syntax_data: {"ast_node_type":"other","tokens":[{"end":49,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:         //    `for<'a> &'a i32` becomes `&0 i32`.
context_before:         // 1. Instantiate those regions to placeholder regions (e.g.,
context_after:         // 2. Produce something like `&'0 i32 : Copy`