These are awesome, thanks.

By 'declarative' I mean some way of telling the shell how my bin can provide completions itself.

E.g. for the AWS util, I am envisioning something like

    aws ec2 describe-instances --instance-id __INSTANCEID__
    __INSTANCEID__: $(aws ec2 list-instances --query Instance.ID)

Syntax here is not anything like a well thought out proposal, just a demonstration of what I believe to be a shell agnostic method of describing both 'where can I complete?' and 'how can I compete it?'