The choice of microcontroller matters. Functional safety qualified MCUs might have the same CPU core but is built in a way that minimizes interference or common cause of failures between peripherals. The software needs to be written closely following the safety manual of such a MCU to make maximum use of those safety guarantees.
There are several safety critical Cortex implementations around, with features like multi-core lock-step operation that is largely transparent to the RTOS (or whatever) beyond fault handling. There isn't some vast gulf between the ATSAMV71Q21 they've piloted this on and whatever space rated device and requirements you imagine.
While our RTOS in fact doesn't care about it, because it's up to the user to implement the abstraction for the platform, the bigger issue is Rust support for more exotic architectures used in space - for example, SPARC (LEON3), which we'd like to consider for a Rust project, but it's compiler support seems relatively bad.
That seems like an insoluble problem: for every fault tolerant SPARC device someone slices off a wafer the ARM crowd ships a truck load of chips. SPARC will never have the mindshare necessary to polish the LLVM/Rust stack to parity.