syntax_data: {"ast_node_type":"other","tokens":[{"end":86,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:                     // this is the `x @ P` case; have to keep matching against `P` now
context_before:                 if let Some(subpattern) = subpattern.as_ref() {
context_after:                     MatchPairTree::for_pattern(