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Rollup merge of #107339 - aliemjay:covariant, r=lcnr

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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`
This commit is contained in:
Matthias Krüger 2023-01-28 05:20:18 +01:00 • committed by GitHub
commit a5caa989c9
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18 changed files with 43 additions and 52 deletions
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8
compiler/rustc_hir_analysis/src/variance/constraints.rs
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@ -225,8 +225,7 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
}
ty::Ref(region, ty, mutbl) => {
let contra = self.contravariant(variance);
self.add_constraints_from_region(current, region, contra);
self.add_constraints_from_region(current, region, variance);
self.add_constraints_from_mt(current, &ty::TypeAndMut { ty, mutbl }, variance);
}
@ -258,9 +257,8 @@ impl<'a, 'tcx> ConstraintContext<'a, 'tcx> {
}
ty::Dynamic(data, r, _) => {
// The type `Foo<T+'a>` is contravariant w/r/t `'a`:
let contra = self.contravariant(variance);
self.add_constraints_from_region(current, r, contra);
// The type `dyn Trait<T> +'a` is covariant w/r/t `'a`:
self.add_constraints_from_region(current, r, variance);
if let Some(poly_trait_ref) = data.principal() {
self.add_constraints_from_invariant_substs(