Rollup merge of #130714 - compiler-errors:try-structurally-resolve-const, r=BoxyUwU

Introduce `structurally_normalize_const`, use it in `rustc_hir_typeck`

Introduces `structurally_normalize_const` to typecking to separate the "eval a const" step from the "try to turn a valtree into a target usize" in HIR typeck, where we may still have infer vars and stuff around.

I also changed `check_expr_repeat` to move a double evaluation of a const into a single one. I'll leave inline comments.

r? ```@BoxyUwU```

I hesitated to really test this on the new solver where it probably matters for unevaluated consts. If you're worried about the side-effects, I'd be happy to craft some more tests 😄

compiler/rustc_trait_selection/src/traits/structural_normalize.rs

@@ -46,4 +46,44 @@ impl<'tcx> At<'_, 'tcx> {
             Ok(self.normalize(ty).into_value_registering_obligations(self.infcx, fulfill_cx))
         }
     }
+
+    fn structurally_normalize_const<E: 'tcx>(
+        &self,
+        ct: ty::Const<'tcx>,
+        fulfill_cx: &mut dyn TraitEngine<'tcx, E>,
+    ) -> Result<ty::Const<'tcx>, Vec<E>> {
+        assert!(!ct.is_ct_infer(), "should have resolved vars before calling");
+
+        if self.infcx.next_trait_solver() {
+            let ty::ConstKind::Unevaluated(..) = ct.kind() else {
+                return Ok(ct);
+            };
+
+            let new_infer_ct = self.infcx.next_const_var(self.cause.span);
+
+            // We simply emit an `alias-eq` goal here, since that will take care of
+            // normalizing the LHS of the projection until it is a rigid projection
+            // (or a not-yet-defined opaque in scope).
+            let obligation = Obligation::new(
+                self.infcx.tcx,
+                self.cause.clone(),
+                self.param_env,
+                ty::PredicateKind::AliasRelate(
+                    ct.into(),
+                    new_infer_ct.into(),
+                    ty::AliasRelationDirection::Equate,
+                ),
+            );
+
+            fulfill_cx.register_predicate_obligation(self.infcx, obligation);
+            let errors = fulfill_cx.select_where_possible(self.infcx);
+            if !errors.is_empty() {
+                return Err(errors);
+            }
+
+            Ok(self.infcx.resolve_vars_if_possible(new_infer_ct))
+        } else {
+            Ok(self.normalize(ct).into_value_registering_obligations(self.infcx, fulfill_cx))
+        }
+    }
 }