Valid punctuation characters generated by:

    use proc_macro::{Punct, Spacing, TokenStream};
    use std::panic;

    #[proc_macro]
    pub fn punct_chars(_input: TokenStream) -> TokenStream {
        for ch in '\0'..=char::MAX {
            if let Ok(_) = panic::catch_unwind(|| {
                let _ = Punct::new(ch, Spacing::Alone);
            }) {
                eprintln!("{:?}", ch);
            }
        }
        TokenStream::new()
    }