Macro return_mode_expression

macro_rules! return_mode_expression {
    (
        (copy, $maybe_backdate:ident, $maybe_default:ident),
        $field_ty:ty,
        $field_ref_expr:expr,
    ) => { ... };
    (
        (clone, $maybe_backdate:ident, $maybe_default:ident),
        $field_ty:ty,
        $field_ref_expr:expr,
    ) => { ... };
    (
        (ref, $maybe_backdate:ident, $maybe_default:ident),
        $field_ty:ty,
        $field_ref_expr:expr,
    ) => { ... };
    (
        (deref, $maybe_backdate:ident, $maybe_default:ident),
        $field_ty:ty,
        $field_ref_expr:expr,
    ) => { ... };
    (
        (as_ref, $maybe_backdate:ident, $maybe_default:ident),
        $field_ty:ty,
        $field_ref_expr:expr,
    ) => { ... };
    (
        (as_deref, $maybe_backdate:ident, $maybe_default:ident),
        $field_ty:ty,
        $field_ref_expr:expr,
    ) => { ... };
}

Generate the expression for the return type, depending on the return mode defined in [salsa-macros::options::Options::returns]

Used when generating field getters.