Focusing on ownership

13 May 2014

Over time, I’ve become convinced that it would be better to drop the distinction between mutable and immutable local variables in Rust. Many people are highly skeptical, to say the least. I wanted to lay out my argument in public. I’ll give various motivations: a philosophical one, an eductional one, and a practical one, and also address the main defense of the current system. (Note: I considered submitting this as a Rust RFC, but decided that the tone was better suited to a blog post, and I don’t have the time to rewrite it now.)

Just to be clear

I’ve written this article rather forcefully, and I do think that the path I’m advocating would be the right one. That said, if we wind up keeping the current system, it’s not a disaster or anything like that. It has its advantages and overall I find it pretty nice. I just think we can improve it.

One sentence summary

I would like to remove the distinction between immutable and mutable locals and rename &mut pointers to &my, &only, or &uniq (I don’t care). There would be no mut keyword.

Philosophical motivation

The main reason I want to do this is because I believe it makes the language more coherent and easier to understand. Basically, it refocuses us from talking about mutability to talking about aliasing (which I will call “sharing”, see below for more on that).

Mutability becomes a sideshow that is derived from uniqueness: “You can always mutate anything that you have unique access to. Shared data is generally immutable, but if you must, you can mutable it using some kind of cell type.”

Put another way, it’s become clear to me over time that the problems with data races and memory safety arise when you have both aliasing and mutability. The functional approach to solving this problem is to remove mutability. Rust’s approach would be to remove aliasing. This gives us a story to tell and helps to set us apart.

A note on terminology: I think we should refer to aliasing as sharing. In the past, we’ve avoided this because of its multithreaded connotations. However, if/when we implement the data parallelism plans I have proposed, then this connotation is not at all inappropriate. In fact, given the close relationship between memory safety and data races, I actually want to promote this connotation.

Eductional motivation

I think that the current rules are harder to understand than they have to be. It’s not obvious, for example, that &mut T implies no aliasing. Moreover, the notation &mut T suggests that &T implies no mutability, which is not entirely accurate, due to types like Cell. And nobody can agree on what to call them (“mutable/immutable reference” is the most common thing to say, but it’s not quite right).

In contrast, a type like &my T or &only T seems to make explanations much easier. This is a unique reference – of course you can’t make two of them pointing at the same place. And mutability is an orthogonal thing: it comes from uniqueness, but also cells. And the type &T is precisely its opposite, a shared reference. RFC PR #58 makes a number of similar arguments. I won’t repeat them here.

Practical motivation

Currently there is a disconnect between borrowed pointers, which can be either shared or mutable+unique, and local variables, which are always unique, but may be mutable or immutable. The end result of this is that users have to place mut declarations on things that are not directly mutated.

Locals can’t be modeled using references

This phenomenon arises from the fact that references are just not as expressive as local variables. In general, this hinders abstraction. Let me give you a few examples to explain what I mean. Imagine I have an environment struct that stores a pointer to an error counter:

struct Env { errors: &mut int }

Now I might create this structure (and use it) like so:

let mut errors = 0;
let env = Env { errors: &mut errors };
if some_condition {
    *env.errors += 1;

OK, now imagine that I want to extract out the code that mutates env.errors into a separate function. I might think that, since env is not declared as mutable above, I can use a & reference:

let mut errors = 0;
let env = Env { errors: &mut errors };

fn helper(env: &Env) {
  if some_condition {
      *env.errors += 1; // ERROR

But that is wrong. The problem is that &Env is an aliasable type, and hence env.errors appears in an aliasable location. To make this code work, I have to declare env as mutable and use an &mut reference:

let mut errors = 0;
let mut env = Env { errors: &mut errors };
helper(&mut env);

This problem arises because we know about locals being unique, but we can’t put that knowledge into a borrowed reference without making it mutable.

This problem arises in a number of other places. Until now, we’ve papered over it in a variety of ways, but I continue to feel like we’re papering over a disconnect that just shouldn’t be there.

Type-checking closures

We had to work around this limitation with closures. Closures are mostly desugarable into structs like Env, but not quite. This is because I didn’t want to require that &mut locals be declared mut if they are used in closures. In other words, given some code like:

fn foo(errors: &mut int) {
    do_something(|| *errors += 1)

The closure expression will in fact create an Env struct like:

struct ClosureEnv<'a, 'b> {
    errors: &uniq &mut int

Note the &uniq reference. That’s not something an end-user can type. It means a “unique but not necessarily mutable” pointer. It’s needed to make this all type check. If the user tried to write that struct manually, they’d have to write &mut &mut int, which would in turn require that the errors parameter be declared mut errors: &mut int.

