> There is one owner, and the owner can not free the memory as long as there is a borrower (it doesn't matter whether the borrower is mutable or not). So that's the task of the borrow checker.

Yes, but what about:

``` let mut x = &mut some_vec; some_vec.push(10); ```

Sure the vec has an owner, but the second mutable borrow causes it to invalidate the first pointer. You need a _third_ category, “unstable mut”, which is exclusive as it can cause an object’s internal data to move. You can then collapse mut and immutable to one… but you end up with the exact some colouring problem between unstable mut and the others

▲ thomasmg 4 days ago | parent [-]

Yes, I see your point. In Rust, the owner, and the mutable borrower can change the pointer itself (like C realloc). If multiple "mut" borrows are allowed, then this would be unsafe, so I understand "unstable mut" would solve this problem - but result in a new colouring problem.

My solution to this would be: in a new language, do not allow reallocation. Not for owners, and not for mut borrow. This is what Java does: an ArrayList is a wrapper around an array, and so adding entries will not move the ArrayList object, just the (wrapped) private array. In Java the programmer never sees dangling references. Java prevents the issue by design, at the cost of always paying for the wrapper. If you want to avoid this price, you need to use the array directly.

(I have to admit I was not aware that in Rust, push moves the memory... so thanks a lot for explaining! Always learning something new.)

I don’t have any problem with Rust’s focus on performance. But its design does make life harder for developers than it needs to be. It doesn't match my vision of a programming language that is at the same time easy to use, safe, and nearly as fast as C.

▲ Dagonfly 4 days ago | parent [-]

> ArrayList is a wrapper around an array, and so adding entries will not move the ArrayList object, just the (wrapped) private array.

That's also how Vec works in Rust. Vec is just (buf_ptr, capacity, len) where capacity is the allocated size of the buffer.

The problem still exists though.

``` let mut v = vec![1, 2 ,3]; let x: &i32 = &v[0]; v.push(4); println!("First element {x}"); ```

The `push` might realloc the array. Then, x points into invalid memory. This is caused by the projection: You can create a reference to a field (aka member) from a reference to the base object.

A language without realloc sounds painful. Any growing container would lead to stale data.

▲ thomasmg 4 days ago | parent [-]

In Rust, a vector is a "fat pointer" (with pointer to the array and length / capacity) which lives on the stack. In Java, the pointer to the array and length / capacity lives in the heap. So in Java, there is an indirection, and it is allowed to have multiple pointers to the ArrayList object.

> A language without realloc sounds painful. Any growing container would lead to stale data.

I think it's not so much about realloc, but about whether it's a fat pointer or not. (I could imagine that Java uses something like realloc for the array, if there is only one pointer to the array for sure).

Fat pointers have some advantages and some disadvantages. Rust chose fat pointers, I assume for performance reasons. That's fine. Java doesn't. But I don't think that's a _huge_ performance disadvantage for Java. What I'm arguing is not so much that one is better and the other is worse, just that there are advantages and disadvantages. Rust might be slightly faster, but a language without (this kind of) fat pointers could potentially be easier to use.

▲ Dagonfly 3 days ago | parent [-]

That's true.

Though, for my example, the storage location of the array metadata/fat pointer is not relevant.

In Rust you can hold references directly into the buffer backing the array (`&v[0]`).

My Java knowledge is quite rusty at this point. AFAIK, a `ArrayList<MyType>` in Java stores pointers to MyType in the buffer. So, when indexing into a ArrayList, you never hold references into the actual buffer. Instead you get a shared-pointer to the class data. It's the indirection of the array entry that saves you during reallocation of the buffer.

Also because Java is a GC'ed VM, it wont dealloc the elements references by the array, as long as there are other references to an element.

The equivalent in Rust is `Vec<Rc<MyType>>` where holding an `&Rc<MyType>` referencing into the vec's buffer is problematic during reallocation. But, cloning the Rc and holding on to it is perfectly fine.

The initial point of this thread was that you can have a Rust-like language where you can hold multiple mutating (aliasing) references and prevent use-after-free. This won't work. Without a GC or RC, you can use one reference to "rug-pull" the memory that is aliases by the other reference.

▲ thomasmg 3 days ago | parent [-]

> In Rust you can hold references directly into the buffer backing the array

Yes! But I am arguing that this prevents having multiple mutable references

> My Java knowledge is quite rusty > Also because Java is a GC'ed VM ...

Your Java knowledge is fine :-) But I'm arguing that you don't strictly need a GC'ed, or RC'ed language: if done "correctly", multiple mutable references are possible. Just not with fat pointers! The programming language I'm building allows this even today. You can try it in the playground [1]:

    fun main()
        list := List+(int[4]) # <<= owned list
        borrow : &list        # <<= mutable borrow
        for i := until(4)
            borrow.add(i)
            list.add(10 * i)
        for i := until(8)
            println(borrow.array[i])
    
    type List
        array int[]
        size int
    
    fun List+ add(x int)
        if size >= array.len
            n : int[array.len * 2]
            for i := until(array.len)
                n[i] = array[i]
            array = n
        array[size] = x
        size += 1

So "List+" is owned type (just "List" without "+" would be reference counted). You may want to look at the generated C code at the end of the page.

[1]: https://thomasmueller.github.io/bau-lang/

	▲	john-h-k 3 days ago \| parent [-]
		You are borrowing the entire list. That’s fine. The problem occurs if you borrow a reference into the list. Java/C# solve this by making that operation impossible. You cannot hold a reference into a vector/list