{"ast_node_type":"variable","tokens":[{"end":54,"kind":"keyword","start":0,"text":"let"}]}
    let sm = SourceMap::new(FilePathMapping::empty());
fn single_backtick() {
    sm.new_source_file(PathBuf::from("foo.rs").into(), r#"#[doc = "`"] fn foo() {}"#.to_string());