Originally, I required a GNU toolchain from the RISC-V organization for both assembling and linking. Now, I don't need any external tools (except QEMU to run it). Everything you need can be downloaded using Rust's configuration tool rustup for RISC-V. If you take a look at my github, you'll see that I added a .cargo/config file that will build AND run the operating system using cargo.

Wait, we can now host a whole OS w/o directly requiring a C compiler?! I mean Rust still requires one to build clang/llvm, but maybe someday when alternative backends land.

There are several projects that do this, yes. They're generally not production ready, but it's absolutely possible.