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

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`

Author: matthiaskrgr

compiler/rustc_hir_analysis/src/variance/constraints.rs

@@ -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(

compiler/rustc_infer/src/infer/glb.rs

@@ -79,7 +79,8 @@ impl<'tcx> TypeRelation<'tcx> for Glb<'_, '_, 'tcx> {
         debug!("{}.regions({:?}, {:?})", self.tag(), a, b);
 
         let origin = Subtype(Box::new(self.fields.trace.clone()));
-        Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().glb_regions(
+        // GLB(&'static u8, &'a u8) == &RegionLUB('static, 'a) u8 == &'static u8
+        Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().lub_regions(
             self.tcx(),
             origin,
             a,

compiler/rustc_infer/src/infer/lub.rs

@@ -79,7 +79,8 @@ impl<'tcx> TypeRelation<'tcx> for Lub<'_, '_, 'tcx> {
         debug!("{}.regions({:?}, {:?})", self.tag(), a, b);
 
         let origin = Subtype(Box::new(self.fields.trace.clone()));
-        Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().lub_regions(
+        // LUB(&'static u8, &'a u8) == &RegionGLB('static, 'a) u8 == &'a u8
+        Ok(self.fields.infcx.inner.borrow_mut().unwrap_region_constraints().glb_regions(
             self.tcx(),
             origin,
             a,

compiler/rustc_infer/src/infer/nll_relate/mod.rs

@@ -663,13 +663,13 @@ where
         debug!(?v_b);
 
         if self.ambient_covariance() {
-            // Covariance: a <= b. Hence, `b: a`.
-            self.push_outlives(v_b, v_a, self.ambient_variance_info);
+            // Covariant: &'a u8 <: &'b u8. Hence, `'a: 'b`.
+            self.push_outlives(v_a, v_b, self.ambient_variance_info);
         }
 
         if self.ambient_contravariance() {
-            // Contravariant: b <= a. Hence, `a: b`.
-            self.push_outlives(v_a, v_b, self.ambient_variance_info);
+            // Contravariant: &'b u8 <: &'a u8. Hence, `'b: 'a`.
+            self.push_outlives(v_b, v_a, self.ambient_variance_info);
         }
 
         Ok(a)

compiler/rustc_infer/src/infer/sub.rs

@@ -191,12 +191,13 @@ impl<'tcx> TypeRelation<'tcx> for Sub<'_, '_, 'tcx> {
         // from the "cause" field, we could perhaps give more tailored
         // error messages.
         let origin = SubregionOrigin::Subtype(Box::new(self.fields.trace.clone()));
+        // Subtype(&'a u8, &'b u8) => Outlives('a: 'b) => SubRegion('b, 'a)
         self.fields
             .infcx
             .inner
             .borrow_mut()
             .unwrap_region_constraints()
-            .make_subregion(origin, a, b);
+            .make_subregion(origin, b, a);
 
         Ok(a)
     }

compiler/rustc_middle/src/ty/relate.rs

@@ -443,12 +443,7 @@ pub fn super_relate_tys<'tcx, R: TypeRelation<'tcx>>(
             if a_repr == b_repr =>
         {
             let region_bound = relation.with_cause(Cause::ExistentialRegionBound, |relation| {
-                relation.relate_with_variance(
-                    ty::Contravariant,
-                    ty::VarianceDiagInfo::default(),
-                    a_region,
-                    b_region,
-                )
+                relation.relate(a_region, b_region)
             })?;
             Ok(tcx.mk_dynamic(relation.relate(a_obj, b_obj)?, region_bound, a_repr))
         }
@@ -497,12 +492,7 @@ pub fn super_relate_tys<'tcx, R: TypeRelation<'tcx>>(
         }
 
         (&ty::Ref(a_r, a_ty, a_mutbl), &ty::Ref(b_r, b_ty, b_mutbl)) => {
-            let r = relation.relate_with_variance(
-                ty::Contravariant,
-                ty::VarianceDiagInfo::default(),
-                a_r,
-                b_r,
-            )?;
+            let r = relation.relate(a_r, b_r)?;
             let a_mt = ty::TypeAndMut { ty: a_ty, mutbl: a_mutbl };
             let b_mt = ty::TypeAndMut { ty: b_ty, mutbl: b_mutbl };
             let mt = relate_type_and_mut(relation, a_mt, b_mt, a)?;

tests/ui/error-codes/E0208.rs

@@ -1,7 +1,7 @@
 #![feature(rustc_attrs)]
 
 #[rustc_variance]
-struct Foo<'a, T> { //~ ERROR [-, o]
+struct Foo<'a, T> { //~ ERROR [+, o]
     t: &'a mut T,
 }
 

tests/ui/error-codes/E0208.stderr

@@ -1,4 +1,4 @@
-error: [-, o]
+error: [+, o]
   --> $DIR/E0208.rs:4:1
    |
 LL | struct Foo<'a, T> {

tests/ui/variance/variance-associated-types.rs

@@ -10,7 +10,7 @@ trait Trait<'a> {
 }
 
 #[rustc_variance]
-struct Foo<'a, T : Trait<'a>> { //~ ERROR [-, +]
+struct Foo<'a, T : Trait<'a>> { //~ ERROR [+, +]
     field: (T, &'a ())
 }
 

tests/ui/variance/variance-associated-types.stderr

@@ -1,4 +1,4 @@
-error: [-, +]
+error: [+, +]
   --> $DIR/variance-associated-types.rs:13:1
    |
 LL | struct Foo<'a, T : Trait<'a>> {