Auto merge of #62429 - cuviper:iter-closures, r=cramertj

Reduce the genericity of closures in the iterator traits

By default, closures inherit the generic parameters of their scope,
including `Self`. However, in most cases, the closures used to implement
iterators don't need to be generic on the iterator type, only its `Item`
type. We can reduce this genericity by redirecting such closures through
local functions.

This does make the closures more cumbersome to write, but it will
hopefully reduce duplication in their monomorphizations, as well as
their related type lengths.

src/libcore/iter/adapters/flatten.rs

@@ -229,16 +229,16 @@ where
                 if let elt@Some(_) = inner.next() { return elt }
             }
             match self.iter.next() {
-                None => return self.backiter.as_mut().and_then(|it| it.next()),
+                None => return self.backiter.as_mut()?.next(),
                 Some(inner) => self.frontiter = Some(inner.into_iter()),
             }
         }
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let (flo, fhi) = self.frontiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());
-        let (blo, bhi) = self.backiter.as_ref().map_or((0, Some(0)), |it| it.size_hint());
+        let (flo, fhi) = self.frontiter.as_ref().map_or((0, Some(0)), U::size_hint);
+        let (blo, bhi) = self.backiter.as_ref().map_or((0, Some(0)), U::size_hint);
         let lo = flo.saturating_add(blo);
         match (self.iter.size_hint(), fhi, bhi) {
             ((0, Some(0)), Some(a), Some(b)) => (lo, a.checked_add(b)),
@@ -250,20 +250,25 @@ where
     fn try_fold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
     {
+        #[inline]
+        fn flatten<'a, T: IntoIterator, Acc, R: Try<Ok = Acc>>(
+            frontiter: &'a mut Option<T::IntoIter>,
+            fold: &'a mut impl FnMut(Acc, T::Item) -> R,
+        ) -> impl FnMut(Acc, T) -> R + 'a {
+            move |acc, x| {
+                let mut mid = x.into_iter();
+                let r = mid.try_fold(acc, &mut *fold);
+                *frontiter = Some(mid);
+                r
+            }
+        }
+
         if let Some(ref mut front) = self.frontiter {
             init = front.try_fold(init, &mut fold)?;
         }
         self.frontiter = None;
 
-        {
-            let frontiter = &mut self.frontiter;
-            init = self.iter.try_fold(init, |acc, x| {
-                let mut mid = x.into_iter();
-                let r = mid.try_fold(acc, &mut fold);
-                *frontiter = Some(mid);
-                r
-            })?;
-        }
+        init = self.iter.try_fold(init, flatten(&mut self.frontiter, &mut fold))?;
         self.frontiter = None;
 
         if let Some(ref mut back) = self.backiter {
@@ -275,13 +280,20 @@ where
     }
 
     #[inline]
-    fn fold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
+    fn fold<Acc, Fold>(self, init: Acc, ref mut fold: Fold) -> Acc
         where Fold: FnMut(Acc, Self::Item) -> Acc,
     {
+        #[inline]
+        fn flatten<U: Iterator, Acc>(
+            fold: &mut impl FnMut(Acc, U::Item) -> Acc,
+        ) -> impl FnMut(Acc, U) -> Acc + '_ {
+            move |acc, iter| iter.fold(acc, &mut *fold)
+        }
+
         self.frontiter.into_iter()
             .chain(self.iter.map(IntoIterator::into_iter))
             .chain(self.backiter)
-            .fold(init, |acc, iter| iter.fold(acc, &mut fold))
+            .fold(init, flatten(fold))
     }
 }
 
@@ -297,7 +309,7 @@ where
                 if let elt@Some(_) = inner.next_back() { return elt }
             }
             match self.iter.next_back() {
-                None => return self.frontiter.as_mut().and_then(|it| it.next_back()),
+                None => return self.frontiter.as_mut()?.next_back(),
                 next => self.backiter = next.map(IntoIterator::into_iter),
             }
         }
@@ -307,20 +319,27 @@ where
     fn try_rfold<Acc, Fold, R>(&mut self, mut init: Acc, mut fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
     {
-        if let Some(ref mut back) = self.backiter {
-            init = back.try_rfold(init, &mut fold)?;
-        }
-        self.backiter = None;
-
+        #[inline]
+        fn flatten<'a, T: IntoIterator, Acc, R: Try<Ok = Acc>>(
+            backiter: &'a mut Option<T::IntoIter>,
+            fold: &'a mut impl FnMut(Acc, T::Item) -> R,
+        ) -> impl FnMut(Acc, T) -> R + 'a where
+            T::IntoIter: DoubleEndedIterator,
         {
-            let backiter = &mut self.backiter;
-            init = self.iter.try_rfold(init, |acc, x| {
+            move |acc, x| {
                 let mut mid = x.into_iter();
-                let r = mid.try_rfold(acc, &mut fold);
+                let r = mid.try_rfold(acc, &mut *fold);
                 *backiter = Some(mid);
                 r
-            })?;
+            }
         }
+
+        if let Some(ref mut back) = self.backiter {
+            init = back.try_rfold(init, &mut fold)?;
+        }
+        self.backiter = None;
+
+        init = self.iter.try_rfold(init, flatten(&mut self.backiter, &mut fold))?;
         self.backiter = None;
 
         if let Some(ref mut front) = self.frontiter {
@@ -332,12 +351,19 @@ where
     }
 
     #[inline]
-    fn rfold<Acc, Fold>(self, init: Acc, mut fold: Fold) -> Acc
+    fn rfold<Acc, Fold>(self, init: Acc, ref mut fold: Fold) -> Acc
         where Fold: FnMut(Acc, Self::Item) -> Acc,
     {
+        #[inline]
+        fn flatten<U: DoubleEndedIterator, Acc>(
+            fold: &mut impl FnMut(Acc, U::Item) -> Acc,
+        ) -> impl FnMut(Acc, U) -> Acc + '_ {
+            move |acc, iter| iter.rfold(acc, &mut *fold)
+        }
+
         self.frontiter.into_iter()
             .chain(self.iter.map(IntoIterator::into_iter))
             .chain(self.backiter)
-            .rfold(init, |acc, iter| iter.rfold(acc, &mut fold))
+            .rfold(init, flatten(fold))
     }
 }