vec![Vec::with_capacity(x); n] has unintuitive behaviour.

[agentsim]

What it does

Warns the user when that for n > 1 only the last of the Vec created by the macro will actually have the requested capacity.

Advantage

Alerts the user to some likely unintended behaviour.

[Alexendoo]

std::iter::repeat would run into a similar thing, except none of the vecs would have the desired capacity

[Centri3]

Can you elaborate on why this is the case? That is quite unintuitive ^^

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=b0444384a560393618dd26639192ca49

Basically, why does only the final element have the correct capacity?

[Alexendoo]

(the unspecialised) vec![foo; n] works by cloning the result of foo, it uses the result directly for the last one since it can avoid an extra clone - https://github.com/rust-lang/rust/blob/62d9034a0d571b78e518727d6cb4b090569e5238/library/alloc/src/vec/mod.rs#L2529-L2555

Cloning a vec doesn't preserve its capacity