Echo code to progress bar stream?

[hadley]

When you're rendering an RMarkdown in remote contexts, it can be hard to debug warnings and errors. As well as the changes suggested in #2215 and #2219, I wonder if it would also be useful to echo all code to the progress bar stream, so that you see exactly what is executing. (Most logging platforms also add a timestamp to each line so this would also help you learn what code is taking a particularly long time). If this sounds interesting, I'm happy to help out either here or in evaluate.

---

By filing an issue to this repo, I promise that

• I have fully read the issue guide at https://yihui.org/issue/.
• I have provided the necessary information about my issue.
  • If I'm asking a question, I have already asked it on Stack Overflow or RStudio Community, waited for at least 24 hours, and included a link to my question there.
  • If I'm filing a bug report, I have included a minimal, self-contained, and reproducible example, and have also included xfun::session_info('knitr'). I have upgraded all my packages to their latest versions (e.g., R, RStudio, and R packages), and also tried the development version: remotes::install_github('yihui/knitr').
  • If I have posted the same issue elsewhere, I have also mentioned it in this issue.
• I have learned the Github Markdown syntax, and formatted my issue correctly.

I understand that my issue may be closed if I don't fulfill my promises.

[hadley]

Necessary changes in evaluate done in r-lib/evaluate#118