syntax_data: {"ast_node_type":"other","tokens":[{"end":22,"kind":"keyword","start":0,"text":"#[gate_rustc_only]"}]}
source_snippet:     #[gate_rustc_only]
context_before:     #[debug_format = "'?{}"]
context_after:     #[cfg_attr(feature = "nightly", derive(HashStable_NoContext))]