @@
-use crate::ops::{ControlFlow, Try};
+use crate::ops::{ControlFlow, NeverShortCircuit, Try};
 
 /// An iterator able to yield elements from both ends.
 ///
@@ -292,16 +292,17 @@ 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);
+        #[inline]
+        fn call<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> NeverShortCircuit<B> {
+            move |accum, item| NeverShortCircuit(f(accum, item))
         }
-        accum
+
+        self.try_rfold(init, call(f)).0
     }
 
     /// Searches for an element of an iterator from the back that satisfies a predicate.

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

@@ -1,5 +1,5 @@
 use crate::cmp::{self, Ordering};
-use crate::ops::{ControlFlow, Try};
+use crate::ops::{ControlFlow, NeverShortCircuit, Try};
 
 use super::super::TrustedRandomAccessNoCoerce;
 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
@@ -2161,16 +2161,17 @@ 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);
+        #[inline]
+        fn call<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> NeverShortCircuit<B> {
+            move |accum, item| NeverShortCircuit(f(accum, item))
         }
-        accum
+
+        self.try_fold(init, call(f)).0
     }
 
     /// Reduces the elements to a single one, by repeatedly applying a reducing

library/core/src/ops/mod.rs

@@ -184,6 +184,8 @@ pub use self::range::{Bound, RangeBounds, RangeInclusive, RangeToInclusive};
 #[unstable(feature = "try_trait_v2", issue = "84277")]
 pub use self::try_trait::{FromResidual, Try};
 
+pub(crate) use self::try_trait::NeverShortCircuit;
+
 #[unstable(feature = "generator_trait", issue = "43122")]
 pub use self::generator::{Generator, GeneratorState};
 

library/core/src/ops/try_trait.rs

@@ -338,3 +338,37 @@ pub trait FromResidual<R = <Self as Try>::Residual> {
     #[unstable(feature = "try_trait_v2", issue = "84277")]
     fn from_residual(residual: R) -> Self;
 }
+
+/// An adapter for implementing non-try methods via the `Try` implementation.
+///
+/// Conceptually the same as `Result<T, !>`, but requiring less work in trait
+/// solving and inhabited-ness checking and such, by being an obvious newtype
+/// and not having `From` bounds lying around.
+///
+/// Not currently planned to be exposed publicly, so just `pub(crate)`.
+#[repr(transparent)]
+pub(crate) struct NeverShortCircuit<T>(pub T);
+
+pub(crate) enum NeverShortCircuitResidual {}
+
+impl<T> Try for NeverShortCircuit<T> {
+    type Output = T;
+    type Residual = NeverShortCircuitResidual;
+
+    #[inline]
+    fn branch(self) -> ControlFlow<NeverShortCircuitResidual, T> {
+        ControlFlow::Continue(self.0)
+    }
+
+    #[inline]
+    fn from_output(x: T) -> Self {
+        NeverShortCircuit(x)
+    }
+}
+
+impl<T> FromResidual for NeverShortCircuit<T> {
+    #[inline]
+    fn from_residual(never: NeverShortCircuitResidual) -> Self {
+        match never {}
+    }
+}

src/test/ui/issues/issue-3044.stderr

@@ -11,7 +11,7 @@ LL | |     });
 note: associated function defined here
   --> $SRC_DIR/core/src/iter/traits/iterator.rs:LL:COL
    |
-LL |     fn fold<B, F>(mut self, init: B, mut f: F) -> B
+LL |     fn fold<B, F>(mut self, init: B, f: F) -> B
    |        ^^^^
 
 error: aborting due to previous error