Unboxed closures and procs

I foresee this limitation being an issue for unboxed closures. Let me elaborate on the design I was thinking of. Basically, the idea would be that a || expression is equivalent to some fresh struct type that implements one of the Fn traits:

trait Fn<A,R> { fn call(&self, ...); }
trait FnMut<A,R> { fn call(&mut self, ...); }
trait FnOnce<A,R> { fn call(self, ...); }

The precise trait would be selected by the expected type, as today. In this case, consumers of closures can write one of two things:

fn foo(&self, closure: FnMut<int,int>) { ... }
fn foo<T:FnMut<int,int>>(&self, closure: T) { ... }

We’ll … probably want to bikeshed the syntax, maybe add sugar like FnMut(int) -> int or retain |int| -> int, etc. That’s not so important, what matters is that we’d be passing in the closure by value. Note that with current DST rules it is legal to pass in a trait type by value as an argument, so the FnMut<int,int> argument is legal in DST and not an issue.

An aside: This design isn’t complete and I will describe the full details in a separate post.

The problem is that calling the closure will require an &mut reference. Since the closure is passed by value, users will again have to write a mut where it doesn’t seem to belong:

fn foo(&self, mut closure: FnMut<int,int>) {
    let x =;

This is the same problem as the Env example above: what’s really happening here is that the FnMut trait just wants a unique reference, but since that is not part of the type system, it requests a mutable reference.

Now, we can probably work around this in various ways. One thing we could do is to have the || syntax not expand to “some struct type” but rather “a struct type or a pointer to a struct type, as dictated by inference”. In that case, the callee could write:

fn foo(&self, closure: &mut FnMut<int,int>) {
    let x =;

I don’t mean to say this is the end of the world. But it’s one more in a growing of contortions we have to go through to retain this split between locals and references.

Other parts of the API

I haven’t done an exhaustive search, but naturally this distinction creeps in elsewhere. For example, to read from a Socket, I need a unique pointer, so I have to declare it mutable. Therefore, sometime like this doesn’t work:

let socket = Socket::new(); // ERROR: need a mutable reference

Naturally, in my proposal, code like this would work fine. You’d still get an error if you tried to read from a &Socket, but then it would say something like “can’t create a unique reference to a shared reference”, which I personally find more clear.

But don’t we need mut for safety?

No, we don’t. Rust programs would be equally sound if you just declared all bindings as mut. The compiler is perfectly capable of tracking which locals are being mutated at any point in time – precisely because they are local to the current function. What the type system really cares about is uniqueness.

The value I see in the current mut rules, and I won’t deny there is value, is primarily that they help to declare intent. That is, when I’m reading the code, I know which variables may be reassigned. On the other hand, I spend a lot of time reading C++ code too, and to be honest I’ve never noticed this as a major stumbling block. (Same goes for the time I’ve spent reading Java, JavaScript, Python, or Ruby code.)

It is also true that I have occasionally found bugs because I declared a variable as mut and failed to mutate it. I think we could get similar benefits via other, more aggressive lints (e.g., none of the variables used in the loop condition are mutated in the loop body). I personally cannot recall having encountered the opposite situation: that is, if the compiler says something must be mutable, that basically always means I forgot a mut keyword somewhere. (Think: when was the last time you responded to a compiler error about illegal mutation by doing anything other than restructuring the code to make the mutation legal?)


I see three alternatives to the current system:

  1. The one I have given, where you just drop “mutability” and track only uniqueness.

  2. One where you have three reference types: &, &uniq, and &mut. (As I wrote, this is in fact the type system we have today, at least from the borrow checker’s point of view.)

  3. A stricter variant in which “non-mut” variables are always considered aliased. That would mean that you’d have to write:

     let mut errors = 0;
     let mut p = &mut errors; // Note that `p` must be declared `mut`
     *p += 1;

    You’d need to declare p as mut because otherwise it’d be considered aliased, even though it’s a local, and hence mutating *p would be illegal. What feels weird about this scheme is that the local variable is not aliased, and we clearly know that, since we will allow it to be moved, run destructors on it and so forth. That is, we still have a notion of “owned” that is distinct from “not aliased”.

    On the other hand, if we described this system by saying that mutability inherits through &mut pointers, and not by talking about aliasing at all, it might make sense.

Of these three, I definitely prefer #1. It’s the simplest, and right now I am most concerned with how we can simplify Rust while retaining its character. Failing that, I think I prefer what we have right now.


Basically, I feel like the current rules around mutability have some value, but they come at a cost. They are presenting a kind of leaky abstraction: that is, they present a simple story that turns out to be incomplete. This causes confusion for people as they transition from the initial understanding, in which &mut is how mutability works, into the full understanding: sometimes mut is needed just to get uniqueness, and sometimes mutability comes without the mut keyword.

Moreover, we have to bend over backwards to maintain the fiction that mut means mutable and not unique. We had to add special cases to borrowck to check closures. We have to make the rules around &mut mutability more complex in general. We have to either add mut to closures so that we can call them, or make closure expressions have a less obvious desugaring. And so forth.

Finally, we wind up with a more complicated language overall. Instead of just having to think about aliasing and uniqueness, the user has to think about both aliasing and mutability, and the two are somehow tangled up together.

I don’t think it’s worth it.