Macro bidirectional_lang_item_map 

macro_rules! bidirectional_lang_item_map {
    (
        $solver_ty:ident, fn $to_solver:ident, fn $from_solver:ident;
        $($name:ident),+ $(,)?
    ) => { ... };
}

Defines trivial conversion functions between the main LangItem enum, and some other lang-item enum that is a subset of it.