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In normal user-mode rust, not running inside the kernel at all, you can open /dev/mem and write whatever you want to any process's memory (assuming you are root). This does not require "unsafe" at all.

Another thing you can do from rust without "unsafe": output some buggy source code that invokes UB in a language like C to a file, then shell out to the compiler to compile that file and run it.

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Sure, but those are non-central to what the program is doing. Writing to the wrong register offset and hosing main memory is a thing that happens when developing drivers (though usually it breaks obviously during testing).

Right, you're not wrong that this is a possible failure mode which Rust's guarantees don't prevent.

I'm just pointing out that "your program manipulates the external system in such a way that UB is caused" is outside the scope of Rust's guarantees, and kernel development doesn't fundamentally change that (even though it might make it easier to trigger). Rust's guarantees are only about what the Rust code does; anything else would be hard or impossible to guarantee and Rust doesn't try to do so.

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