[beta] [1.57] Disable LLVM newPM by default

[nagisa]

This is a temporary solution until we manage to get some sort of a workaround for the rampant inlining behaviour and other issues such as #91128 in place. Expect significant compilation time regressions in other areas, though.

[rust-highfive]

r? @wesleywiser

(rust-highfive has picked a reviewer for you, use r? to override)

[rust-highfive]

⚠️ Warning ⚠️

• Pull requests are usually filed against the master branch for this repo, but this one is against beta. Please double check that you specified the right target!

[nagisa]

I went ahead and unilaterally specified the beta-accepted tag based on conversation from here onwards.

[camelid]

(Just noting it might be helpful to add some explanation of why this change is being made to the PR description.)

[camelid]

Thanks for the description ❤️

Expect significant compilation time regressions in other areas, though.

@bors rollup=never

[nagisa]

One pretty concerning thing I just realised now is that we pretty much had almost no testing of LLVM 13.0 with the oldPM, unless people were really heavily using the -Z flag to disable newPM, which I doubt they did.

[camelid]

I went ahead and unilaterally specified the beta-accepted tag based on conversation from here onwards.

This PR is filed against beta and marked as beta-accepted. Usually, one would just open a PR against master (#91190) and mark that as beta-accepted. Then someone from the release team will backport it.

[nikic]

One pretty concerning thing I just realised now is that we pretty much had almost no testing of LLVM 13.0 with the oldPM, unless people were really heavily using the -Z flag to disable newPM, which I doubt they did.

I believe the LLVM 13 upgrade landed in 1.56 already, so it was tested in that release cycle. Though you're right that 1.57 did not receive any testing with the legacy PM.

[nikic]

This has been landed in #91263.