As Miguel said, the commitment from the Rust project is making Rust for Linux build on stable Rust, rather than using nightly. We care about that for a variety of reasons: we don't want folks to get a negative impression of Rust's stability based on nightly Rust, and Rust for Linux is a good exemplar of projects needing certain features, and it's an important effort in its own right that demonstrates the ability to incrementally add Rust to an established project.

It's up to the awesome Rust for Linux folks to work on the integration and upstreaming and policy needed to ship the kernel with Rust. And they're doing incredible work, based on the ability to write more and more kinds of drivers/filesystems/etc in Rust.