Rollup merge of #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.

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/rustc_index/src/bit_set.rs b/compiler/rustc_index/src/bit_set.rs
index a4885aabe1f..645d95b1dba 100644
--- a/compiler/rustc_index/src/bit_set.rs
+++ b/compiler/rustc_index/src/bit_set.rs
@@ -1744,13 +1744,13 @@ impl<R: Idx, C: Idx> SparseBitMatrix<R, C> {
 
 #[inline]
 fn num_words<T: Idx>(domain_size: T) -> usize {
-    (domain_size.index() + WORD_BITS - 1) / WORD_BITS
+    domain_size.index().div_ceil(WORD_BITS)
 }
 
 #[inline]
 fn num_chunks<T: Idx>(domain_size: T) -> usize {
     assert!(domain_size.index() > 0);
-    (domain_size.index() + CHUNK_BITS - 1) / CHUNK_BITS
+    domain_size.index().div_ceil(CHUNK_BITS)
 }
 
 #[inline]