Use placeholders to prevent using inferred RPITIT types to imply their own WF-ness

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -14,6 +14,7 @@ use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKi
 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
 use rustc_infer::traits::util;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
+use rustc_middle::ty::fold::BottomUpFolder;
 use rustc_middle::ty::util::ExplicitSelf;
 use rustc_middle::ty::{
     self, GenericArgs, Ty, TypeFoldable, TypeFolder, TypeSuperFoldable, TypeVisitableExt,
@@ -692,9 +693,9 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
     let ocx = ObligationCtxt::new(infcx);
 
     // Normalize the impl signature with fresh variables for lifetime inference.
-    let norm_cause = ObligationCause::misc(return_span, impl_m_def_id);
+    let misc_cause = ObligationCause::misc(return_span, impl_m_def_id);
     let impl_sig = ocx.normalize(
-        &norm_cause,
+        &misc_cause,
         param_env,
         tcx.liberate_late_bound_regions(
             impl_m.def_id,
@@ -725,12 +726,68 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
         );
     }
 
-    let trait_sig = ocx.normalize(&norm_cause, param_env, unnormalized_trait_sig);
+    let trait_sig = ocx.normalize(&misc_cause, param_env, unnormalized_trait_sig);
     trait_sig.error_reported()?;
     let trait_return_ty = trait_sig.output();
 
+    // RPITITs are allowed to use the implied predicates of the method that
+    // defines them. This is because we want code like:
+    // ```
+    // trait Foo {
+    //     fn test<'a, T>(_: &'a T) -> impl Sized;
+    // }
+    // impl Foo for () {
+    //     fn test<'a, T>(x: &'a T) -> &'a T { x }
+    // }
+    // ```
+    // .. to compile. However, since we use both the normalized and unnormalized
+    // inputs and outputs from the substituted trait signature, we will end up
+    // seeing the hidden type of an RPIT in the signature itself. Naively, this
+    // means that we will use the hidden type to imply the hidden type's own
+    // well-formedness.
+    //
+    // To avoid this, we replace the infer vars used for hidden type inference
+    // with placeholders, which imply nothing about outlives bounds, and then
+    // prove below that the hidden types are well formed.
+    let universe = infcx.create_next_universe();
+    let mut idx = 0;
+    let mapping: FxHashMap<_, _> = collector
+        .types
+        .iter()
+        .map(|(_, &(ty, _))| {
+            assert!(
+                infcx.resolve_vars_if_possible(ty) == ty && ty.is_ty_var(),
+                "{ty:?} should not have been constrained via normalization",
+                ty = infcx.resolve_vars_if_possible(ty)
+            );
+            idx += 1;
+            (
+                ty,
+                Ty::new_placeholder(
+                    tcx,
+                    ty::Placeholder {
+                        universe,
+                        bound: ty::BoundTy {
+                            var: ty::BoundVar::from_usize(idx),
+                            kind: ty::BoundTyKind::Anon,
+                        },
+                    },
+                ),
+            )
+        })
+        .collect();
+    let mut type_mapper = BottomUpFolder {
+        tcx,
+        ty_op: |ty| *mapping.get(&ty).unwrap_or(&ty),
+        lt_op: |lt| lt,
+        ct_op: |ct| ct,
+    };
     let wf_tys = FxIndexSet::from_iter(
-        unnormalized_trait_sig.inputs_and_output.iter().chain(trait_sig.inputs_and_output.iter()),
+        unnormalized_trait_sig
+            .inputs_and_output
+            .iter()
+            .chain(trait_sig.inputs_and_output.iter())
+            .map(|ty| ty.fold_with(&mut type_mapper)),
     );
 
     match ocx.eq(&cause, param_env, trait_return_ty, impl_return_ty) {
@@ -787,6 +844,20 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
         }
     }
 
+    // FIXME: This has the same issue as #108544, but since this isn't breaking
+    // existing code, I'm not particularly inclined to do the same hack as above
+    // where we process wf obligations manually. This can be fixed in a forward-
+    // compatible way later.
+    let collected_types = collector.types;
+    for (_, &(ty, _)) in &collected_types {
+        ocx.register_obligation(traits::Obligation::new(
+            tcx,
+            misc_cause.clone(),
+            param_env,
+            ty::ClauseKind::WellFormed(ty.into()),
+        ));
+    }
+
     // Check that all obligations are satisfied by the implementation's
     // RPITs.
     let errors = ocx.select_all_or_error();
@@ -795,8 +866,6 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
         return Err(reported);
     }
 
-    let collected_types = collector.types;
-
     // Finally, resolve all regions. This catches wily misuses of
     // lifetime parameters.
     let outlives_env = OutlivesEnvironment::with_bounds(