Hi,
I want to build a Rust library that exposes a C API, and then write some tests in C for that API. Ideally cargo test would build and run those tests.
It looks like the gcc crate is only for building C libraries that are linked into Rust programs. I guess I could have a Rust program that links in a C library that links in a Rust library. Then the top-level Rust program would just call one function in the C library, to run all the tests.
Is there some simpler way?
Yeah, I think cargo test is just for testing Rust code. Your work-around sounds like it might work.
I personally just avoided that and wrote a simple compile script and a test runner in C and then hooked it up to travis. It's definitely not as convenient as just running cargo test though. 
Hm, gcc complains about an undefined symbol that should be coming from my Rust library, but I can't see a way to tell it to link that in, and I'm not sure if it's a good idea...
Just hardcoding a gcc command as you did might be simplest.
ETA: I ended up adding a makefile in a subdirectory similar to yours,. And, an integration test that runs make, so cargo test does the whole job.