Fix incosistent usage of push_lifetime_outlives

The code that relates Adt's was using `push_lifetime_outlives_goals` in
a way that assumed it would push a outlives b when the variance is
covariant. The code that relates two references assumed that it would
push a constraint b outlives a when the variance was covariant. This
latter behavior was what the function actually did, despite its name
implying the opposite.

This commit changes the behavior of `push_lifetime_outlives_goals` to
push a outlives b when the variance is covariant, and updates the code
that relates references.

We also updated an incorrect test case where a reference was used in a
struct. This test case would have failed (because
`push_lifetime_outlives_goals` was inverted), but the constraints in the
expected output were also incorrectly inverted.

The output of two additional test cases also were updated because the
constraints were emitted in a different order after making the update.

Co-authored-by: David Ross <daboross@daboross.net>

chalk-solve/src/infer/test.rs

@@ -413,10 +413,10 @@ fn lifetime_constraint_indirect() {
     assert_eq!(goals.len(), 2);
     assert_eq!(
         format!("{:?}", goals[0]),
-        "InEnvironment { environment: Env([]), goal: \'?2: \'!1_0 }",
+        "InEnvironment { environment: Env([]), goal: \'!1_0: \'?2 }",
     );
     assert_eq!(
         format!("{:?}", goals[1]),
-        "InEnvironment { environment: Env([]), goal: \'!1_0: \'?2 }",
+        "InEnvironment { environment: Env([]), goal: \'?2: \'!1_0 }",
     );
 }

chalk-solve/src/infer/unify.rs

@@ -275,13 +275,8 @@ impl<'t, I: Interner> Unifier<'t, I> {
                 if mutability_a != mutability_b {
                     return Err(NoSolution);
                 }
-                // The lifetime is `Contravariant`
-                Zip::zip_with(
-                    self,
-                    variance.xform(Variance::Contravariant),
-                    lifetime_a,
-                    lifetime_b,
-                )?;
+                // Zipping lifetimes implies a is a subtype of b, meaning a outlives b
+                Zip::zip_with(self, variance, lifetime_a, lifetime_b)?;
                 // The type is `Covariant` when not mut, `Invariant` otherwise
                 let output_variance = match mutability_a {
                     Mutability::Not => Variance::Covariant,
@@ -1035,16 +1030,16 @@ impl<'t, I: Interner> Unifier<'t, I> {
             self.goals.push(InEnvironment::new(
                 self.environment,
                 WhereClause::LifetimeOutlives(LifetimeOutlives {
-                    a: a.clone(),
-                    b: b.clone(),
+                    a: b.clone(),
+                    b: a.clone(),
                 })
                 .cast(self.interner),
             ));
         }
         if matches!(variance, Variance::Invariant | Variance::Covariant) {
             self.goals.push(InEnvironment::new(
                 self.environment,
-                WhereClause::LifetimeOutlives(LifetimeOutlives { a: b, b: a }).cast(self.interner),
+                WhereClause::LifetimeOutlives(LifetimeOutlives { a, b }).cast(self.interner),
             ));
         }
     }

tests/test/misc.rs

@@ -259,8 +259,8 @@ fn basic_region_constraint_from_positive_impl() {
             forall<'a, 'b, T> { Ref<'a, 'b, T>: Foo }
         } yields_all[SolverChoice::slg(3, None)] {
             "substitution [], lifetime constraints [\
-            InEnvironment { environment: Env([]), goal: '!1_0: '!1_1 }, \
-            InEnvironment { environment: Env([]), goal: '!1_1: '!1_0 } \
+            InEnvironment { environment: Env([]), goal: '!1_1: '!1_0 }, \
+            InEnvironment { environment: Env([]), goal: '!1_0: '!1_1 } \
             ]"
         }
     }

tests/test/projection.rs

@@ -799,10 +799,10 @@ fn normalize_under_binder_multi() {
         } yields_all {
             "substitution [?0 := I32], lifetime constraints []",
             "for<?U0,?U0> { substitution [?0 := (Deref::Item)<Ref<'^0.0, I32>, '^0.1>], lifetime constraints [\
-            InEnvironment { environment: Env([]), goal: '!1_0: '^0.1 }, \
             InEnvironment { environment: Env([]), goal: '^0.1: '!1_0 }, \
-            InEnvironment { environment: Env([]), goal: '!1_0: '^0.0 }, \
-            InEnvironment { environment: Env([]), goal: '^0.0: '!1_0 }] }"
+            InEnvironment { environment: Env([]), goal: '!1_0: '^0.1 }, \
+            InEnvironment { environment: Env([]), goal: '^0.0: '!1_0 }, \
+            InEnvironment { environment: Env([]), goal: '!1_0: '^0.0 }] }"
         }
 
         goal {

tests/test/subtype.rs

@@ -45,7 +45,7 @@ fn struct_lifetime_variance() {
             }
         } yields {
             "Unique; substitution [], lifetime constraints [\
-                InEnvironment { environment: Env([]), goal: '!1_1: '!1_0 } \
+                InEnvironment { environment: Env([]), goal: '!1_0: '!1_1 } \
             ]"
         }
     }