Add an optimizer hint for the capacity that with_capacity_in returns

library/alloc/src/raw_vec.rs

@@ -423,7 +423,13 @@ impl<A: Allocator> RawVecInner<A> {
  #[inline]
  fn with_capacity_in(capacity: usize, alloc: A, elem_layout: Layout) -> Self {
  match Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc, elem_layout) {
- Ok(res) => res,
+ Ok(this) => {
+ unsafe {
+ // Make it more obvious that a subsquent Vec::reserve(capacity) will not allocate.
+ hint::assert_unchecked(!this.needs_to_grow(0, capacity, elem_layout));
+ }
+ this
+ }
  Err(err) => handle_error(err),
  }
  }