A kernel's interface isn't necessarily syscalls. On macOS libSystem's C API is the API of the kernel, because syscall stability is not guaranteed.

On the other hand, pretty much every kernel has a C API.

I think the parent comment was less about the capability of Rust, but more about the overwhelming momentum that C has.

And on that I'd fully agree with him.

My comment also isn't really a comment about rust. It's about a common misconception that the unix/linux kernel somehow talks in "C".

Libc talks in C, most of the other libraries talking for interfacing with the kernel talk in C. The kernel speaks it's own language that has no special relation to C at all.

Except that you'll be hard pressed to find a syscall without an accompanying C library. It's common for other languages to build upon that rather than call the syscalls directly. Specially during the early days of a new kernel feature.