cd /Development/source/libc/libc-test
cargo test
In output:
Compiling libc v0.2.142
...
Compiling libc v0.2.142 (/Development/source/rust/libc)
At least on Haiku, this causes a linking error as the current libc on crates.io hasn't been updated with a fix to a linking issue, which has been fixed in my local repo (and has landed here, a new release hasn't been cut yet).