Rollup merge of rust-lang#143477 - folkertdev:use-is-multiple-of, r=joshtriplett

use `is_multiple_of` and `div_ceil`

In tricky logic, these functions are much more informative than the manual implementations. They also catch subtle bugs:

- the manual `is_multiple_of` often does not handle division by zero
- manual `div_ceil` often does not consider overflow

The transformation is free for `is_multiple_of` if the divisor is compile-time known to be non-zero. For `div_ceil` there is a small cost to considering overflow. Here is some assembly https://godbolt.org/z/5zP8KaE1d.

Author: matthiaskrgr

core/src/slice/mod.rs

@@ -1316,7 +1316,7 @@ impl<T> [T] {
         assert_unsafe_precondition!(
             check_language_ub,
             "slice::as_chunks_unchecked requires `N != 0` and the slice to split exactly into `N`-element chunks",
-            (n: usize = N, len: usize = self.len()) => n != 0 && len % n == 0,
+            (n: usize = N, len: usize = self.len()) => n != 0 && len.is_multiple_of(n),
         );
         // SAFETY: Caller must guarantee that `N` is nonzero and exactly divides the slice length
         let new_len = unsafe { exact_div(self.len(), N) };
@@ -1512,7 +1512,7 @@ impl<T> [T] {
         assert_unsafe_precondition!(
             check_language_ub,
             "slice::as_chunks_unchecked requires `N != 0` and the slice to split exactly into `N`-element chunks",
-            (n: usize = N, len: usize = self.len()) => n != 0 && len % n == 0
+            (n: usize = N, len: usize = self.len()) => n != 0 && len.is_multiple_of(n)
         );
         // SAFETY: Caller must guarantee that `N` is nonzero and exactly divides the slice length
         let new_len = unsafe { exact_div(self.len(), N) };
@@ -4866,7 +4866,7 @@ impl<T> [T] {
 
         let byte_offset = elem_start.wrapping_sub(self_start);
 
-        if byte_offset % size_of::<T>() != 0 {
+        if !byte_offset.is_multiple_of(size_of::<T>()) {
             return None;
         }
 
@@ -4920,7 +4920,7 @@ impl<T> [T] {
 
         let byte_start = subslice_start.wrapping_sub(self_start);
 
-        if byte_start % size_of::<T>() != 0 {
+        if !byte_start.is_multiple_of(size_of::<T>()) {
             return None;
         }
 

core/src/slice/sort/shared/smallsort.rs

@@ -823,7 +823,7 @@ unsafe fn bidirectional_merge<T: FreezeMarker, F: FnMut(&T, &T) -> bool>(
         let right_end = right_rev.wrapping_add(1);
 
         // Odd length, so one element is left unconsumed in the input.
-        if len % 2 != 0 {
+        if !len.is_multiple_of(2) {
             let left_nonempty = left < left_end;
             let last_src = if left_nonempty { left } else { right };
             ptr::copy_nonoverlapping(last_src, dst, 1);

core/src/slice/sort/stable/drift.rs

@@ -158,7 +158,7 @@ fn merge_tree_scale_factor(n: usize) -> u64 {
         panic!("Platform not supported");
     }
 
-    ((1 << 62) + n as u64 - 1) / n as u64
+    (1u64 << 62).div_ceil(n as u64)
 }
 
 // Note: merge_tree_depth output is < 64 when left < right as f*x and f*y must
@@ -182,7 +182,7 @@ fn sqrt_approx(n: usize) -> usize {
     // Finally we note that the exponentiation / division can be done directly
     // with shifts. We OR with 1 to avoid zero-checks in the integer log.
     let ilog = (n | 1).ilog2();
-    let shift = (1 + ilog) / 2;
+    let shift = ilog.div_ceil(2);
     ((1 << shift) + (n >> shift)) / 2
 }
 

core/src/str/count.rs

@@ -52,7 +52,7 @@ fn do_count_chars(s: &str) -> usize {
     // Check the properties of `CHUNK_SIZE` and `UNROLL_INNER` that are required
     // for correctness.
     const _: () = assert!(CHUNK_SIZE < 256);
-    const _: () = assert!(CHUNK_SIZE % UNROLL_INNER == 0);
+    const _: () = assert!(CHUNK_SIZE.is_multiple_of(UNROLL_INNER));
 
     // SAFETY: transmuting `[u8]` to `[usize]` is safe except for size
     // differences which are handled by `align_to`.

core/src/str/iter.rs

@@ -102,7 +102,7 @@ impl<'a> Iterator for Chars<'a> {
         // `(len + 3)` can't overflow, because we know that the `slice::Iter`
         // belongs to a slice in memory which has a maximum length of
         // `isize::MAX` (that's well below `usize::MAX`).
-        ((len + 3) / 4, Some(len))
+        (len.div_ceil(4), Some(len))
     }
 
     #[inline]
@@ -1532,11 +1532,11 @@ impl<'a> Iterator for EncodeUtf16<'a> {
         // belongs to a slice in memory which has a maximum length of
         // `isize::MAX` (that's well below `usize::MAX`)
         if self.extra == 0 {
-            ((len + 2) / 3, Some(len))
+            (len.div_ceil(3), Some(len))
         } else {
             // We're in the middle of a surrogate pair, so add the remaining
             // surrogate to the bounds.
-            ((len + 2) / 3 + 1, Some(len + 1))
+            (len.div_ceil(3) + 1, Some(len + 1))
         }
     }
 }

core/src/str/validations.rs

@@ -219,7 +219,7 @@ pub(super) const fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
             // Ascii case, try to skip forward quickly.
             // When the pointer is aligned, read 2 words of data per iteration
             // until we find a word containing a non-ascii byte.
-            if align != usize::MAX && align.wrapping_sub(index) % USIZE_BYTES == 0 {
+            if align != usize::MAX && align.wrapping_sub(index).is_multiple_of(USIZE_BYTES) {
                 let ptr = v.as_ptr();
                 while index < blocks_end {
                     // SAFETY: since `align - index` and `ascii_block_size` are

portable-simd/crates/core_simd/src/lane_count.rs

@@ -8,7 +8,7 @@ pub struct LaneCount<const N: usize>;
 
 impl<const N: usize> LaneCount<N> {
     /// The number of bytes in a bitmask with this many lanes.
-    pub const BITMASK_LEN: usize = (N + 7) / 8;
+    pub const BITMASK_LEN: usize = N.div_ceil(8);
 }
 
 /// Statically guarantees that a lane count is marked as supported.