This is inevitable: Rust is proposed as a safe language, but there is no way to have a "half-secure" kernel. The only option for people who believe in Rust is to have its own kernel, and Linux should have no part on this.
There is, and this is how Rust naturally works. If you look at its standard library, you will see a lot of unsafe code or libc calls hidden away under safe interfaces.
In fact, this is how all memory safe languages work, including Java, Python, etc: A small trusted base written in an unsafe language that exposes a safe interface (i.e. the interpreter, the JVM, etc), with the large majority of the code written over that safe interface (i.e. the Java/Python code).
Rust is used to make kernel drivers secure by providing a safe interface for them to use.
