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The basic point I'm getting at is that in a kernel, I'd be surprised if at least 50% of all the code isn't unsafe (or even just raw assembly). Because the nature of the beast is that you're manipulating raw memory and hardware at its very lowest level.
Checked & safe languages like Rust are nice, but the idea that using such a language for a kernel suddenly makes the kernel less liable to errors is largely unfounded. They can eliminate certain types of errors, but not the errors that are the most likely to bite you in the ass when you're in ring 0.
Having said all that I'd love to see someone expand on the concept and perhaps write a proper kernel in Rust. Edited 2013-05-28 15:31 UTC
I advise you to inform yourself of operating systems done in such languages, in case you find the subject interesting.
Lilith was done in Modula-2. AS/400 also had a part of Modula-2 on its kernel on the first versions, but it has been replaced by C++ in more recent versions.
A very good reading about such systems is the book Niklaus Wirth wrote about how Native Oberon was designed, freely available from Zurich's university:
It was quite usable system back in the late 90's.
Or the Mirage OS done by Citrix and the Xen Project,
Back when C appeared, kernel safe languages were already starting to be explored, but like everything in computing, many concepts take decades before they become accepted in the industry.
Granted, others never managed to mostly due to the famous "Worse is Better" axiom.