--fix with -Zthreads=8 on large workspace hangs

[CinchBlue]

Summary

I was compiling recently on my 1.3k crate + dep repo that is 1 giant Rust workspace, and it would hang on some of these crates during our build process:

   Checking quinn-proto v0.10.6
    Checking cast v0.3.0
    Checking flapigen v0.6.0
    Checking walkdir v2.5.0
    Checking num-bigint v0.4.4
    Checking num-iter v0.1.44
    Checking num-rational v0.3.2
    Checking num-rational v0.4.1
    Checking p256 v0.11.1
    Checking num-bigint v0.2.6
    Checking criterion-plot v0.5.0
    Checking chrono-tz v0.8.6
    Checking simba v0.8.1
    Checking nix v0.28.0
    Checking coreaudio-sys v0.2.15
    Checking plotters v0.3.5
    Checking tinytemplate v1.2.1
    Checking anes v0.1.6
    Checking bytesize v1.3.0
    Checking pathdiff v0.2.1
    Checking pkg-config v0.3.30
    Checking ciborium-ll v0.2.2

It would stop progressing the clippy command (but the rest of the cargo test and cargo build targets are okay -- only cargo clippy has the issue).

Reproducer

<cannot reproduce, is private code>

Version

rustc 1.79.0-nightly (385fa9d84 2024-04-04)
binary: rustc
commit-hash: 385fa9d845dd326c6bbfd58c22244215e431948a
commit-date: 2024-04-04
host: aarch64-apple-darwin
release: 1.79.0-nightly
LLVM version: 18.1.2

Additional Labels

No response

[blyxyas]

Could you check if cargo check --fix also has this issue? We rely on it and knowing this would reveal what part of the engine is failing.