I'm evaluating different options to use C++ code from rust, and an important point for me is that error handling should be seamless. It would be convenient if what happens when an exception crosses the boundary was documented somewhere.
The only relevant information I found was #21.

If anybody else wonders as well, I've now tried it myself, and it results in this crash:

let result = unsafe {
    cpp!([] -> i32 as "std::int32_t" {
            throw std::runtime_error("oops");
            return 0;
        })
    };

fatal runtime error: Rust cannot catch foreign exceptions
Aborted (core dumped)

In hindsight, this is the obvious behaviour, but it's still useful to have it explicitly mentioned.