Fall back to bidirectional normalizes-to if no subst-eq in alias-eq goal

compiler/rustc_trait_selection/src/solve/alias_relate.rs

@@ -79,12 +79,22 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
                     // them. This is necessary for inference during typeck.
                     //
                     // As this is incomplete, we must not do so during coherence.
-                    match (self.solver_mode(), subst_relate_response) {
-                        (SolverMode::Normal, Ok(response)) => Ok(response),
-                        (SolverMode::Normal, Err(NoSolution)) | (SolverMode::Coherence, _) => {
+                    match self.solver_mode() {
+                        SolverMode::Normal => {
+                            if let Ok(subst_relate_response) = subst_relate_response {
+                                Ok(subst_relate_response)
+                            } else if let Ok(bidirectional_normalizes_to_response) = self
+                                .assemble_bidirectional_normalizes_to_candidate(
+                                    param_env, lhs, rhs, direction,
+                                )
+                            {
+                                Ok(bidirectional_normalizes_to_response)
+                            } else {
                                 self.flounder(&candidates)
                             }
                         }
+                        SolverMode::Coherence => self.flounder(&candidates),
+                    }
                 }
             }
         }
@@ -100,27 +110,40 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         invert: Invert,
     ) -> QueryResult<'tcx> {
         self.probe(|ecx| {
+            ecx.normalizes_to_inner(param_env, alias, other, direction, invert)?;
+            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+        })
+    }
+
+    fn normalizes_to_inner(
+        &mut self,
+        param_env: ty::ParamEnv<'tcx>,
+        alias: ty::AliasTy<'tcx>,
+        other: ty::Term<'tcx>,
+        direction: ty::AliasRelationDirection,
+        invert: Invert,
+    ) -> Result<(), NoSolution> {
         let other = match direction {
             // This is purely an optimization.
             ty::AliasRelationDirection::Equate => other,
 
             ty::AliasRelationDirection::Subtype => {
-                    let fresh = ecx.next_term_infer_of_kind(other);
+                let fresh = self.next_term_infer_of_kind(other);
                 let (sub, sup) = match invert {
                     Invert::No => (fresh, other),
                     Invert::Yes => (other, fresh),
                 };
-                    ecx.sub(param_env, sub, sup)?;
+                self.sub(param_env, sub, sup)?;
                 fresh
             }
         };
-            ecx.add_goal(Goal::new(
-                ecx.tcx(),
+        self.add_goal(Goal::new(
+            self.tcx(),
             param_env,
             ty::Binder::dummy(ty::ProjectionPredicate { projection_ty: alias, term: other }),
         ));
-            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-        })
+
+        Ok(())
     }
 
     fn assemble_subst_relate_candidate(
@@ -143,4 +166,30 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         })
     }
+
+    fn assemble_bidirectional_normalizes_to_candidate(
+        &mut self,
+        param_env: ty::ParamEnv<'tcx>,
+        lhs: ty::Term<'tcx>,
+        rhs: ty::Term<'tcx>,
+        direction: ty::AliasRelationDirection,
+    ) -> QueryResult<'tcx> {
+        self.probe(|ecx| {
+            ecx.normalizes_to_inner(
+                param_env,
+                lhs.to_alias_ty(ecx.tcx()).unwrap(),
+                rhs,
+                direction,
+                Invert::No,
+            )?;
+            ecx.normalizes_to_inner(
+                param_env,
+                rhs.to_alias_ty(ecx.tcx()).unwrap(),
+                lhs,
+                direction,
+                Invert::Yes,
+            )?;
+            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+        })
+    }
 }

tests/ui/traits/new-solver/tait-eq-proj-2.rs

@@ -0,0 +1,21 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+// Similar to tests/ui/traits/new-solver/tait-eq-proj.rs
+// but check the alias-sub relation in the other direction.
+
+type Tait = impl Iterator<Item = impl Sized>;
+
+fn mk<T>() -> T { todo!() }
+
+fn a() {
+    let x: Tait = mk();
+    let mut array = mk();
+    let mut z = IntoIterator::into_iter(array);
+    z = x;
+    array = [0i32; 32];
+}
+
+fn main() {}

tests/ui/traits/new-solver/tait-eq-proj.rs

@@ -0,0 +1,35 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+type Tait = impl Iterator<Item = impl Sized>;
+
+/*
+
+Consider the goal - AliasRelate(Tait, <[i32; 32] as IntoIterator>::IntoIter)
+which is registered on the line above.
+
+A. SubstRelate - fails (of course).
+
+B. NormalizesToRhs - Tait normalizes-to <[i32; 32] as IntoIterator>::IntoIter
+    * infer definition - Tait := <[i32; 32] as IntoIterator>::IntoIter
+
+C. NormalizesToLhs - <[i32; 32] as IntoIterator>::IntoIter normalizes-to Tait
+    * Find impl candidate, after substitute - std::array::IntoIter<i32, 32>
+    * Equate std::array::IntoIter<i32, 32> and Tait
+        * infer definition - Tait := std::array::IntoIter<i32, 32>
+
+B and C are not equal, but they are equivalent modulo normalization.
+
+We get around this by evaluating both the NormalizesToRhs and NormalizesToLhs
+goals together. Essentially:
+    A alias-relate B if A normalizes-to B and B normalizes-to A.
+
+*/
+
+fn a() {
+    let _: Tait = IntoIterator::into_iter([0i32; 32]);
+}
+
+fn main() {}

tests/ui/traits/new-solver/tait-eq-tait.rs

@@ -0,0 +1,17 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+// Not exactly sure if this is the inference behavior we *want*,
+// but it is a side-effect of the lazy normalization of TAITs.
+
+#![feature(type_alias_impl_trait)]
+
+type Tait = impl Sized;
+type Tait2 = impl Sized;
+
+fn mk<T>() -> T { todo!() }
+
+fn main() {
+    let x: Tait = 1u32;
+    let y: Tait2 = x;
+}