compiletest: Implement an experimental --new-output-capture mode

[Zalathar]

Thanks to the efforts on #140192, compiletest no longer has an unstable dependency on libtest, but it still has an unstable dependency on #![feature(internal_output_capture)]. That makes building compiletest more complicated than for most other bootstrap tools.

This PR therefore adds opt-in support for an experimental compiletest mode that avoids the use of internal_output_capture APIs, and instead uses more mundane means to capture the output of individual test runners.

Each TestCx now contains &dyn ConsoleOut references for stdout and stderr. All print statements in compiletests::runtest have been replaced with write! or writeln! calls that explicitly write to one of those trait objects. The underlying implementation then forwards to print! or eprint! (for --no-capture or old-output-capture mode), or writes to a separate buffer (in new-output-capture mode).

---

Currently, new-output-capture is disabled by default. It can be explicitly enabled in one of two ways:

• When running x test, pass --new-output-capture=on as a compiletest argument (after --).
  • E.g. x test ui -- --new-output-capture=on.
  • The short form is -Non or -Ny.
• Set environment variable COMPILETEST_NEW_OUTPUT_CAPTURE=on.

After some amount of opt-in testing, new-output-capture will become the default (with a temporary opt-out). Eventually, old-output-capture and #![feature(internal_output_capture)] will be completely removed from compiletest.

r? jieyouxu

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

This PR modifies bootstrap.example.toml.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[Kobzol]

I think that if this is an experimental feature designed for testing with a handful of people, I wouldn't add it to bootstrap.toml (nor add a change tracker entry), and just use some magic environment variable for it. bootstrap.toml should ideally be used for configuration that will last long-term, as it's rather annoying to remove stuff from it.

[Zalathar]

Fair enough; I'll switch over to an environment variable instead.

[jieyouxu]

Thanks for working on this! Other changes look good to me, just a tiny nit regarding "please don't support so many ways to spell a true/false" of the flag lol

View changes since this review

[jieyouxu]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[jieyouxu]

@bors r+ rollup

[bors]

📌 Commit 9c5e1cb has been approved by jieyouxu

It is now in the queue for this repository.

[Zalathar]

Added a short comment to parse_bool_option, since the topic of which values to accept came up in review.

@bors r=jieyouxu

[bors]

📌 Commit c2ae2e0 has been approved by jieyouxu

It is now in the queue for this repository.