syntax_data: {"ast_node_type":"impl","tokens":[{"end":62,"kind":"keyword","start":0,"text":"f:"}]}
source_snippet:         f: impl FnOnce() -> CanonicalParamEnvCacheEntry<Self>,
context_before:         param_env: Self::ParamEnv,
context_after:         from_entry: impl FnOnce(&CanonicalParamEnvCacheEntry<Self>) -> R,