The problem is not “a length of a vector”, Rust is not that fine grained. As far as Rust is concerned, what can happen is that you acquire an immutable reference to an object while there’s a mutable (unique) reference outstanding, you can invalidate the immutable reference via the mutable one, and thus have a dangling reference, which is UB.

Hell, even here one could imagine two scoped threads (so they can work with references), one thread keeps modifying the vector, and on some ISAs the other might get torn values for the length, or possibly worse (I don’t know if worse is an option but I wouldn’t be surprised).