Test `extern crate proc_macro2 as proc_macro;`
APACHE-2.0 License
Group::span_open
/span_close
on Rust 1.55+ to reflect their stabilization in rustc (#305, https://github.com/rust-lang/rust/pull/86136)impl FromStr for Literal
(#286, https://github.com/rust-lang/rust/pull/84717)