cargo fix with -Zthreads hangs

[Nemo157]

Problem

Running cargo fix on a large project I noticed it reliably hang with a bunch of subprocesses not doing anything (23 cargo rustc, 4 rustc). CARGO_LOG=trace didn't really seem to output anything useful near where it hung.

Playing around I managed to reproduce it on a blank project, seems like it's some bad interaction of the job server with -Zthreads under cargo-fix.

I'm not certain if the issue lies in cargo-fix or is a bug in rustc with -Zthreads that cargo-fix just happens to trigger.

Steps

> cargo new foo && cd foo
> RUSTFLAGS=-Zthreads=2 cargo fix --allow-dirty -j1
    Building [                             ] 0/2: foo(bin test), foo(bin)
…hangs…

Note that a check works in this configuration:

> RUSTFLAGS=-Zthreads=2 cargo check -j1
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.09s

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.86.0-nightly (088d49608 2025-01-10)
release: 1.86.0-nightly
commit-hash: 088d496082726091024f1689c124a0c3dccbd775
commit-date: 2025-01-10
host: x86_64-unknown-linux-gnu
libgit2: 1.9.0 (sys:0.20.0 vendored)
libcurl: 8.9.0-DEV (sys:0.4.74+curl-8.9.0 vendored ssl:OpenSSL/1.1.1w)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: NixOS 24.11.0 [64-bit]

[ehuss]

Thanks for the report! I'm going to close as a duplicate of rust-lang/rust#133873.