Add flag to opt out one test from parallel testing

[dns2utf8]

Hi all

I am not certain this is the right place, because the parsing of the test files appears to be done in rustc as well.

I had a problem while testing the threadpool-crate because all test run in parallel and all of them use the whole cpu.
Would it be possible to add a additional flag for test like this? Maybe something like this:

#[test]
#[no_parallel]
fn test_heavy_load_on_cpu_or_gpu_or_global_state() {
  // working hard here ...
}

The implementation would have to queue these test and run them sequencally.

The flag no_parallel seems very expressiv, but I prefere a positive keyword like single or individual or something like this.

What do you think?
Regards

[alexcrichton]

Thanks for the report! This is actually more of a rustc bug than a Cargo bug (as the compiler is what processes these annotations), so you may want to open a bug over at rust-lang/rust.

I know I've wanted the ability to specify something like this before, though! One trick is to set RUST_TEST_THREADS=1 in the environment and that'll make sure the tests are single-threaded, but ideally there'd be a way to encode that in the source itself.

[dns2utf8]

Okay, i will open another issue in Rust. In my case running all tests in a chain kills the performance oft the tests. So flagging just 2 out of 20+ tests would be very handy.

[dns2utf8]

Added Issue to Rust: rust-lang/rust#33519

[alexcrichton]

Thanks!