Macro macro_attr::macro_attr_callback

macro_rules! macro_attr_callback {
    (
        ($cb:ident ! { $($cb_fixed:tt)* }),
        $($args:tt)*
    ) => { ... };
    (
        ($cb:ident ! [ $($cb_fixed:tt)* ]),
        $($args:tt)*
    ) => { ... };
    (
        ($cb:ident ! ( $($cb_fixed:tt)* )),
        $($args:tt)*
    ) => { ... };
}

This macro invokes a "callback" macro, merging arguments together.

It takes an arbitrary macro call (name!(args...)), plus some sequence of new_args..., and expands name!(args... new_args...).

Importantly, it works irrespective of the kind of grouping syntax used for the macro arguments, simplifying macros which need to capture callbacks.