Macro param_key 

macro_rules! param_key {
    ($(#[$meta:meta])* $vis:vis $name:ident = $param:literal) => { ... };
}

Declare a route-parameter name for BoundAs.

param_key!(pub AuthorParam = "author");

pub async fn show(author: BoundAs<User, AuthorParam>) -> Result<Response> {
    Ok(Response::json(&author.into_inner()))
}