--fix hangs with parallel rustc

[matthiaskrgr]

repro:

git clone https://github.com/rust-lang/rust-analyzer
cd rust-analyzer
git checkout 723121e5958cf282db3fdb06970776724a7326d5
RUSTFLAGS="-Zthreads=2" cargo clippy --fix    -- -Aclippy::all  -Wclippy::clone_on_copy

At some point there will be no more progress during the builds, it looks like we get stuck on build.rs scripts?
Works fine without the -Zthreads=2 and also works fine without the --fix

What is weird that we don't seem to hang with active cpu load, but I can still see some rustc processes running

rustc 1.88.0-nightly (077cedc2a 2025-04-19)
binary: rustc
commit-hash: 077cedc2afa8ac0b727b7a6cbe012940ba228deb
commit-date: 2025-04-19
host: x86_64-unknown-linux-gnu
release: 1.88.0-nightly
LLVM version: 20.1.2

[matthiaskrgr]

also reproducible on stable rustc 1.86.0

[matthiaskrgr]

also happens with just RUSTFLAGS="-Zthreads=2" cargo fix 🤔