Do binder folding eagerly in bound_coroutine_hidden_types

I refuse to fix this in the old solver; its lazy instantiation of
binders will be the end of me.

compiler/rustc_middle/src/ty/util.rs

@@ -682,6 +682,9 @@ impl<'tcx> TyCtxt<'tcx> {
 
     /// Return the set of types that should be taken into account when checking
     /// trait bounds on a coroutine's internal state.
+    // FIXME(compiler-errors): We should remove this when the old solver goes away;
+    // and all other usages of this function should go through `bound_coroutine_hidden_types`
+    // instead.
     pub fn coroutine_hidden_types(
         self,
         def_id: DefId,
@@ -694,6 +697,33 @@ impl<'tcx> TyCtxt<'tcx> {
             .map(|decl| ty::EarlyBinder::bind(decl.ty))
     }
 
+    /// Return the set of types that should be taken into account when checking
+    /// trait bounds on a coroutine's internal state. This properly replaces
+    /// `ReErased` with new existential bound lifetimes.
+    pub fn bound_coroutine_hidden_types(
+        self,
+        def_id: DefId,
+    ) -> impl Iterator<Item = ty::EarlyBinder<ty::Binder<'tcx, Ty<'tcx>>>> {
+        let coroutine_layout = self.mir_coroutine_witnesses(def_id);
+        coroutine_layout
+            .as_ref()
+            .map_or_else(|| [].iter(), |l| l.field_tys.iter())
+            .filter(|decl| !decl.ignore_for_traits)
+            .map(move |decl| {
+                let mut vars = vec![];
+                let ty = self.fold_regions(decl.ty, |re, debruijn| {
+                    assert_eq!(re, self.lifetimes.re_erased);
+                    let var = ty::BoundVar::from_usize(vars.len());
+                    vars.push(ty::BoundVariableKind::Region(ty::BrAnon));
+                    ty::Region::new_bound(self, debruijn, ty::BoundRegion { var, kind: ty::BrAnon })
+                });
+                ty::EarlyBinder::bind(ty::Binder::bind_with_vars(
+                    ty,
+                    self.mk_bound_variable_kinds(&vars),
+                ))
+            })
+    }
+
     /// Expands the given impl trait type, stopping if the type is recursive.
     #[instrument(skip(self), level = "debug", ret)]
     pub fn try_expand_impl_trait_type(
@@ -998,8 +1028,10 @@ impl<'tcx> OpaqueTypeExpander<'tcx> {
                 Some(expanded_ty) => *expanded_ty,
                 None => {
                     if matches!(self.inspect_coroutine_fields, InspectCoroutineFields::Yes) {
-                        for bty in self.tcx.coroutine_hidden_types(def_id) {
-                            let hidden_ty = bty.instantiate(self.tcx, args);
+                        for bty in self.tcx.bound_coroutine_hidden_types(def_id) {
+                            let hidden_ty = self.tcx.instantiate_bound_regions_with_erased(
+                                bty.instantiate(self.tcx, args),
+                            );
                             self.fold_ty(hidden_ty);
                         }
                     }

compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs

@@ -73,8 +73,8 @@ pub(in crate::solve) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
 
         ty::CoroutineWitness(def_id, args) => Ok(ecx
             .tcx()
-            .coroutine_hidden_types(def_id)
-            .map(|bty| replace_erased_lifetimes_with_bound_vars(tcx, bty.instantiate(tcx, args)))
+            .bound_coroutine_hidden_types(def_id)
+            .map(|bty| bty.instantiate(tcx, args))
             .collect()),
 
         // For `PhantomData<T>`, we pass `T`.
@@ -93,26 +93,6 @@ pub(in crate::solve) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
     }
 }
 
-pub(in crate::solve) fn replace_erased_lifetimes_with_bound_vars<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    ty: Ty<'tcx>,
-) -> ty::Binder<'tcx, Ty<'tcx>> {
-    let mut counter = 0;
-    let ty = tcx.fold_regions(ty, |r, current_depth| match r.kind() {
-        ty::ReErased => {
-            let br = ty::BoundRegion { var: ty::BoundVar::from_u32(counter), kind: ty::BrAnon };
-            counter += 1;
-            ty::Region::new_bound(tcx, current_depth, br)
-        }
-        // All free regions should be erased here.
-        r => bug!("unexpected region: {r:?}"),
-    });
-    let bound_vars = tcx.mk_bound_variable_kinds_from_iter(
-        (0..counter).map(|_| ty::BoundVariableKind::Region(ty::BrAnon)),
-    );
-    ty::Binder::bind_with_vars(ty, bound_vars)
-}
-
 #[instrument(level = "debug", skip(ecx), ret)]
 pub(in crate::solve) fn instantiate_constituent_tys_for_sized_trait<'tcx>(
     ecx: &EvalCtxt<'_, 'tcx>,
@@ -240,13 +220,8 @@ pub(in crate::solve) fn instantiate_constituent_tys_for_copy_clone_trait<'tcx>(
         // impl Copy/Clone for CoroutineWitness where T: Copy/Clone forall T in coroutine_hidden_types
         ty::CoroutineWitness(def_id, args) => Ok(ecx
             .tcx()
-            .coroutine_hidden_types(def_id)
-            .map(|bty| {
-                replace_erased_lifetimes_with_bound_vars(
-                    ecx.tcx(),
-                    bty.instantiate(ecx.tcx(), args),
-                )
-            })
+            .bound_coroutine_hidden_types(def_id)
+            .map(|bty| bty.instantiate(tcx, args))
             .collect()),
     }
 }

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -1438,10 +1438,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
                 ty::CoroutineWitness(def_id, args) => {
                     let tcx = self.tcx();
-                    stack.extend(tcx.coroutine_hidden_types(def_id).map(|bty| {
-                        let ty = bty.instantiate(tcx, args);
-                        debug_assert!(!ty.has_bound_regions());
-                        ty
+                    stack.extend(tcx.bound_coroutine_hidden_types(def_id).map(|bty| {
+                        self.infcx.enter_forall_and_leak_universe(bty.instantiate(tcx, args))
                     }))
                 }