Auto merge of #110204 - compiler-errors:new-solver-hir-typeck-hacks, r=lcnr

Deal with unnormalized projections when structurally resolving types with new solver

1. Normalize types in `structurally_resolved_type` when the new solver is enabled
2. Normalize built-in autoderef targets in `Autoderef` when the new solver is enabled
3. Normalize-erasing-regions in `resolve_type` in writeback

This is motivated by the UI test provided, which currently fails with:

```
error[E0609]: no field `x` on type `<usize as SliceIndex<[Foo]>>::Output`
 --> <source>:9:11
  |
9 |     xs[0].x = 1;
  |           ^
```

 I'm pretty happy with the approach in (1.) and (2.) and think we'll inevitably need something like this in the long-term, but (3.) seems like a hack to me. It's a *lot* of work to add tons of new calls to every user of these typeck results though (mir build, late lints, etc). Happy to discuss further.

r? `@lcnr`

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -17,6 +17,7 @@ pub mod query;
 mod select;
 mod specialize;
 mod structural_match;
+mod structural_normalize;
 mod util;
 mod vtable;
 pub mod wf;
@@ -62,6 +63,7 @@ pub use self::specialize::{
 pub use self::structural_match::{
     search_for_adt_const_param_violation, search_for_structural_match_violation,
 };
+pub use self::structural_normalize::StructurallyNormalizeExt;
 pub use self::util::elaborate;
 pub use self::util::{expand_trait_aliases, TraitAliasExpander};
 pub use self::util::{get_vtable_index_of_object_method, impl_item_is_final, upcast_choices};

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))
+        }
+    }
+}