Rollup merge of #76492 - fusion-engineering-forks:int-bits, r=dtolnay

Add associated constant `BITS` to all integer types

Recently I've regularly come across this snippet (in a few different crates, including `core` and `std`):
```rust
std::mem::size_of<usize>() * 8
```

I think it's time for a `usize::BITS`.

library/alloc/src/collections/binary_heap.rs

@@ -146,7 +146,7 @@
 
 use core::fmt;
 use core::iter::{FromIterator, FusedIterator, InPlaceIterable, SourceIter, TrustedLen};
-use core::mem::{self, size_of, swap, ManuallyDrop};
+use core::mem::{self, swap, ManuallyDrop};
 use core::ops::{Deref, DerefMut};
 use core::ptr;
 
@@ -617,7 +617,7 @@ impl<T: Ord> BinaryHeap<T> {
 
         #[inline(always)]
         fn log2_fast(x: usize) -> usize {
-            8 * size_of::<usize>() - (x.leading_zeros() as usize) - 1
+            (usize::BITS - x.leading_zeros() - 1) as usize
         }
 
         // `rebuild` takes O(len1 + len2) operations

library/alloc/src/lib.rs

@@ -101,6 +101,7 @@
 #![feature(fn_traits)]
 #![feature(fundamental)]
 #![feature(inplace_iteration)]
+#![feature(int_bits_const)]
 #![feature(lang_items)]
 #![feature(layout_for_ptr)]
 #![feature(libc)]

library/alloc/src/raw_vec.rs

@@ -528,7 +528,7 @@ unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
 
 #[inline]
 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
-    if mem::size_of::<usize>() < 8 && alloc_size > isize::MAX as usize {
+    if usize::BITS < 64 && alloc_size > isize::MAX as usize {
         Err(CapacityOverflow)
     } else {
         Ok(())