{"ast_node_type":"function","tokens":[{"end":73,"kind":"keyword","start":0,"text":"fn"}]}
    fn parse_fn_block_decl(&mut self) -> PResult<'a, (P<FnDecl>, Span)> {
    /// Parses the `|arg, arg|` header of a closure.
        let arg_start = self.token.span.lo();