That's the tyranny of Gödel incompleteness (or maybe Rice's theorem, or even both): useful formal systems can be either sound or complete. Rust makes the choice of being sound, with the price of course being that some valid operations not being expressible in the language. C of course works the other way around; all valid programs can be expressed, but there's no (general) way to distinguish invalid programs from valid programs.
For your concrete example of subdividing matrixes, that seems like it should be fairly straightforward in Rust too, if you convert your mutable reference to the data into a pointer, wrap your pointer arithmetic shenanigans in an unsafe block and add a comment at the top saying more or less "this is safe because the different subprograms are always operating on disjoint subsets of the data, and therefore no mutable aliasing can occur"?
For your concrete example of subdividing matrixes, that seems like it should be fairly straightforward in Rust too, if you convert your mutable reference to the data into a pointer, wrap your pointer arithmetic shenanigans in an unsafe block and add a comment at the top saying more or less "this is safe because the different subprograms are always operating on disjoint subsets of the data, and therefore no mutable aliasing can occur"?