The Rust in kernel doesn't use Cargo, does it? (Genuine question - someone do confirm)

That being said, it depends on how well the two languages integrate with each other - I think.

Some of the best programming experience I had so far was when using Qt C++ with QML for the UI. The separation of concerns was so good, QML was really well suited for what it was designed for - representing the Ui state graph and scripting their interactions etc ... And it had a specific role to fill.

Rust in the kernel - does it have any specific places where it would fit well?