Vec and Box documentation do not state maximum allocation size guarantee

[aticu]

In the documentation for pointer::add, it is mentioned that

Vec and Box ensure they never allocate more than isize::MAX bytes

however, this is not stated in documentation of Vec, the module documentation of vec, the documentation of Box or the module documentation of boxed.

This can cause doubt, whether this statement actually holds true, since the places where one would expect such guarantees to be made (especially the "Guarantees" section of the Vec documentation) does not mention these guarantees.

[cuviper]

Along these lines, I want to point out that this a real system limit now, beyond just what Rust wants to do, at least if you're using the system allocator. This is mentioned in the glibc 2.30 release notes:

* Memory allocation functions malloc, calloc, realloc, reallocarray, valloc,
  pvalloc, memalign, and posix_memalign fail now with total object size
  larger than PTRDIFF_MAX.  This is to avoid potential undefined behavior with
  pointer subtraction within the allocated object, where results might
  overflow the ptrdiff_t type.

C and C++ ptrdiff_t is essentially the same as Rust's isize, so PTRDIFF_MAX == isize::MAX.

[aticu]

I want to point out that this a real system limit now, beyond just what Rust wants to do, at least if you're using the system allocator.

If that's the case, it might make sense to document that constraint on every allocation based API in the standard library, since it should apply to all of them.

[poliorcetics]

This has been fixed by #68711 and could be closed