message: Checked capacity cast usize -> u32
  Summary:
  Investigating assertion in T132564771.
  This unchecked cast is one possibility.
diff: @@ -604,8 +604,10 @@ impl Heap {
  return FrozenValueTyped::new_repr(VALUE_EMPTY_ARRAY.repr()).to_value_typed();
  }
  + let cap: u32 = cap.try_into().expect("capacity overflows u32::MAX");
  +
  unsafe {
  - let (avalue, _) = self.arena.borrow().alloc_extra(array_avalue(cap as u32));
  + let (avalue, _) = self.arena.borrow().alloc_extra(array_avalue(cap));
  ValueTyped::new_repr(&*avalue)
  }
  }
  