Macro extension

macro_rules! extension {
    (
        $(#[$meta: meta])*
        pub extension $name: ident $(<$($generic: ident),*>)?: $ty: ident$(<$($generic1: ident),*>)? {
            $($tokens: tt)*
        }
    ) => { ... };
}

Available on crate feature extension only.

Generate a sealed trait with a given name and content.