If you rewrite the Linux kernel in Rust, one module at a time, you'll just end up with C written in Rust syntax, with raw pointers all over the place.
In order to provide realtime guarantees that cover small latencies, you need infrastructure that generates as few instructions as possible. (If you have lots of instructions you can hit targets of a hundreds of microseconds since you can just wait. Start inching toward nanoseconds, though, and the scheduler's own execution will get in the way unless it's crazy compact.)
The reason I mention this is that QNX runs on a variety of embedded devices (http://qnx.symmetry.com.au/ has an interesting list, universal remotes particularly jumped out at me) which are traditionally quite slow.
What's Rust like in terms of generating code that's consistently compact?
What's Rust like in ...
I don't quite follow why speed needs to be sacrificed to get predictable performance though.
And I don't know about writing a (whole new?!) Rust compiler (!) - if the current Rust compiler wouldn't be a good fit for this purpose, well, I definitely don't have the resources to anything about that, considering the unbelievable amount of attention to detail and wrangling that's gone into getting rustc to where it is today.