Normalize field types before checking validity

compiler/rustc_const_eval/src/transform/validate.rs

@@ -810,13 +810,18 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                     let adt_def = self.tcx.adt_def(def_id);
                     assert!(adt_def.is_union());
                     assert_eq!(idx, FIRST_VARIANT);
-                    let dest = adt_def.non_enum_variant().fields[field].ty(self.tcx, args);
-                    if fields.len() != 1 {
+                    let dest_ty = self.tcx.normalize_erasing_regions(
+                        self.param_env,
+                        adt_def.non_enum_variant().fields[field].ty(self.tcx, args),
+                    );
+                    if fields.len() == 1 {
+                        let src_ty = fields.raw[0].ty(self.body, self.tcx);
+                        if !self.mir_assign_valid_types(src_ty, dest_ty) {
+                            self.fail(location, "union field has the wrong type");
+                        }
+                    } else {
                         self.fail(location, "unions should have one initialized field");
                     }
-                    if !self.mir_assign_valid_types(fields.raw[0].ty(self.body, self.tcx), dest) {
-                        self.fail(location, "union field has the wrong type");
-                    }
                 }
                 AggregateKind::Adt(def_id, idx, args, _, None) => {
                     let adt_def = self.tcx.adt_def(def_id);
@@ -826,10 +831,10 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
                         self.fail(location, "adt has the wrong number of initialized fields");
                     }
                     for (src, dest) in std::iter::zip(fields, &variant.fields) {
-                        if !self.mir_assign_valid_types(
-                            src.ty(self.body, self.tcx),
-                            dest.ty(self.tcx, args),
-                        ) {
+                        let dest_ty = self
+                            .tcx
+                            .normalize_erasing_regions(self.param_env, dest.ty(self.tcx, args));
+                        if !self.mir_assign_valid_types(src.ty(self.body, self.tcx), dest_ty) {
                             self.fail(location, "adt field has the wrong type");
                         }
                     }

tests/ui/type-alias-impl-trait/struct-assignment-validity.rs

@@ -0,0 +1,28 @@
+// compile-flags: -Zvalidate-mir
+// check-pass
+
+// Check that we don't cause cycle errors when validating pre-`Reveal::All` MIR
+// that assigns opaques through normalized projections.
+
+#![feature(impl_trait_in_assoc_type)]
+
+struct Bar;
+
+trait Trait {
+    type Assoc;
+    fn foo() -> Foo;
+}
+
+impl Trait for Bar {
+    type Assoc = impl std::fmt::Debug;
+    fn foo() -> Foo {
+        let x: <Bar as Trait>::Assoc = ();
+        Foo { field: () }
+    }
+}
+
+struct Foo {
+    field: <Bar as Trait>::Assoc,
+}
+
+fn main() {}