It's less specified than rust. For example, you can bind references to individual collection elements here. It doesn't seem to prohibit borrowing a dynamically indexed element either, which is one hell of a static analysis lift. Seems like you can also have simultaneous references to distinct static indices (e.g. &mut arr[0] and &mut arr[1]) as long as you don't do invalidating operations, which isn't expressible in NLL or polonius. That's feasible for a borrowchecker at least.