syntax_data: {"ast_node_type":"variable","tokens":[{"end":54,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:                 let pat = &self.thir[snd_arm].pattern;
context_before:                 // the for loop pattern is not irrefutable
context_after:                 // `pat` should be `Some(<pat_field>)` from a desugared for loop.