internally change regions to be covariant
Surprisingly, we consider the reference type `&'a T` to be contravaraint in its lifetime parameter. This is confusing and conflicts with the documentation we have in the reference, rustnomicon, and rustc-dev-guide. This also arguably not the correct use of terminology since we can use `&'static u8` in a place where `&' a u8` is expected, this implies that `&'static u8 <: &' a u8` and consequently `'static <: ' a`, hence covariance.
Because of this, when relating two types, we used to switch the argument positions in a confusing way:
`Subtype(&'a u8 <: &'b u8) => Subtype('b <: 'a) => Outlives('a: 'b) => RegionSubRegion('b <= 'a)`
The reason for the current behavior is probably that we wanted `Subtype('b <: 'a)` and `RegionSubRegion('b <= 'a)` to be equivalent, but I don' t think this is a good reason since these relations are sufficiently different in that the first is a relation in the subtyping lattice and is intrinsic to the type-systems, while the the second relation is an implementation detail of regionck.
This PR changes this behavior to use covariance, so..
`Subtype(&'a u8 <: &'b u8) => Subtype('a <: 'b) => Outlives('a: 'b) => RegionSubRegion('b <= 'a) `
Resolves#103676
r? `@lcnr`
a5caa989c9