rustc build broken after #95906

[klensy]

After #95906 (0516711), building rustc (stage0 stdlib) with python x.py --stage=0 b fails with

$ python x.py build --stage=0
Updating only changed submodules
  Submodules updated in 0.02 seconds
Building rustbuild
    Finished dev [unoptimized] target(s) in 0.21s
thread 'main' panicked at '`should_run.paths` should correspond to real on-disk paths - use `alias` if there is no relevant path: src/llvm-project/llvm', src\bootstrap\builder.rs:391:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Build completed unsuccessfully in 0:00:01

The same true for perf runner: https://perf.rust-lang.org/status.html (https://perf.rust-lang.org/compare.html?start=6fd7e9010db6be7605241c39eab7c5078ee2d5bd&end=0516711ab057d9731f55511f00f9d426bc9db724)
cc @jyn514
@rustbot label +A-rustbuild

[jyn514]

Thanks for catching this. The issue is that src/llvm-project/llvm doesn't exist if the llvm-project submodule isn't checked out locally. As a workaround, you can run git submodule update --init src/llvm-project.

I am not quite sure the right fix - I guess I could check in Llvm::should_run whether the submodule is checked out? But it seems confusing to have the possible paths be different based on your git state ... alternatively, I could revert the hard requirement, but then this will regress in the future.

I suppose a third alternative is to add a ShouldRun::submodule_path function that calls either path or alias depending on whether the submodule is checked out. But that seems a little overcomplicated.

cc @Mark-Simulacrum - happy to revert this assertion for now to unbreak things

@rustbot label +C-bug

[klensy]

As consequence of that, every next perf run missing bootstrap timings.

P.S. Bootstrap timings in perf run missing, but no indication of error on the page (for example https://perf.rust-lang.org/compare.html?start=8305398d7ae6128811ec2b3223939bcd067530c2&end=311e2683e1bad87715b1558f7900e294d24ce491), except for first time (https://perf.rust-lang.org/compare.html?start=6fd7e9010db6be7605241c39eab7c5078ee2d5bd&end=0516711ab057d9731f55511f00f9d426bc9db724, but there bootstrap time show only partially)

[jyn514]

Fixed in #96196