Vec::reserve() (etc) panic conditions don't apply to zero-sized or empty types

[bsilver8192]

Location

https://doc.rust-lang.org/std/vec/struct.Vec.html#method.reserve (and several other methods)

Summary

Vec::<()>::new().capacity() is usize::MAX, which is greater than isize::MAX. Calling reserve(1) on this vector does not panic, and the capacity afterwards is still greater than isize::MAX. The docs say this should unconditionally panic.

I would find it reasonable to rely on this documentation as part of ensuring soundness of some unsafe code interacting with Vec, without duplicating the assertion in my code.

This same comment is repeated on several other Vec methods, and the same thing applies.

For reference, #117437 and #99790 are another issue and a PR touching on this same sentence of the documentation.

[Jules-Bertholet]

The docs say (emphasis mine):

Panics

Panics if the new capacity exceeds isize::MAX bytes.

A Vec::<()> will never have more than 0 bytes of capacity, and therefore reserve will never panic. The documentation is correct.

[bsilver8192]

Oh you're right, I read that many times and never processed the last word. That's the only place I see which talks about "capacity" as a number of bytes instead of a number of elements, which is easy to miss IMHO.

[Noratrieb]

Might make sense to make thebytes bold.

[gurry]

Might make sense to make thebytes bold.

I have raised PR #119374 doing just that (except italicising it instead making it bold)