Rust building ignores Makefile job control

[chrysn]

Description

@maribu has observed that Rust behaves badly w/rt job numbers; @kaspar030 pointed me toward Make's job servers.

Even though cargo should (and is tested to) adhere to make's job server protocol, a simple test shows it's not.

Simplified to reproduce the issue

This does not directly depend on RIOT, and is probably best tested in a temporary directory

$ cat Makefile
all:
	cargo install cargo-crev -j2
$ make -j100

This happily builds two crates at a time, while according to the above-mentioned tests, it should show an error message that its own -j2 is ignored because it's running under a Make job server.

Next steps

Find out why it doesn't work; possibly try running the cargo test manually 1:1, and maybe just report as bug in cargo (but I'm not confident enough it's not something in our setup to do that right now).

[kaspar030]

I tracked this down to GNU make using a different method since 4.4. jobserver-rs issue.

[kaspar030]

(it might be that our build system also misses the "+" prefix for cargo calls)