syntax_data: {"ast_node_type":"other","tokens":[{"end":83,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:                 // Note that the len being WF is implicitly checked while visiting.
context_before:                 self.require_sized(subty, ObligationCauseCode::SliceOrArrayElem);
context_after:                 // Here we just check that it's of type usize.