compiletest: show affected architecture in error summary

[RalfJung]

When compiletest fails, it prints an error summary like this at the end:

failures:
    [mir-opt] mir-opt/issue-73223.rs

test result: FAILED. 139 passed; 1 failed; 3 ignored; 0 measured; 0 filtered out

However, there is one very important bit of information that I cannot gleam from this information: what are the host and target architectures that were tested! Figuring this out requires scrolling up quite a bit to where this test beings (and carefully not scrolling up too far because then one might look at the wrong test).

See #78343 for an example where this confused people. I have also been confused by this multiple times in the past, e.g. using the name of the CI runner to incorrectly deduce what the architecture(s) are (and not realizing that some of our runners run tests for quite a few different targets).

So I think in case of a failure, that summary at the end should repeat the important information that is printed at the top of the test run:

compiletest suite=mir-opt mode=mir-opt (x86_64-unknown-linux-gnu -> armv5te-unknown-linux-gnueabi)

[RalfJung]

Cc @Mark-Simulacrum

[camelid]

And also it would be nice if when it ignored a test because it was the wrong platform, it showed some indication of that, because otherwise it looks like it ran the test and it passed. Perhaps show a question mark (?)? (We already use i for tests that were ignored because they were already run and passed.)

[RalfJung]

AFAIK i should already be used for all kinds of "ignored", including // ignore-XX directives.

[RalfJung]

Turns out these messages are printed by bootstrap, not compiletest:

rust/src/bootstrap/test.rs

Line 1299 in a6403b0

"Check compiletest suite={} mode={} ({} -> {})",

However, detecting failures and exiting is done a few layers deeper:

rust/src/build_helper/lib.rs

Line 66 in a6403b0

std::process::exit(1);

In case of failure, try_run never returns or unwinds (except if fail-fast is off).

I am not sure how to best adjust this to print an error before exiting (or to always print an error after the process finished, whether it succeeded or failed). @Mark-Simulacrum any advice? If we just want this to happen when fail-fast is off, that would be easy, but I am not sure if that ever applies.