Macro lifelink::cov

macro_rules! cov {
    (<$($tv:ident $(: $bound:tt $(+ $bounds:tt)*)?),*> $thing:ty) => { ... };
    ($thing:ty) => { ... };
}

Macro that implements Cov safely for a Ctor type.

Cov::cov is implemented by simply returning the input reference and letting Rust coerce it. This should work automatically for most types that are actually covariant. In case cov!(MyType) does not compile, users would need to provide their own unsafe Cov implementation.

See Cov for an example.