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Subtyping and Variance

Rust uses lifetimes to track the relationships between borrows and ownership. However, a naive implementation of lifetimes would be either too restrictive, or permit undefined behavior.

In order to allow flexible usage of lifetimes while also preventing their misuse, Rust uses subtyping and variance.

Let's start with an example.

// Note: debug expects two parameters with the *same* lifetime
fn debug<'a>(a: &'a str, b: &'a str) {
    println!("a = {a:?} b = {b:?}");
}

fn main() {
    let hello: &'static str = "hello";
    {
        let world = String::from("world");
        let world = &world; // 'world has a shorter lifetime than 'static
        debug(hello, world);
    }
}

In a conservative implementation of lifetimes, since hello and world have different lifetimes, we might see the following error:

error[E0308]: mismatched types
 --> src/main.rs:10:16
   |
10 |         debug(hello, world);
   |                      ^
   |                      |
   |                      expected `&'static str`, found struct `&'world str`

This would be rather unfortunate. In this case, what we want is to accept any type that lives at least as long as 'world. Let's try using subtyping with our lifetimes.

Subtyping

Subtyping is the idea that one type can be used in place of another.

Let's define that Sub is a subtype of Super (we'll be using the notation Sub <: Super throughout this chapter).

What this is suggesting to us is that the set of requirements that Super defines are completely satisfied by Sub. Sub may then have more requirements.

Now, in order to use subtyping with lifetimes, we need to define the requirement of a lifetime:

'a defines a region of code.

Now that we have a defined set of requirements for lifetimes, we can define how they relate to each other:

'long <: 'short if and only if 'long defines a region of code that completely contains 'short.

'long may define a region larger than 'short, but that still fits our definition.

As we will see throughout the rest of this chapter, subtyping is a lot more complicated and subtle than this, but this simple rule is a very good 99% intuition. And unless you write unsafe code, the compiler will automatically handle all the corner cases for you.

But this is the Rustonomicon. We're writing unsafe code, so we need to understand how this stuff really works, and how we can mess it up.

Going back to our example above, we can say that 'static <: 'world. For now, let's also accept the idea that subtypes of lifetimes can be passed through references (more on this in Variance), e.g. &'static str is a subtype of &'world str, then we can "downgrade" &'static str into a &'world str. With that, the example above will compile:

fn debug<'a>(a: &'a str, b: &'a str) {
    println!("a = {a:?} b = {b:?}");
}

fn main() {
    let hello: &'static str = "hello";
    {
        let world = String::from("world");
        let world = &world; // 'world has a shorter lifetime than 'static
        debug(hello, world); // hello silently downgrades from `&'static str` into `&'world str`
    }
}

Variance

Above, we glossed over the fact that 'static <: 'b implied that &'static T <: &'b T. This uses a property known as variance. It's not always as simple as this example, though. To understand that, let's try to extend this example a bit:

fn assign<T>(input: &mut T, val: T) {
    *input = val;
}

fn main() {
    let mut hello: &'static str = "hello";
    {
        let world = String::from("world");
        assign(&mut hello, &world);
    }
    println!("{hello}"); // use after free 😿
}

In assign, we are setting the hello reference to point to world. But then world goes out of scope, before the later use of hello in the println!

This is a classic use-after-free bug!

Our first instinct might be to blame the assign impl, but there's really nothing wrong here. It shouldn't be surprising that we might want to assign a T into a T.

The problem is that we cannot assume that &mut &'static str and &mut &'b str are compatible. This means that &mut &'static str cannot be a subtype of &mut &'b str, even if 'static is a subtype of 'b.

Variance is the concept that Rust borrows to define relationships about subtypes through their generic parameters.

NOTE: For convenience we will define a generic type F<T> so that we can easily talk about T. Hopefully this is clear in context.

The type F's variance is how the subtyping of its inputs affects the subtyping of its outputs. There are three kinds of variance in Rust. Given two types Sub and Super, where Sub is a subtype of Super:

F is covariant if F<Sub> is a subtype of F<Super> (the subtype property is passed through)
F is contravariant if F<Super> is a subtype of F<Sub> (the subtype property is "inverted")
F is invariant otherwise (no subtyping relationship exists)

If we remember from the above examples, it was ok for us to treat &'a T as a subtype of &'b T if 'a <: 'b, therefore we can say that &'a T is covariant over 'a.

Also, we saw that it was not ok for us to treat &mut &'a U as a subtype of &mut &'b U, therefore we can say that &mut T is invariant over T

Here is a table of some other generic types and their variances:

	'a	T	U
`&'a T`	covariant	covariant
`&'a mut T`	covariant	invariant
`Box<T>`		covariant
`Vec<T>`		covariant
`UnsafeCell<T>`		invariant
`Cell<T>`		invariant
`fn(T) -> U`		contravariant	covariant
`*const T`		covariant
`*mut T`		invariant

Some of these can be explained simply in relation to the others:

Vec<T> and all other owning pointers and collections follow the same logic as Box<T>
Cell<T> and all other interior mutability types follow the same logic as UnsafeCell<T>
UnsafeCell<T> having interior mutability gives it the same variance properties as &mut T
*const T follows the logic of &T
*mut T follows the logic of &mut T (or UnsafeCell<T>)

For more types, see the "Variance" section on the reference.

NOTE: the only source of contravariance in the language is the arguments to a function, which is why it really doesn't come up much in practice. Invoking contravariance involves higher-order programming with function pointers that take references with specific lifetimes (as opposed to the usual "any lifetime", which gets into higher rank lifetimes, which work independently of subtyping).

