Implement as_chunks with split_at_unchecked

Author: scottmcm

library/core/src/slice/mod.rs

@@ -1337,8 +1337,10 @@ impl<T> [T] {
     #[must_use]
     pub const fn as_chunks<const N: usize>(&self) -> (&[[T; N]], &[T]) {
         assert!(N != 0, "chunk size must be non-zero");
-        let len = self.len() / N;
-        let (multiple_of_n, remainder) = self.split_at(len * N);
+        let len_rounded_down = self.len() / N * N;
+        // SAFETY: The rounded-down value is always the same or smaller than the
+        // original length, and thus must be in-bounds of the slice.
+        let (multiple_of_n, remainder) = unsafe { self.split_at_unchecked(len_rounded_down) };
         // SAFETY: We already panicked for zero, and ensured by construction
         // that the length of the subslice is a multiple of N.
         let array_slice = unsafe { multiple_of_n.as_chunks_unchecked() };
@@ -1487,8 +1489,10 @@ impl<T> [T] {
     #[must_use]
     pub const fn as_chunks_mut<const N: usize>(&mut self) -> (&mut [[T; N]], &mut [T]) {
         assert!(N != 0, "chunk size must be non-zero");
-        let len = self.len() / N;
-        let (multiple_of_n, remainder) = self.split_at_mut(len * N);
+        let len_rounded_down = self.len() / N * N;
+        // SAFETY: The rounded-down value is always the same or smaller than the
+        // original length, and thus must be in-bounds of the slice.
+        let (multiple_of_n, remainder) = unsafe { self.split_at_mut_unchecked(len_rounded_down) };
         // SAFETY: We already panicked for zero, and ensured by construction
         // that the length of the subslice is a multiple of N.
         let array_slice = unsafe { multiple_of_n.as_chunks_unchecked_mut() };