Use a macro to not have to copy-paste ConstFnMutClosure::new(&mut fold, NeverShortCircuit::wrap_mut_2_imp)).0 everywhere

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

@@ -1,7 +1,6 @@
 use crate::array;
-use crate::const_closure::ConstFnMutClosure;
 use crate::iter::{ByRefSized, FusedIterator, Iterator};
-use crate::ops::{ControlFlow, NeverShortCircuit, Try};
+use crate::ops::{ControlFlow, Try};
 
 /// An iterator over `N` elements of the iterator at a time.
 ///
@@ -83,13 +82,7 @@ where
         }
     }
 
-    fn fold<B, F>(mut self, init: B, mut f: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        self.try_fold(init, ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp)).0
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
@@ -127,13 +120,7 @@ where
         try { acc }
     }
 
-    fn rfold<B, F>(mut self, init: B, mut f: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        self.try_rfold(init, ConstFnMutClosure::new(&mut f, NeverShortCircuit::wrap_mut_2_imp)).0
-    }
+    impl_fold_via_try_fold! { rfold -> try_rfold }
 }
 
 impl<I, const N: usize> ArrayChunks<I, N>

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

@@ -64,19 +64,7 @@ where
         .into_try()
     }
 
-    #[inline]
-    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Self: Sized,
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[unstable(issue = "none", feature = "inplace_iteration")]

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

@@ -1,6 +1,5 @@
-use crate::const_closure::ConstFnMutClosure;
 use crate::iter::{InPlaceIterable, Iterator};
-use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, NeverShortCircuit, Residual, Try};
+use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
 
 mod array_chunks;
 mod by_ref_sized;
@@ -204,13 +203,7 @@ where
             .into_try()
     }
 
-    fn fold<B, F>(mut self, init: B, mut fold: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        self.try_fold(init, ConstFnMutClosure::new(&mut fold, NeverShortCircuit::wrap_mut_2_imp)).0
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[unstable(issue = "none", feature = "inplace_iteration")]

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

@@ -74,19 +74,7 @@ where
         self.iter.try_fold(init, scan(state, f, fold)).into_try()
     }
 
-    #[inline]
-    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Self: Sized,
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[unstable(issue = "none", feature = "inplace_iteration")]

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

@@ -206,17 +206,7 @@ where
         if n == 0 { try { init } } else { self.iter.try_rfold(init, check(n, fold)).into_try() }
     }
 
-    fn rfold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<Acc, T>(mut f: impl FnMut(Acc, T) -> Acc) -> impl FnMut(Acc, T) -> Result<Acc, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_rfold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { rfold -> try_rfold }
 
     #[inline]
     fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {

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

@@ -98,19 +98,7 @@ where
         }
     }
 
-    #[inline]
-    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Self: Sized,
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 
     #[inline]
     #[rustc_inherit_overflow_checks]

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

@@ -94,19 +94,7 @@ where
         }
     }
 
-    #[inline]
-    fn fold<Acc, Fold>(mut self, init: Acc, fold: Fold) -> Acc
-    where
-        Self: Sized,
-        Fold: FnMut(Acc, Self::Item) -> Acc,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(fold)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]

library/core/src/iter/mod.rs

@@ -352,6 +352,29 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
+// This needs to be up here in order to be usable in the child modules
+macro_rules! impl_fold_via_try_fold {
+    (fold -> try_fold) => {
+        impl_fold_via_try_fold! { @internal fold -> try_fold }
+    };
+    (rfold -> try_rfold) => {
+        impl_fold_via_try_fold! { @internal rfold -> try_rfold }
+    };
+    (@internal $fold:ident -> $try_fold:ident) => {
+        #[inline]
+        fn $fold<AAA, FFF>(mut self, init: AAA, mut fold: FFF) -> AAA
+        where
+            FFF: FnMut(AAA, Self::Item) -> AAA,
+        {
+            use crate::const_closure::ConstFnMutClosure;
+            use crate::ops::NeverShortCircuit;
+
+            let fold = ConstFnMutClosure::new(&mut fold, NeverShortCircuit::wrap_mut_2_imp);
+            self.$try_fold(init, fold).0
+        }
+    };
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::traits::Iterator;
 

library/core/src/iter/range.rs

@@ -1150,19 +1150,7 @@ impl<A: Step> Iterator for ops::RangeInclusive<A> {
         self.spec_try_fold(init, f)
     }
 
-    #[inline]
-    fn fold<B, F>(mut self, init: B, f: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_fold(init, ok(f)).unwrap()
-    }
+    impl_fold_via_try_fold! { fold -> try_fold }
 
     #[inline]
     fn last(mut self) -> Option<A> {
@@ -1230,19 +1218,7 @@ impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> {
         self.spec_try_rfold(init, f)
     }
 
-    #[inline]
-    fn rfold<B, F>(mut self, init: B, f: F) -> B
-    where
-        Self: Sized,
-        F: FnMut(B, Self::Item) -> B,
-    {
-        #[inline]
-        fn ok<B, T>(mut f: impl FnMut(B, T) -> B) -> impl FnMut(B, T) -> Result<B, !> {
-            move |acc, x| Ok(f(acc, x))
-        }
-
-        self.try_rfold(init, ok(f)).unwrap()
-    }
+    impl_fold_via_try_fold! { rfold -> try_rfold }
 }
 
 // Safety: See above implementation for `ops::Range<A>`