rust lifetime variance (fail mains does note compile)

rust_variance.rs

use std::marker::PhantomData;

// CONTRAVARIANT
struct Contra<'contra> (PhantomData<
  fn(&'contra ()) -> ()
>);
// succeeds (contravariant "lengthening")
fn contra_1<'a>( contra : Contra<'a>) -> Contra<'static> {
  contra
}
// fails
fn contra_2<'a>( contra : Contra<'static>) -> Contra<'a> 
{
  contra
}


// COVARIANT
struct Co<'co> (PhantomData<
  fn() -> &'co ()
>);

// fails
fn co_1<'a>( co : Co<'a>) -> Co<'static> {
  co
}
// succeeds (covariant "shortening")
fn co_2<'a>( co : Co<'static>) -> Co<'a> 
{
  co
}



// INVARIANT
struct Inv<'inv> (PhantomData<
  fn(&'inv ()) -> &'inv ()
>);

// fails
fn inv_1<'a>( inv : Inv<'a>) -> Inv<'static> {
  inv
}
// fails
fn inv_2<'a>( inv : Inv<'static>) -> Inv<'a> 
{
  inv
} // succeeds (invariant "same")
fn inv_3<'a>( inv : Inv<'a>) -> Inv<'a> 
{
  inv
}