Rollup merge of #58730 - timvermeulen:internal_iteration, r=scottmcm

Have all methods of Filter and FilterMap use internal iteration

This PR changes `Filter::{next, next_back, count}` and `FilterMap::{next, next_back}` to all use internal iteration. The `next` and `next_back` methods are changed to directly forward to `try_for_each` and `try_rfold` respectively, using `Result` as the `Try` type. I think that's okay? Alternatively, I could change their implementations to use `LoopState` instead if there's any benefit in doing so.

r? @scottmcm

src/libcore/benches/iter.rs

@@ -185,13 +185,13 @@ bench_sums! {
 bench_sums! {
     bench_filter_sum,
     bench_filter_ref_sum,
-    (0i64..1000000).filter(|x| x % 2 == 0)
+    (0i64..1000000).filter(|x| x % 3 == 0)
 }
 
 bench_sums! {
     bench_filter_chain_sum,
     bench_filter_chain_ref_sum,
-    (0i64..1000000).chain(0..1000000).filter(|x| x % 2 == 0)
+    (0i64..1000000).chain(0..1000000).filter(|x| x % 3 == 0)
 }
 
 bench_sums! {
@@ -306,3 +306,31 @@ fn bench_skip_then_zip(b: &mut Bencher) {
         assert_eq!(s, 2009900);
     });
 }
+
+#[bench]
+fn bench_filter_count(b: &mut Bencher) {
+    b.iter(|| {
+        (0i64..1000000).map(black_box).filter(|x| x % 3 == 0).count()
+    })
+}
+
+#[bench]
+fn bench_filter_ref_count(b: &mut Bencher) {
+    b.iter(|| {
+        (0i64..1000000).map(black_box).by_ref().filter(|x| x % 3 == 0).count()
+    })
+}
+
+#[bench]
+fn bench_filter_chain_count(b: &mut Bencher) {
+    b.iter(|| {
+        (0i64..1000000).chain(0..1000000).map(black_box).filter(|x| x % 3 == 0).count()
+    })
+}
+
+#[bench]
+fn bench_filter_chain_ref_count(b: &mut Bencher) {
+    b.iter(|| {
+        (0i64..1000000).chain(0..1000000).map(black_box).by_ref().filter(|x| x % 3 == 0).count()
+    })
+}

src/libcore/iter/adapters/mod.rs

@@ -681,12 +681,7 @@ impl<I: Iterator, P> Iterator for Filter<I, P> where P: FnMut(&I::Item) -> bool
 
     #[inline]
     fn next(&mut self) -> Option<I::Item> {
-        for x in &mut self.iter {
+        self.try_for_each(Err).err()
-            if (self.predicate)(&x) {
-                return Some(x);
-            }
-        }
-        None
     }
 
     #[inline]
@@ -707,12 +702,9 @@ impl<I: Iterator, P> Iterator for Filter<I, P> where P: FnMut(&I::Item) -> bool
     // Using the branchless version will also simplify the LLVM byte code, thus
     // leaving more budget for LLVM optimizations.
     #[inline]
-    fn count(mut self) -> usize {
+    fn count(self) -> usize {
-        let mut count = 0;
+        let mut predicate = self.predicate;
-        for x in &mut self.iter {
+        self.iter.map(|x| predicate(&x) as usize).sum()
-            count += (self.predicate)(&x) as usize;
-        }
-        count
     }
 
     #[inline]
@@ -746,12 +738,7 @@ impl<I: DoubleEndedIterator, P> DoubleEndedIterator for Filter<I, P>
 {
     #[inline]
     fn next_back(&mut self) -> Option<I::Item> {
-        for x in self.iter.by_ref().rev() {
+        self.try_rfold((), |_, x| Err(x)).err()
-            if (self.predicate)(&x) {
-                return Some(x);
-            }
-        }
-        None
     }
 
     #[inline]
@@ -820,12 +807,7 @@ impl<B, I: Iterator, F> Iterator for FilterMap<I, F>
 
     #[inline]
     fn next(&mut self) -> Option<B> {
-        for x in self.iter.by_ref() {
+        self.try_for_each(Err).err()
-            if let Some(y) = (self.f)(x) {
-                return Some(y);
-            }
-        }
-        None
     }
 
     #[inline]
@@ -863,12 +845,7 @@ impl<B, I: DoubleEndedIterator, F> DoubleEndedIterator for FilterMap<I, F>
 {
     #[inline]
     fn next_back(&mut self) -> Option<B> {
-        for x in self.iter.by_ref().rev() {
+        self.try_rfold((), |_, x| Err(x)).err()
-            if let Some(y) = (self.f)(x) {
-                return Some(y);
-            }
-        }
-        None
     }
 
     #[inline]