syntax_data: {"ast_node_type":"other","tokens":[{"end":34,"kind":"keyword","start":0,"text":"Placeholder(I::PlaceholderTy),"}]}
source_snippet:     Placeholder(I::PlaceholderTy),
context_before:     Param(I::ParamTy),
context_after:     UnresolvedInferenceVariable(ty::InferTy),