{"ast_node_type":"function","tokens":[{"end":85,"kind":"keyword","start":0,"text":"fn"}]}
    fn check_foreign_fn(&mut self, def_id: LocalDefId, decl: &'tcx hir::FnDecl<'_>) {
    /// Check if a function's argument types and result type are "ffi-safe".
        let sig = self.cx.tcx.fn_sig(def_id).instantiate_identity();