Covariant and Invariant - Rust Example

variance.rs

struct CovariantExample<'a, T> {
    a: &'a  T
}

struct InvariantExample<'a, T> {
    a: &'a mut T
}

fn covarient() {
    let a = 1;
    let v2: &i32 = &a;

    let c1 = CovariantExample::<&'static i32> {a: &&1};
    let c2 = CovariantExample::<&i32> {a: &v2};

    let mut c3 = &c2;
    c3 = &c1;   // ==> This will works as CovariantExample<T> is covariant to T
}

fn invarient() {
    let  a = 1;
    let mut v2: &i32 = &a;

    let c1 = InvariantExample::<&'static i32> {a: &mut &1 };
    let c2 = InvariantExample::<&i32> {a: &mut v2};

    let mut c3 = &c2;
    // the below will fail to compile
    c3 = &c1;          // ==> This will fail to compile as InvariantExample<T> is invariant to T
}

fn main() {
    covarient();
    invarient();
}

The compilation fails with the compiler complaining about "variable a 'not live long enough'"

error[E0597]: `a` does not live long enough
  --> src/main.rs:27:24
   |
27 |     let mut v2: &i32 = &a;
   |                        ^^
   |                        |
   |                        borrowed value does not live long enough
   |                        assignment requires that `a` is borrowed for `'static`
...
35 | }
   | - `a` dropped here while still borrowed

It is due to the reason that, due to the line 34, the compile tries to make c2 type as InvariantExample<&'static i32>. It does it because InvariantExample<T> is invariant over T. So, it can't pass InvariantExample::<&'static i32> where InvariantExample::<&i32> is needed.

PS:

1. Theoretical Explanation https://doc.rust-lang.org/nomicon/subtyping.html
2. Compiler version used: rustc 1.48.0 (7eac88abb 2020-11-16)