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`

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)
     }