Now that we have some more formal understanding of variance, let's go through some more examples in more detail.

fn assign<T>(input: &mut T, val: T) {
    *input = val;
}

fn main() {
    let mut hello: &'static str = "hello";
    {
        let world = String::from("world");
        assign(&mut hello, &world);
    }
    println!("{hello}");
}

And what do we get when we run this?

error[E0597]: `world` does not live long enough
  --> src/main.rs:9:28
   |
6  |     let mut hello: &'static str = "hello";
   |                    ------------ type annotation requires that `world` is borrowed for `'static`
...
9  |         assign(&mut hello, &world);
   |                            ^^^^^^ borrowed value does not live long enough
10 |     }
   |     - `world` dropped here while still borrowed

Good, it doesn't compile! Let's break down what's happening here in detail.

First let's look at the assign function:

fn assign<T>(input: &mut T, val: T) {
    *input = val;
}

All it does is take a mutable reference and a value and overwrite the referent with it. What's important about this function is that it creates a type equality constraint. It clearly says in its signature the referent and the value must be the exact same type.

Meanwhile, in the caller we pass in &mut &'static str and &'world str.

Because &mut T is invariant over T, the compiler concludes it can't apply any subtyping to the first argument, and so T must be exactly &'static str.

This is counter to the &T case:

fn debug<T: std::fmt::Debug>(a: T, b: T) {
    println!("a = {a:?} b = {b:?}");
}

where similarly a and b must have the same type T. But since &'a T is covariant over 'a, we are allowed to perform subtyping. So the compiler decides that &'static str can become &'b str if and only if &'static str is a subtype of &'b str, which will hold if 'static <: 'b. This is true, so the compiler is happy to continue compiling this code.

As it turns out, the argument for why it's ok for Box (and Vec, HashMap, etc.) to be covariant is pretty similar to the argument for why it's ok for lifetimes to be covariant: as soon as you try to stuff them in something like a mutable reference, they inherit invariance and you're prevented from doing anything bad.

However Box makes it easier to focus on the by-value aspect of references that we partially glossed over.

Unlike a lot of languages which allow values to be freely aliased at all times, Rust has a very strict rule: if you're allowed to mutate or move a value, you are guaranteed to be the only one with access to it.

Consider the following code:

let hello: Box<&'static str> = Box::new("hello");

let mut world: Box<&'b str>;
world = hello;

There is no problem at all with the fact that we have forgotten that hello was alive for 'static, because as soon as we moved hello to a variable that only knew it was alive for 'b, we destroyed the only thing in the universe that remembered it lived for longer!

Only one thing left to explain: function pointers.

To see why fn(T) -> U should be covariant over U, consider the following signature:

fn get_str() -> &'a str;

This function claims to produce a str bound by some liftime 'a. As such, it is perfectly valid to provide a function with the following signature instead:

fn get_static() -> &'static str;

So when the function is called, all it's expecting is a &str which lives at least the lifetime of 'a, it doesn't matter if the value actually lives longer.

However, the same logic does not apply to arguments. Consider trying to satisfy:

fn store_ref(&'a str);

with:

fn store_static(&'static str);

The first function can accept any string reference as long as it lives at least for 'a, but the second cannot accept a string reference that lives for any duration less than 'static, which would cause a conflict. Covariance doesn't work here. But if we flip it around, it actually does work! If we need a function that can handle &'static str, a function that can handle any reference lifetime will surely work fine.

Let's see this in practice

# use std::cell::RefCell;
thread_local! {
    pub static StaticVecs: RefCell<Vec<&'static str>> = RefCell::new(Vec::new());
}

/// saves the input given into a thread local `Vec<&'static str>`
fn store(input: &'static str) {
    StaticVecs.with_borrow_mut(|v| v.push(input));
}

/// Calls the function with it's input (must have the same lifetime!)
fn demo<'a>(input: &'a str, f: fn(&'a str)) {
    f(input);
}

fn main() {
    demo("hello", store); // "hello" is 'static. Can call `store` fine

    {
        let smuggle = String::from("smuggle");

        // `&smuggle` is not static. If we were to call `store` with `&smuggle`,
        // we would have pushed an invalid lifetime into the `StaticVecs`.
        // Therefore, `fn(&'static str)` cannot be a subtype of `fn(&'a str)`
        demo(&smuggle, store);
    }

    // use after free 😿
    StaticVecs.with_borrow(|v| println!("{v:?}"));
}

And that's why function types, unlike anything else in the language, are contravariant over their arguments.

Now, this is all well and good for the types the standard library provides, but how is variance determined for types that you define? A struct, informally speaking, inherits the variance of its fields. If a struct MyType has a generic argument A that is used in a field a, then MyType's variance over A is exactly a's variance over A.

However if A is used in multiple fields:

If all uses of A are covariant, then MyType is covariant over A
If all uses of A are contravariant, then MyType is contravariant over A
Otherwise, MyType is invariant over A

use std::cell::Cell;

struct MyType<'a, 'b, A: 'a, B: 'b, C, D, E, F, G, H, In, Out, Mixed> {
    a: &'a A,     // covariant over 'a and A
    b: &'b mut B, // covariant over 'b and invariant over B

    c: *const C,  // covariant over C
    d: *mut D,    // invariant over D

    e: E,         // covariant over E
    f: Vec<F>,    // covariant over F
    g: Cell<G>,   // invariant over G

    h1: H,        // would also be covariant over H except...
    h2: Cell<H>,  // invariant over H, because invariance wins all conflicts

    i: fn(In) -> Out,       // contravariant over In, covariant over Out

    k1: fn(Mixed) -> usize, // would be contravariant over Mixed except..
    k2: Mixed,              // invariant over Mixed, because invariance wins all conflicts
}

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Subtyping and Variance

Subtyping

Variance

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