The Vec should not needlessly overallocate capacity if it is guaranteed to fail.

[ghost]

Current Vec / RawVec allocation strategy is to at least double the capacity on
reallocation. This leads to unnecessary panic when the new overallocated
capacity exceeds std::isize::MAX. If allocating std::isize::MAX would be
sufficient it should do so instead.

For example I would expect the following to work on 32-bit platform (provided
that there is sufficient amount of memory):

fn main() {
    let mut v = std::vec::from_elem(1 as u8, (std::isize::MAX / 2 + 1) as usize);
    v.push(1);
}

[nagisa]

Would’ve been fixed-ish by #29848?

[Gankra]

I'm curious: what languages succeed or fail by this metric?

[ghost]

@nagisa as far as I can see that PR would have exactly the same problem.
Generally, the source of problem lies in a fact that the calculation of new
capacity is unaware on requirement that it should be less than std::isize::MAX,
which is checked only later in alloc_guard.

@gankro the GNU libstdc++ works fine in this respect. It seems more like
quality of implementation issue, than some strong requirement.

[steveklabnik]

Triage: I don't think anyone has addressed this issue.