If I'm understanding correct, the major change here for Rust users (rather than people who hack on the compiler) is that mutable references will not be considered to be "interfering" with other references being made at the same time until they're actually written to for the first time. This makes intuitive sense to me, but I suspect that there may be a bit of concern that this will make things more confusing when reading code and trying to understand what's going on. I'd be lying if I said that thought didn't occur to me, but at this point being surprised at how much I end up liking the way things turned out has become the norm for me; I remember having misgivings about nested import paths (rather than only being able to use `{`...`}` at the very end), match ergonomics, and `.await` as a postfix keyword but pretty quickly became glad they decided things the way they did after using each of them a bit when they finally got stabilized. I think I did realize that I'd like NLL (i.e. the borrow checker detecting the final use of a reference and not considering it as conflicting for the remainder of the scope) before it landed, but I know a lot of people had misgivings about that as well. I imagine this will be one of those things that in a few years will seem weird it wasn't always how it worked!

> ...by the time x.len() gets executed, arg0 already exists...

So, I realize that this is the way that Java does it--and, presumably, one still doesn't get fired for doing whatever Java does ;P--but, would it not actually make more sense for the arguments to be evaluated before the target reference, making the argument order more like Haskell/Erlang (but very sadly not Elixir, which makes it awkwardly incompatible with Erlang and breaks some of the basic stuff like fold/reduce)? Particularly so, given that, as far as I can tell from this example, what makes arg0 have the type that it does is the type of the function that hasn't even been called yet? (As in, the semantic gap I am seeing between what the user probably meant and what the compiler wants to do is that "x" shouldn't really be mutably-borrowed until the call happens, and the call here clearly shouldn't happen until after the arguments are evaluated.) (Note: I do not program in Rust currently; I just have spent a number of decades analyzing languages and at times teaching college language design courses. I might be missing something obvious elsewhere that forces Rust to do this, but that one example, in isolation, at least feels like an unforced error.)

I have not programmed Rust, yet. But this article gives me a feeling that this looks similar to database transactions. This might be wildly wrong but for me I see an analogy:

Once you get the &mut reference, you have your tree, which then looks to me like you have created a transaction. An in this transaction context you do your things.

    fn two_phase(mut x: Vec<usize>) {
        let arg0 = &mut x;
        let arg1 = Vec::len(&x);
        Vec::push(arg0, arg1);
    }

> This code clearly violates the regular borrow checking rules since x is mutably borrowed to arg0 when we call x.len()! And yet, the compiler will accept this code

Does anybody else wish the compiler wouldn't and would be even more verbose? I know one of the biggest learning curves (personally) for Rust is the borrow checker complaining hardcore and "getting in your way" preventing you from basically doing anything you're used to (passing around pointers in C or objects in JavaScript (even though you should be following immutable practices and not doing object mutation... most of the time))

I'm sure there's probably been discussions on how to make the borrow checker less "mean/rigid/obtuse" but silently passing something as "non mut" and it actually does "mut" stuff, I wouldn't have guessed Rust allowed that.

Edit: gah, I did not realize the function signature is (mut x), I thought it was just (x) and the mut was implied which is what I was trying to call out, apologies.

Interesting, and a nice demonstration of how a language gets more complicated "all by itself".

In Rust, `reciever.some_method(whatever)` is supposed to be relatively thin sugar for `TypeOfReciever::some_method(receiver, whatever)`. So the evaluation order should be the same for those two forms.

Doesn't Rust support having a function as a field of a struct?

If it does, then the order of evaluation of a.foo(b) would depend on whether foo is a field or a "free-standing" function of a, which seems horrible.

Also, there is a simple elegance in having the order of evaluation match the order the symbols are written that should require a very hight bar to reverse, in my opinion at least.

interesting. Can you expand on Elixir incompatibility? How is it different?

  Enum.map(list, func)

  def mapper(func), do: Enum.map(func)
  incrementer = mapper(fn x -> x + 1 end)
  incrementer(list)

The original code (which desugars to the snippet you posted) is:

    fn two_phase(mut x: Vec<usize>) {
        x.push(x.len());
    }

This should clearly be accepted (this is self evident in my opinion); if you need to jump through loops to write code like this then the language is too restrictive to write normal code.

The standard implementation of Rust does indeed accept this, and there is no soundness hole here.

The existing semantics for aliasing and borrowing from MPI (Stacked Borrows) don’t allow this, which means the semantics are overly restrictive; we want this to be accepted.

This work “fixes” this issue by extending the semantics to admit the behaviour exhibited by the standard implementation.

The rules for the borrow checker are not fully formalised and to some extent the rustc implementation is the specification; formalising the rules (i.e. RustBelt, Stacked Borrows, etc.) is important, but we don’t want to formalise something that is strictly more restrictive than the reference implementation, especially if there’s no soundness hole.

The borrow checker was made for correctness, not correctness for the borrow checker.

You have ownership of a Vec, you get its length, then you push to it through a mutable reference; nothing evil happens here except the order of the statements (which is an implementation detail that people might not think about when writing the short form x.push(x.len())). The code above is perfectly safe if written in C, which is why the borrow checker was extended to also allow it in Rust. You could make the argument that simpler borrow checker rules lead to a simpler mental model. The counterargument (that won in the end) is that "if it's safe, the borrow checker allows it" is a mental model worth pursuing.

> silently passing something as "non mut" and it actually does "mut" stuff

No, it's the opposite that's happening here: a mutable borrow of the vector is made, and then a non-mutable thing is done with it (getting the length), before finally mutating it (pushing).

I’ve been learning rust and I spend the vast majority of my time dealing with lifetimes and borrow checking. Common ways in used to doing things simply don’t work in rust and a lot of effort has to go into keeping track of how and where data is used.

I’ve worked in OOP languages, functional languages, and dynamic languages but all of them were essentially garbage collected, so having to keep track in my head of how data ownership is managed is a big learning curve.

>"Does anybody else wish the compiler wouldn't"

Compiler being obtuse and not being able to figure when it is safe to "break rules" is the problem. Not twisting brain of the programmer into being "safe compiler". This sounds like a Stockholm syndrome.

>"you should be following immutable practices"

No I should not. I should do what makes sense in particular situation and not bending over for some zealots trying to enforce one and the only way.