Macro parameters 

macro_rules! parameters {
    (
        $($key:expr => $value:tt),* $(,)?
    ) => { ... };
    (@val [$($val:expr),* $(,)?]) => { ... };
    (@val $val:expr) => { ... };
}

Easier Parameters construction.

let vec = vec!["".into()];
parameters! {
    "language" => "en",
    String::from("sort-as") => [String::from("foo"), "bar"],
    "vec" => vec,
};