Auto merge of #111781 - the8472:filter-map-chunk, r=thomcc

optimize next_chunk impls for Filter and FilterMap

```
OLD:

benchmarks:
    iter::bench_next_chunk_filter_even                 104.00ns/iter  +/- 1.00ns
    iter::bench_next_chunk_filter_map_even             101.00ns/iter  +/- 1.00ns
    iter::bench_next_chunk_filter_map_mostly_false       1.99µs/iter +/- 10.00ns
    iter::bench_next_chunk_filter_map_predictably_true  56.00ns/iter  +/- 0.00ns
    iter::bench_next_chunk_filter_mostly_false           1.15µs/iter  +/- 6.00ns
    iter::bench_next_chunk_filter_predictably_true      65.00ns/iter  +/- 1.00ns

NEW:

benchmarks:
    iter::bench_next_chunk_filter_even                  42.00ns/iter  +/- 0.00ns
    iter::bench_next_chunk_filter_map_even              49.00ns/iter  +/- 1.00ns
    iter::bench_next_chunk_filter_map_mostly_false     501.00ns/iter  +/- 3.00ns
    iter::bench_next_chunk_filter_map_predictably_true  31.00ns/iter  +/- 0.00ns
    iter::bench_next_chunk_filter_mostly_false         534.00ns/iter +/- 13.00ns
    iter::bench_next_chunk_filter_predictably_true      28.00ns/iter  +/- 1.00ns
```

library/core/benches/iter.rs

@@ -404,7 +404,7 @@ fn bench_trusted_random_access_adapters(b: &mut Bencher) {
 
 /// Exercises the iter::Copied specialization for slice::Iter
 #[bench]
-fn bench_copied_chunks(b: &mut Bencher) {
+fn bench_next_chunk_copied(b: &mut Bencher) {
     let v = vec![1u8; 1024];
 
     b.iter(|| {
@@ -421,7 +421,7 @@ fn bench_copied_chunks(b: &mut Bencher) {
 
 /// Exercises the TrustedRandomAccess specialization in ArrayChunks
 #[bench]
-fn bench_trusted_random_access_chunks(b: &mut Bencher) {
+fn bench_next_chunk_trusted_random_access(b: &mut Bencher) {
     let v = vec![1u8; 1024];
 
     b.iter(|| {
@@ -437,3 +437,45 @@ fn bench_trusted_random_access_chunks(b: &mut Bencher) {
             .sum::<Wrapping<u64>>()
     })
 }
+
+#[bench]
+fn bench_next_chunk_filter_even(b: &mut Bencher) {
+    let a = (0..1024).next_chunk::<1024>().unwrap();
+
+    b.iter(|| black_box(&a).iter().filter(|&&i| i % 2 == 0).next_chunk::<32>())
+}
+
+#[bench]
+fn bench_next_chunk_filter_predictably_true(b: &mut Bencher) {
+    let a = (0..1024).next_chunk::<1024>().unwrap();
+
+    b.iter(|| black_box(&a).iter().filter(|&&i| i < 100).next_chunk::<32>())
+}
+
+#[bench]
+fn bench_next_chunk_filter_mostly_false(b: &mut Bencher) {
+    let a = (0..1024).next_chunk::<1024>().unwrap();
+
+    b.iter(|| black_box(&a).iter().filter(|&&i| i > 900).next_chunk::<32>())
+}
+
+#[bench]
+fn bench_next_chunk_filter_map_even(b: &mut Bencher) {
+    let a = (0..1024).next_chunk::<1024>().unwrap();
+
+    b.iter(|| black_box(&a).iter().filter_map(|&i| (i % 2 == 0).then(|| i)).next_chunk::<32>())
+}
+
+#[bench]
+fn bench_next_chunk_filter_map_predictably_true(b: &mut Bencher) {
+    let a = (0..1024).next_chunk::<1024>().unwrap();
+
+    b.iter(|| black_box(&a).iter().filter_map(|&i| (i < 100).then(|| i)).next_chunk::<32>())
+}
+
+#[bench]
+fn bench_next_chunk_filter_map_mostly_false(b: &mut Bencher) {
+    let a = (0..1024).next_chunk::<1024>().unwrap();
+
+    b.iter(|| black_box(&a).iter().filter_map(|&i| (i > 900).then(|| i)).next_chunk::<32>())
+}

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

@@ -1,6 +1,9 @@
 use crate::fmt;
 use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
 use crate::ops::Try;
+use core::array;
+use core::mem::{ManuallyDrop, MaybeUninit};
+use core::ops::ControlFlow;
 
