syntax_data: {"ast_node_type":"other","tokens":[{"end":74,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:         // FIXME: this is only mutation of UnstableOptions here, move into
context_before:     if let Ok(graphviz_font) = std::env::var("RUSTC_GRAPHVIZ_FONT") {
context_after:         // UnstableOptions::build?