syntax_data: {"ast_node_type":"variable","tokens":[{"end":45,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:                 let (then_blk, mut else_blk);
context_before:                 // Unpack `BlockAnd<BasicBlock>` into `(then_blk, else_blk)`.
context_after:                 else_blk = unpack!(then_blk = then_and_else_blocks);