Show stderr from code blocks

[mgeisler]

Today, mdbook doesn't show stderr when running Rust code. The problem is that it uses a Playground endpoint which doesn't deliver this information. However, the Playground itself does have this information via an internal endpoint. See rust-lang/mdBook#1315 for details.

Fixing this would allow us to start using dbg!(foo) instead of println!("foo: {foo:?}") in our code samples and thus simplify a lot of code blocks.

With google/mdbook-i18n-helpers#75 merged, many more code blocks would be automatically removed from the translation files, thus reducing churn for our translators.

[emmali01]

I'll try to work on it in the next two weeks.
-- Emma

[mgeisler]

Thanks Emma! You might very likely have to make changes to the upstream mdbook — we should upstream as much as possible, both so it can benefit everybody and so we can avoid carrying patches (which makes it hard to update our version of mdbook).

[Alx-Lai]

Hi @mgeisler, I applied rust-lang/mdBook#1315 in #2397. I think there's a room to discuss whether we should push it to upstream repo. My concern is that the original PR is put aside for a long time so we don't know if i'll be finally adopted or not; however, if we merge it first in downstream, the only possible effort comes iff it's finally adopted in upstream.