But you still get those guarantees and benefits in the Rust implementation itself, even if not at the interface (to non-Rust code). By a similar argument, a standalone Rust program "gives up" its guarantees whenever the process makes a call to libc or to the operating system (syscalls), but this isn't really a practical problem.
Sure, I agree. The point is that if the total corpus of Rust code is too isolated into too small islands, then the boundary effects of each island may eat up a lot of the benefit.
I still think the project is really cool and worthwhile.
