Proc-macros that re-use input spans break our syntax fixup

#[tokio::main]
async fn main() {
    main.
}

gets expanded to

fn main(){
  {
    main.
  }[()]{
    tokio::runtime::Builder::new_multi_thread()()()()()
  }
}

while ideally it should be

fn main() {
    let body = async { main. };
    #[allow(clippy::expect_used, clippy::diverging_sub_expression)]
    {
        return tokio::runtime::Builder::new_multi_thread()
            .enable_all()
            .build()
            .expect("Failed building the Runtime")
            .block_on(body);
    }
}

tokio-macros version 1.8.0

This happens because the tokio macro is applying the span of the final expression of the block to all its extra tokens that it is outputting, so our syntax fixup (given its based on the token id of tokens, and token ids being our current span) completely obliterates the output in this case.

Reproduce the example with tokio-macros 1.8.0 and compare the expanded output with the expected expansion. Trace syntax fixup's use of token IDs and spans, focusing on proc-macro output whose extra tokens reuse the final expression's span. Done means the generated async-main expansion is preserved instead of being obliterated by syntax fixup.