struct CovCov<'a>(fn(fn(&'a ()))); // Two wrongs make a right
struct ConCon<'a>(fn(fn(fn(&'a ())))); // Three wrongs make a left

fn verify_covariance<'a, 'b:'a>(c: CovCov<'b>) -> CovCov<'a> { c }
fn verify_contravariance<'a, 'b:'a>(c: ConCon<'a>) -> ConCon<'b> { c }

struct Struct;
impl Struct {
    fn concon_mut(&mut self) -> ConCon { ConCon(|_| ()) }
    fn covcov_mut(&mut self) -> CovCov { CovCov(|_| ()) }
}

fn main() {
    let mut x = Struct;

    {
        // This is forbidden. `a` and `b` both borrow from `x`.
        let a = x.covcov_mut();
        let b = x.covcov_mut();
    }

    {
        // This is okay. `a` and `b` do not borrow from `x`.
        let _a = x.concon_mut();
        let _b = x.concon_mut();
    }
}