Macro higher_order_closure::higher_order_closure

macro_rules! higher_order_closure {
    (
    $(#![
        with<
            $($(
                $lt:lifetime $(: $super_lt:lifetime)?
            ),+ $(,)?)?
            $($(
                $T:ident $(:
                    $(
                        ?$Sized:ident $(+)?
                    )?
                    $(
                        $super:lifetime $(+)?
                    )?
                    $(
                        $Trait:path
                    )?
                )?
            ),+ $(,)?)?
        >
        $(where
            $($wc:tt)*
        )?
    ])?

    $( for<$($hr:lifetime),* $(,)?> )?
    $( move $(@$move:tt)?)?
    | $($arg_pat:tt : $ArgTy:ty),* $(,)?|
      -> $Ret:ty
    $body:block
) => { ... };
}

See the main docs for more info.