syntax_data: {"ast_node_type":"other","tokens":[{"end":62,"kind":"keyword","start":0,"text":"assert!(arg.layout.is_unsized()"}]}
source_snippet:                 assert!(arg.layout.is_unsized() && !on_stack);
context_before:                 // With metadata. Must be unsized and not on the stack.
context_after:                 // Also, must not be `extern` type.