A spec is not a replacement for #![feature] attributes. It's rather the opposite: #![feature] indicates that you are stepping out of the stable, specified core

A spec is not a replacement for #![feature] attributes. It's rather the opposite: #![feature] indicates that you are stepping out of the stable, specified core language and into an area that is still a work in progress and thus cannot have a committed specification yet. You shouldn't need it at all unless you are actively experimenting with some unfinished proposal.