Alias-xor-mutability is useful for interior pointers (referencing into an enum, referencing into a Vec) because modification may change the memory layout and invalidate the interior pointer. But when the memory layout does not change in singlethread case, alias-xor-mutability can reject safe programs.
I don't understand what "singlethread case" means here. If you mean that only a single reference is active at any given point of the program, Rust allows you to make that a mutable borrowing. If multiple aliased references are active, then you don't really have a "single thread" of control, and modifying one reference may invalidate assertions made by others (even something as simple as n != 0 prior to a division, or n < array_size after a bound check). Shared mutable state must be signaled very clearly if you want to keep a sensible semantics, and Rust does this with the Cell<>, RefCell<>, Atomic* etc. constructs.
