Make cargo test work

[casey]

Currently, cargo test doesn't work out of the box, and extra flags are needed. The simplest way to make this work is probably to just add a mutex to in_temp_dir.

[soenkehahn]

Unfortunately cargo test does not work out of the box, even after merging #125. The reason is that the tests rely on test helper executables that have to be built before running the test suite with e.g. cargo build --all-targets --all-features. But I think this issue was meant to get rid of the need for --test-threads=1. And #125 does that.

[casey]

Ahh, that's too bad. One option to fix that would be something like this:

fn test_helper() -> PathBuf {
  let path = executable_path("cradle_test_helper");
  if !path.exists() {
    cmd_unit!(%"cargo build --bin cradle_test_helper --features test_executables");
  }
  path
}

And have tests get the path to the helper by running test_helper().

[soenkehahn]

Sadly, I think that only works when you don't change the cradle_test_helper. As soon as you start modifying the behavior of that file, this .exists() check will see that the old version is still in place and not cause a recompilation.

We could look at the time stamps, and recompile when the modification time of the source files is newer than the executable. But then we're pretty much in the business of writing a small build system, and I'm a bit hesitant to do that.

Here's two ideas:

1. We make sure (maybe through a test_helper() function using a Once value) that we once call cargo build --bin cradle_test_helper --features test_executables before running all the tests. And then rely on cargo's ability to execute that very quickly if none of the source files have changed.
2. We use rust-script to execute the test-helper. Since hashbangs don't work on windows, it'd have to be something like cmd_unit!("rust-script", test_helper(), ...). And then we would rely on rust-script's ability to recompile quickly if nothing has changed. One thing that I haven't been able to figure out is how to make rust-script a dev-dependency, so that its executables are in the path while the tests are running. Is that possible?

[casey]

I think 1 sounds the best. I'm guessing that it's very fast, and probably what rust-script does itself. Making dev-dependency binaries available isn't yet supported, see this issue. (Opened by me in 2016!)