Structurally normalize in the new solver

compiler/rustc_trait_selection/src/traits/structural_normalize.rs

@@ -0,0 +1,55 @@
+use rustc_infer::infer::at::At;
+use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
+use rustc_infer::traits::{FulfillmentError, TraitEngine};
+use rustc_middle::ty::{self, Ty};
+
+use crate::traits::{query::evaluate_obligation::InferCtxtExt, NormalizeExt, Obligation};
+
+pub trait StructurallyNormalizeExt<'tcx> {
+    fn structurally_normalize(
+        &self,
+        ty: Ty<'tcx>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx>,
+    ) -> Result<Ty<'tcx>, Vec<FulfillmentError<'tcx>>>;
+}
+
+impl<'tcx> StructurallyNormalizeExt<'tcx> for At<'_, 'tcx> {
+    fn structurally_normalize(
+        &self,
+        mut ty: Ty<'tcx>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx>,
+    ) -> Result<Ty<'tcx>, Vec<FulfillmentError<'tcx>>> {
+        assert!(!ty.is_ty_var(), "should have resolved vars before calling");
+
+        if self.infcx.tcx.trait_solver_next() {
+            while let ty::Alias(ty::Projection, projection_ty) = *ty.kind() {
+                let new_infer_ty = self.infcx.next_ty_var(TypeVariableOrigin {
+                    kind: TypeVariableOriginKind::NormalizeProjectionType,
+                    span: self.cause.span,
+                });
+                let obligation = Obligation::new(
+                    self.infcx.tcx,
+                    self.cause.clone(),
+                    self.param_env,
+                    ty::Binder::dummy(ty::ProjectionPredicate {
+                        projection_ty,
+                        term: new_infer_ty.into(),
+                    }),
+                );
+                if self.infcx.predicate_may_hold(&obligation) {
+                    fulfill_cx.register_predicate_obligation(self.infcx, obligation);
+                    let errors = fulfill_cx.select_where_possible(self.infcx);
+                    if !errors.is_empty() {
+                        return Err(errors);
+                    }
+                    ty = self.infcx.resolve_vars_if_possible(new_infer_ty);
+                } else {
+                    break;
+                }
+            }
+            Ok(ty)
+        } else {
+            Ok(self.normalize(ty).into_value_registering_obligations(self.infcx, fulfill_cx))
+        }
+    }
+}