sparseset | A Sparse Set in rust | Dataset library

it's totally fine to get references to different component sets mutably or immutably

This is true: we can safely have multiple mutable, or mutable and immutable references, as long as no mutable reference points to the same data as any other reference.

However, not every means of obtaining those references will be accepted by the compiler's borrow checker. This doesn't mean they're unsound; just that we haven't convinced the compiler that they're safe. In particular, the only way the compiler understands to have simultaneous references is a struct's fields, because the compiler can know those are disjoint using a purely local analysis (looking only at the code of a single function):