 /// An iterator that filters the elements of `iter` with `predicate`.
 ///
@@ -56,6 +59,58 @@ fn next(&mut self) -> Option<I::Item> {
         self.iter.find(&mut self.predicate)
     }
 
+    #[inline]
+    fn next_chunk<const N: usize>(
+        &mut self,
+    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> {
+        let mut array: [MaybeUninit<Self::Item>; N] = MaybeUninit::uninit_array();
+
+        struct Guard<'a, T> {
+            array: &'a mut [MaybeUninit<T>],
+            initialized: usize,
+        }
+
+        impl<T> Drop for Guard<'_, T> {
+            #[inline]
+            fn drop(&mut self) {
+                if const { crate::mem::needs_drop::<T>() } {
+                    // SAFETY: self.initialized is always <= N, which also is the length of the array.
+                    unsafe {
+                        core::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
+                            self.array.get_unchecked_mut(..self.initialized),
+                        ));
+                    }
+                }
+            }
+        }
+
+        let mut guard = Guard { array: &mut array, initialized: 0 };
+
+        let result = self.iter.try_for_each(|element| {
+            let idx = guard.initialized;
+            guard.initialized = idx + (self.predicate)(&element) as usize;
+
+            // SAFETY: Loop conditions ensure the index is in bounds.
+            unsafe { guard.array.get_unchecked_mut(idx) }.write(element);
+
+            if guard.initialized < N { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
+        });
+
+        let guard = ManuallyDrop::new(guard);
+
+        match result {
+            ControlFlow::Break(()) => {
+                // SAFETY: The loop above is only explicitly broken when the array has been fully initialized
+                Ok(unsafe { MaybeUninit::array_assume_init(array) })
+            }
+            ControlFlow::Continue(()) => {
+                let initialized = guard.initialized;
+                // SAFETY: The range is in bounds since the loop breaks when reaching N elements.
+                Err(unsafe { array::IntoIter::new_unchecked(array, 0..initialized) })
+            }
+        }
+    }
+
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let (_, upper) = self.iter.size_hint();

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

@@ -1,6 +1,7 @@
-use crate::fmt;
 use crate::iter::{adapters::SourceIter, FusedIterator, InPlaceIterable};
+use crate::mem::{ManuallyDrop, MaybeUninit};
 use crate::ops::{ControlFlow, Try};
+use crate::{array, fmt};
 
 /// An iterator that uses `f` to both filter and map elements from `iter`.
 ///
@@ -61,6 +62,65 @@ fn next(&mut self) -> Option<B> {
         self.iter.find_map(&mut self.f)
     }
 
+    #[inline]
+    fn next_chunk<const N: usize>(
+        &mut self,
+    ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> {
+        let mut array: [MaybeUninit<Self::Item>; N] = MaybeUninit::uninit_array();
+
+        struct Guard<'a, T> {
+            array: &'a mut [MaybeUninit<T>],
+            initialized: usize,
+        }
+
+        impl<T> Drop for Guard<'_, T> {
+            #[inline]
+            fn drop(&mut self) {
+                if const { crate::mem::needs_drop::<T>() } {
+                    // SAFETY: self.initialized is always <= N, which also is the length of the array.
+                    unsafe {
+                        core::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(
+                            self.array.get_unchecked_mut(..self.initialized),
+                        ));
+                    }
+                }
+            }
+        }
+
+        let mut guard = Guard { array: &mut array, initialized: 0 };
+
+        let result = self.iter.try_for_each(|element| {
+            let idx = guard.initialized;
+            let val = (self.f)(element);
+            guard.initialized = idx + val.is_some() as usize;
+
+            // SAFETY: Loop conditions ensure the index is in bounds.
+
+            unsafe {
+                let opt_payload_at = core::intrinsics::option_payload_ptr(&val);
+                let dst = guard.array.as_mut_ptr().add(idx);
+                crate::ptr::copy_nonoverlapping(opt_payload_at.cast(), dst, 1);
+                crate::mem::forget(val);
+            };
+
+            if guard.initialized < N { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
+        });
+
+        let guard = ManuallyDrop::new(guard);
+
+        match result {
+            ControlFlow::Break(()) => {
+                // SAFETY: The loop above is only explicitly broken when the array has been fully initialized
+                Ok(unsafe { MaybeUninit::array_assume_init(array) })
+            }
+            ControlFlow::Continue(()) => {
+                let initialized = guard.initialized;
+                // SAFETY: The range is in bounds since the loop breaks when reaching N elements.
+                Err(unsafe { array::IntoIter::new_unchecked(array, 0..initialized) })
+            }
+        }
+    }
+
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let (_, upper) = self.iter.size_hint();