Bug in lifetime checker

[bvssvni]

fn main() {
    x := 0
    a := []
    y := 1
    foo(mut a, x, y)
}

fn foo(mut a, x: 'a, y) {
    a = [x, y]
}

[bvssvni]

This can be solved by introducing LifetimeError and LifetimeResult:

#[derive(Debug)]
pub enum LifetimeError {
    None,
    FailedToUnify,
}

pub type LifetimeResult = Result<Lifetime, LifetimeError>;

The LifetimeError::FailedToUnify is used when it is undecidable which lifetime to assign to the expression.

In most cases, one can solve these problems using clone, so this is suggested in the error message.

This design makes a pragmatic tradeoff between practical use and computational complexity.