Macro def_installer 

macro_rules! def_installer {
    (
        $(#[$meta:meta])*
        $fn_name:ident < T: $trait:ident > {
            $( $has:ident => $method:ident, $tramp:ident; )*
        }
    ) => { ... };
    (
        $(#[$meta:meta])*
        $fn_name:ident < T: $trait:ident > {
            $( $has:ident => $method:ident, $tramp:ident; )*
        }
        always { $( $always_method:ident, $always_tramp:ident; )* }
    ) => { ... };
}

Generate a family-specific installer function from a declarative spec.

Each entry maps a HAS_* const to a method on R_altrep_class_t and a trampoline. Optional always entries are installed unconditionally (e.g. required Elt).