> This is commonly done when using Vec as a fixed capacity buffer and you don't want to add another field to whatever owns it that's semantically equivalent to .capacity().
The documentation for Vec already explains exactly what it's offering you, but lets explore, what exactly is the problem? You've said this is "commonly done" so doubtless you can point at examples for reference.
Suppose a Goose is 40 bytes in size, and we aim to store say 4 of them, for some reason we decide to Vec::new() and then Vec::reserve_exact(..., 4) rather than more naturally (but with the same effect) asking Vec::with_capacity(4) but alas the allocator underpinning our system has 128 or 256 byte blocks to give, 4x40 = 160, too big for 128, so a 256 byte block is allocated and (a hypothetical future) Vec sets capacity to 6 anyway.
Now, what disaster awaits in the common code you're talking about? Capacity is 6 and... there's capacity for 6 entries instead of 4
