How about (5) you're trying to learn Rust and you need a project to work on? Irresponsible for production code, but perfectly valid for a weekend project. And in my experience can be a great way to learn the quirks of a particular language!

Why not use ffi for this case? Many languages have robust support for linking to Rust code.

yeah sometimes that makes sense. if the lib in question in small enough it might be more valuable to port, so avoid having a multi language project, and avoid the effort of the ffi.

at least Rust -> C ffi is zero cost (usually)

Having everything available in Rust code also means you don't have to do lots of extra work for cross-compilation.

How does Rust help you if you have to cross-compile?

You can compile rust in one go. If you rely on native libraries, you need to be able to link them during cargo build and that's painful to set up. Not impossible... just really annoying to manage.

I'm not positive, but I think GP intended to refer to idea that you would need to multiple compilers to compile the project rather than cross-compiling to another platform.