Huge compile-time regression in beta/nightly

[bnjbvr]

We have a serious Rust compile-time regression happening on an internal (unfortunately closed-source) crate, happening on both the beta and nightly channels.

Before, compiling the crate would take around 1 minute 30 seconds in automation. Now if I'm using the nightly channel, it seems to take around an hour or so.

Running with RUSTFLAGS="-Z new-llvm-pass-manager=no" makes the regression disappear on the nightly channel, so the new pass manager seems to be at fault here.

I've tried compiling with a rustc build from #89830, but it didn't make the regression disappear, so it might not be the same inlining issue.

Thanks a bunch to @lqd who suggested disabling the pass manager, and trying the above patch. Pinging LLVM people: @nikic @Mark-Simulacrum. Of course I'm happy to run any other diagnostic and try any tool that could help figure out what's going on here.

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

We assign a P-critcal to raise the flag for the team. Further investigation will probably need some way to reproduce

@rustbot label -I-prioritize +P-critical

[bnjbvr]

I've managed to reduce our use case to a small file that reproduces the issue, all the code is accessible in the following repository https://github.com/EmbarkStudios/rustc-compile-time-regress:

• on stable, it takes around 26 seconds to compile on my machine with --release
• on beta and nightly, it takes around 2 minutes to compile with --release

There are multiple variants of the Shim trait in our code base, in some cases with many more functions in it that make use of the same patterns as those shown in this example. I am almost certain that it's the compilation of all these traits and impls that cause the very long compile times somehow, hence the reduced test case.

Note: I've also tried again the build from #89830, and the compile time is still the same with it, so it's definitely not the same issue.

[nikic]

Problematic IR for opt -O3: https://gist.github.com/nikic/cae7dbfe7da91308ba012b4b3172eebc