[KMag]

However, there are real lifetime and thread-safety side effects of passing a reference to the outer structure when you only need a reference to the inner structure.

In a GC'd language without an effects system, passing in a reference to the outer structure would still prevent the GC from freeing the outer structure (depending on the ABI, even if you null out the reference passed outer reference inside the function, the value passed on the stack might be immutable) and would also mean that you need to be careful about later changes causing non-thread-safe mutations to the outer structure. If your language isn't GC'd and doesn't have an effects system, then you need to manually keep track of the borrowing.

These aren't artifacts of Rust's type system; they're genuine side effects that are present but more subtle in other languages.

[tatterdemalion]

In Rust, the outer struct will not be freed either because you cannot pass a reference to a field of a struct before the struct is freed - you can mem::replace to get that field out of the struct, but that's rather different.