The addition of the reserve_index and reserve_index_exact new methods on Vec may actually require a new RFC to add. Recently @aturon and @gankro did some awesome work to create a consistent story for all our collections, and this story currently doesn't include these methods. Could you move these changes to a separate PR? You may also want to discuss with @aturon or @gankro on IRC beforehand to talk about rationale and such.

The addition of the standard capacity, reserve, and reserve_exact functions on VecMap seem great though, and I'd love to whisk them through to bors quickly!

Okay, I'll resubmit a separate PR for each.

reserve and reserve_exact don't super make sense on VecMap, because VecMap's capacity isn't really bound by number of elements. All that matters is that you have space for the largest index you store. Hence reserve_index.

But yes I'm working on an RFC for this.

@gankro Yeah, it was my thought that reserve_index made more sense. I still submitted #19523 with regular reserve at @alexcrichton's request. I will await the RFC for reserve_index :)