Why does a program compile despite an apparent lifetime mismatch?

Question

Given the following Rust program:

struct Value<'v>(&'v ());
struct Container {}

impl Container {
    fn get<'v>(&'v self) -> Value<'v> {
        todo!()
    }
    
    fn set<'v>(&'v self, x: Value<'v>) {
        todo!()
    }
}

fn convert<'v1, 'v2>(x: &'v1 Container, env: &'v2 Container) {
    let root: Value<'v2> = env.get();
    x.set(root);
}

I would expect convert to be a compile time error as Value<'v2> gets passed to x.set() which requires a value of type Value<'v1> - but it successfully compiles. There is no subtyping relationship between 'v1 and 'v2. How has Rust inferred satisfying lifetimes?

Racehorse

There's no way you can implement get and set to make convert unsound. Try it. – Connivent

@trentcl not without using unsafe. But in the case I'm writing, I am using unsafe, but "safely", and it is the case that convert is unsound (but I appreciate that's just reflecting that my unsafe isn't quite safe enough). – Racehorse

Answer 1

The compiler is always allowed to re-borrow with a shorter lifetime.

In this case, what happens in:

fn convert<'v1, 'v2>(x: &'v1 Container, env: &'v2 Container) {
    let root: Value<'v2> = env.get();
    x.set(root);
}

Is that the compiler reborrows x (aka (&*x)) with a lifetime 'v3, shorter than 'v2, which is allowed due to the (inferred) variance of Value<'v> (which matches &'v T).

---

It is possible to change the inferred variance of Value<'v> by changing the inner value:

• &'v () (the current) is covariant.
• Cell<&'v ()> is invariant.
• fn (&'v ()) -> () is contravariant, the inverse of covariant.

Using an invariant Value<'v> prevents unifying the lifetime with a fresh one, while using a contravariant Value<'v> only allows unifying the lifetime with a greater one.

Derbyshire

Thanks! And to stop the variance, I defined Value as Cell<&'v ()> which then causes a compile-time mismatch. – Racehorse

I meant contra, so I'll sneakily remove my comment and integrate it in the answer... with the fix. – Derbyshire