Cargo ignores GNU Make jobserver only when using `make -j1`.

[jnms-me]

Problem

When invoking cargo build from GNU Make through make -j1, Cargo should limit the amount of spawned jobs to 1, as it implements a jobserver protocol client.
Specifically when there is only a single job slot available, Cargo instead spawns as many jobs as there are CPU cores available.
All other cases appear to behave correctly.

Steps

git clone https://github.com/rust-lang/rust-analyzer.git
cd rust-analyzer
cat >Makefile <<EOF
all:
	+cargo build
.PHONY: all
EOF

Observe that make -j2 correctly only spawns at most 2 rustc instances at a time.

Observe that make -j1 or make will spawn many rustc instances that saturate the host cpu.

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.97.0 (c980f4866 2026-06-30)
release: 1.97.0
commit-hash: c980f4866141969fab6254a680546a277789d6f0
commit-date: 2026-06-30
host: x86_64-unknown-linux-gnu
libgit2: 1.9.2 (sys:0.20.4 vendored)
libcurl: 8.20.0-DEV (sys:0.4.88+curl-8.20.0 vendored ssl:OpenSSL/3.6.2)
ssl: OpenSSL 3.6.2 7 Apr 2026
os: Linux [64-bit]

[weihanglo]

not sure if this is related: #7689

[ehuss]

This does look like we're not quite doing the jobserver protocol correctly. Per this and this, make's default is -j1, and in that scenario it does not set up the jobserver auth (because every sub-make will also assume just a single process). However, cargo sees that it is running under make, but does not see the auth, and so assumes it can run with max parallelism.

I'm surprised nobody has really pointed this out (at least that I can find).