Function as_extern_assoc_item 

pub(crate) fn as_extern_assoc_item<'db, ID, DEF, LOC>(
    db: &(dyn HirDatabase + 'db),
    ctor: impl FnOnce(DEF) -> ExternAssocItem,
    id: ID,
) -> Option<ExternAssocItem>
where
    ID: Lookup<Database = dyn DefDatabase, Data = AssocItemLoc<LOC>>,
    DEF: From<ID>,
    LOC: AstIdNode,