As unsafe as C or C++. In fact, safer, because only the ABI surface is unsafe, the rust code behind it can be as safe or unsafe as you want it to be.

I was addressing this portion of your comment: "C's ABI and dynamic linking are the thing that enables the software to get huge". If the C ABI is what enables software to get huge then Rust is already there.

There is a second claim in your comment about a "safe ABI", but that is something that neither C or C++ offers right now.

A safe ABI would be cool, for sure, but in the market (specifically addressing your prediction) I don't know if it's really that big a priority for adoption. The market is obviously fine with an unsafe ABI, seeing how C/C++ is already dominant. Rust with an unsafe ABI might then not be as big an improvement as we would like, but it's still an improvement, and I feel like you're underestimating the benefits of safe Rust code as an application-level frontline of security, even linked to unsafe C code.