macOS 15 fails line-tables-only because of change in C debuginfo

[workingjubilee]

cc @madsmtm If you recognize this off the top of your head: https://github.com/rust-lang/backtrace-rs/actions/runs/17957211691/job/51071470672

A regression has been surfaced by a recent change in the runner images from macOS 14 to macOS 15. The effect of the regression is that when we compile some C code using -g1, which typically adds debuginfo, using an automatically-determined C toolchain by the logic in the cc crate, we no longer have frames for the C code in our backtrace.

The root of the regression might be in backtrace-rs, it might be in cc-rs, and it might be in the Xcode toolchain packaged by https://github.com/actions/runner-images. This might be because of an Apple-specific change, or it might be because of a change in upstream Clang. Currently nothing has been eliminated from consideration.

[madsmtm]

Likely actions/runner-images#12520, try using macos-14?

Unsure why the bug happens, but that should unblock your CI in the meantime.

[workingjubilee]

That's weird.

[workingjubilee]

The trace is failing when it enters C code. It may be a change in available debuginfo for C libraries...?

[workingjubilee]

That doesn't make sense, we build it ourselves and set -g1?

[workingjubilee]

I have updated the issue with the current summary of the situation.

[workingjubilee]

Hmm. Apparently "weird issues we can't pin down but Xcode maybe?" is a recurring problem: #327

[workingjubilee]

GitHub's runner images package macOS 14 with Xcode 16.2, but macOS 15 is packaged with Xcode 16.4, giving us these two release note sets to scan through:

• https://developer.apple.com/documentation/xcode-release-notes/xcode-16_3-release-notes
• https://developer.apple.com/documentation/xcode-release-notes/xcode-16_4-release-notes

But this contains absolutely no information I can identify as useful, here.