Macro implement_generic_index 

macro_rules! implement_generic_index {
    ($index:ident, $optional_index:ident) => { ... };
    (pub $index:ident, pub $optional_index:ident) => { ... };
    (pub(crate) $index:ident, pub(crate) $optional_index:ident) => { ... };
    (pub(super) $index:ident, pub(super) $optional_index:ident) => { ... };
    (pub(in $index_visibility:path) $index:ident, pub(in $optional_index_visibility:path) $optional_index:ident) => { ... };
    ($index:ident, $optional_index:ident, __inner__) => { ... };
}