What you're describing is pointers and value semantics. Rust (and C and C++ and to some degree Go) has those. Java does not.

Pointers and value semantics are nice, but have nothing to do with ADTs. For example, in Rust, you can also do:

    let mut ex: i32 = 42;
    println!("{ex:?}"); // 42

    let ex_ref: &mut i32 = &mut ex;
    *ex_ref = 53;

    println!("{ex:?}"); // 53

And in C:

    int x = 42;
    printf("%d\n", x); // 42

    int* x_ref = &x;
    *x_ref = 53;

    printf("%d\n", x); // 53

In these examples, we haven't mutated the number 42 into the number 53. We've simply stored an entirely new value in the location of `x`. In your Rust example, you're doing the exact same thing with an ADT. The variant case isn't being changed. You're creating a new value and storing it in an existing storage location. Every pointer pointing to that storage location sees the update because they all point to the same storage.