Do we still need the NeverShortCircuit type?

This was added back before `ControlFlow` existed, when it was definitely better than `Result<T, !>` by avoiding the `From::from` calls.  But now that we have `ControlFlow`, maybe we can avoid the extra type and impls?

(Still keeps the helper methods for wrapping closures and such, of course.)

Author: scottmcm

library/core/src/array/iter/iter_inner.rs

@@ -199,7 +199,7 @@ impl<T> PolymorphicIter<[MaybeUninit<T>]> {
 
     #[inline]
     pub(super) fn fold<B>(&mut self, init: B, f: impl FnMut(B, T) -> B) -> B {
-        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
+        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).into_continue()
     }
 
     #[inline]
@@ -257,7 +257,7 @@ impl<T> PolymorphicIter<[MaybeUninit<T>]> {
 
     #[inline]
     pub(super) fn rfold<B>(&mut self, init: B, f: impl FnMut(B, T) -> B) -> B {
-        self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).0
+        self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).into_continue()
     }
 
     #[inline]

library/core/src/array/mod.rs

@@ -109,7 +109,7 @@ pub fn from_fn<T, const N: usize, F>(f: F) -> [T; N]
 where
     F: FnMut(usize) -> T,
 {
-    try_from_fn(NeverShortCircuit::wrap_mut_1(f)).0
+    try_from_fn(NeverShortCircuit::wrap_mut_1(f)).into_continue()
 }
 
 /// Creates an array `[T; N]` where each fallible array element `T` is returned by the `cb` call.
@@ -553,7 +553,7 @@ impl<T, const N: usize> [T; N] {
     where
         F: FnMut(T) -> U,
     {
-        self.try_map(NeverShortCircuit::wrap_mut_1(f)).0
+        self.try_map(NeverShortCircuit::wrap_mut_1(f)).into_continue()
     }
 
     /// A fallible function `f` applied to each element on array `self` in order to
@@ -848,7 +848,7 @@ impl<T, const N: usize> [T; N] {
 /// it easily optimizes away) so it doesn't impact the loop that fills the array.
 #[inline]
 fn from_trusted_iterator<T, const N: usize>(iter: impl UncheckedIterator<Item = T>) -> [T; N] {
-    try_from_trusted_iterator(iter.map(NeverShortCircuit)).0
+    try_from_trusted_iterator(iter.map(NeverShortCircuit::Continue)).into_continue()
 }
 
 #[inline]

library/core/src/iter/adapters/array_chunks.rs

@@ -212,7 +212,7 @@ where
         Self: Sized,
         F: FnMut(B, Self::Item) -> B,
     {
-        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
+        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).into_continue()
     }
 }
 

library/core/src/iter/adapters/by_ref_sized.rs

@@ -42,7 +42,7 @@ impl<I: Iterator> Iterator for ByRefSized<'_, I> {
         F: FnMut(B, Self::Item) -> B,
     {
         // `fold` needs ownership, so this can't forward directly.
-        I::try_fold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).0
+        I::try_fold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).into_continue()
     }
 
     #[inline]
@@ -78,7 +78,7 @@ impl<I: DoubleEndedIterator> DoubleEndedIterator for ByRefSized<'_, I> {
         F: FnMut(B, Self::Item) -> B,
     {
         // `rfold` needs ownership, so this can't forward directly.
-        I::try_rfold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).0
+        I::try_rfold(self.0, init, NeverShortCircuit::wrap_mut_2(f)).into_continue()
     }
 
     #[inline]

library/core/src/iter/adapters/take.rs

@@ -266,7 +266,7 @@ impl<I: Iterator> SpecTake for Take<I> {
         F: FnMut(B, Self::Item) -> B,
     {
         use crate::ops::NeverShortCircuit;
-        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
+        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).into_continue()
     }
 
     #[inline]

library/core/src/iter/mod.rs

@@ -377,7 +377,7 @@ macro_rules! impl_fold_via_try_fold {
         {
             use crate::ops::NeverShortCircuit;
 
-            self.$try_fold(init, NeverShortCircuit::wrap_mut_2(fold)).0
+            self.$try_fold(init, NeverShortCircuit::wrap_mut_2(fold)).into_continue()
         }
     };
 }

library/core/src/ops/index_range.rs

@@ -150,7 +150,7 @@ impl Iterator for IndexRange {
 
     #[inline]
     fn fold<B, F: FnMut(B, usize) -> B>(mut self, init: B, f: F) -> B {
-        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).0
+        self.try_fold(init, NeverShortCircuit::wrap_mut_2(f)).into_continue()
     }
 
     #[inline]
@@ -192,7 +192,7 @@ impl DoubleEndedIterator for IndexRange {
 
     #[inline]
     fn rfold<B, F: FnMut(B, usize) -> B>(mut self, init: B, f: F) -> B {
-        self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).0
+        self.try_rfold(init, NeverShortCircuit::wrap_mut_2(f)).into_continue()
     }
 
     #[inline]

library/core/src/ops/try_trait.rs

@@ -1,3 +1,4 @@
+use crate::convert::Infallible;
 use crate::ops::ControlFlow;
 
 /// The `?` operator and `try {}` blocks.
@@ -384,15 +385,8 @@ pub fn residual_into_try_type<R: Residual<O>, O>(r: R) -> <R as Residual<O>>::Tr
 pub(crate) type ChangeOutputType<T: Try<Residual: Residual<V>>, V> =
     <T::Residual as Residual<V>>::TryType;
 
-/// 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);
+/// A helper alias for implementing non-try methods via the `Try` implementation.
+pub(crate) type NeverShortCircuit<T> = ControlFlow<Infallible, T>;
 
 impl<T> NeverShortCircuit<T> {
     /// Wraps a unary function to produce one that wraps the output into a `NeverShortCircuit`.
@@ -403,43 +397,21 @@ impl<T> NeverShortCircuit<T> {
     pub(crate) fn wrap_mut_1<A>(
         mut f: impl FnMut(A) -> T,
     ) -> impl FnMut(A) -> NeverShortCircuit<T> {
-        move |a| NeverShortCircuit(f(a))
+        move |a| NeverShortCircuit::Continue(f(a))
     }
 
     #[inline]
     pub(crate) fn wrap_mut_2<A, B>(mut f: impl FnMut(A, B) -> T) -> impl FnMut(A, B) -> Self {
-        move |a, b| NeverShortCircuit(f(a, b))
-    }
-}
-
-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)
+        move |a, b| NeverShortCircuit::Continue(f(a, b))
     }
 
     #[inline]
-    fn from_output(x: T) -> Self {
-        NeverShortCircuit(x)
+    pub(crate) fn into_continue(self) -> T {
+        let NeverShortCircuit::Continue(v) = self;
+        v
     }
 }
 
-impl<T> FromResidual for NeverShortCircuit<T> {
-    #[inline]
-    fn from_residual(never: NeverShortCircuitResidual) -> Self {
-        match never {}
-    }
-}
-
-impl<T> Residual<T> for NeverShortCircuitResidual {
-    type TryType = NeverShortCircuit<T>;
-}
-
 /// Implement `FromResidual<Yeet<T>>` on your type to enable
 /// `do yeet expr` syntax in functions returning your type.
 #[unstable(feature = "try_trait_v2_yeet", issue = "96374")]