It is if you want to have automatic deallocation with a strict guarantee of memory safety including no use-after-free bugs.

How does this relate to whether or not you plan to mutate things? Wouldn't plain ole counting work for this, regardless of whether or not they're mutable, rather than what Rust does - allowing multiple readers and no writers, or zero readers and 1 writer?

Refcounting is GC; it automatically detects at runtime that an object has no references. It also adds a substantial cost that Rust is trying to avoid, often by copying immutable objects (which have better cache locality than random access to refcounts everywhere).

I'm not talking about GC. I'm talking about Rust references:

https://doc.rust-lang.org/book/ch04-02-references-and-borrow...

> At any given time, you can have either one mutable reference or any number of immutable references.

Oh, I get it, you're suggesting all references could be mutable, but still limited to the lifetime of the object. I guess the downside would be that you can't design an API like iterator invalidation that relies on "some (mutable) methods can't be called while any other (immutable) references exist".

It would mean that, once you obtain a derived pointer – say, if you start with a Vec<Foo>, a pointer to Foo that refers to the first element – you would have to throw away that pointer as soon as you made any function call whatsoever. After all, that function call might mutate the Vec<Foo> and cause the backing storage to be reallocated.

In practice, this is unworkable enough that it would basically forces you to reference count the Foo, so you could preserve pointers across calls by incrementing the reference count.

On the other hand, Rust's approach is suboptimal in cases where you're using reference counting anyway, or are willing to pay the cost of reference counting.

The immutable borrows also help prevent iterator invalidation in single threaded code when using an iterator.

Yes, exactly. One of my first big fights with the borrow checker was over not realizing I was invalidating an iterator by mutating it by looping over it. Then it clicked for me.