syntax_data: {"ast_node_type":"variable","tokens":[{"end":40,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:         let bb = self.start_new_block();
context_before:     pub(crate) fn start_new_cleanup_block(&mut self) -> BasicBlock {
context_after:         self.block_data_mut(bb).is_cleanup = true;