Auto merge of #116072 - compiler-errors:rpitit-implied-bounds, r=aliemjay

Use placeholders to prevent using inferred RPITIT types to imply their own well-formedness

The issue here is that we use the same signature to do RPITIT inference as we do to compute implied bounds. To fix this, when gathering the assumed wf types for the method, we replace all of the infer vars (that will be eventually used to infer RPITIT types) with type placeholders, which imply nothing about lifetime bounds.

This solution kind of sucks, but I'm not certain there's another feasible way to fix this. If anyone has a better solution, I'd be glad to hear it.

My naive first solution was, instead of using placeholders, to replace the signature with the RPITIT projections that it originally started out with. But turns out that we can't just use the unnormalized signature of the trait method in `implied_outlives_bounds` since we normalize during WF computation -- that would cause a query cycle in `collect_return_position_impl_trait_in_trait_tys`.

idk who to request review...
r? `@lcnr` or `@aliemjay` i guess.

Fixes #116060

Author: bors

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,
@@ -661,8 +662,6 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
     let trait_m = tcx.opt_associated_item(impl_m.trait_item_def_id.unwrap()).unwrap();
     let impl_trait_ref =
         tcx.impl_trait_ref(impl_m.impl_container(tcx).unwrap()).unwrap().instantiate_identity();
-    let param_env = tcx.param_env(impl_m_def_id);
-
     // First, check a few of the same things as `compare_impl_method`,
     // just so we don't ICE during substitution later.
     check_method_is_structurally_compatible(tcx, impl_m, trait_m, impl_trait_ref, true)?;
@@ -688,13 +687,26 @@ pub(super) fn collect_return_position_impl_trait_in_trait_tys<'tcx>(
     let trait_to_placeholder_args =
         impl_to_placeholder_args.rebase_onto(tcx, impl_m.container_id(tcx), trait_to_impl_args);
 
+    let hybrid_preds = tcx
+        .predicates_of(impl_m.container_id(tcx))
+        .instantiate_identity(tcx)
+        .into_iter()
+        .chain(tcx.predicates_of(trait_m.def_id).instantiate_own(tcx, trait_to_placeholder_args))
+        .map(|(clause, _)| clause);
+    let param_env = ty::ParamEnv::new(tcx.mk_clauses_from_iter(hybrid_preds), Reveal::UserFacing);
+    let param_env = traits::normalize_param_env_or_error(
+        tcx,
+        param_env,
+        ObligationCause::misc(tcx.def_span(impl_m_def_id), impl_m_def_id),
+    );
+
     let infcx = &tcx.infer_ctxt().build();
     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 +737,68 @@ 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 +855,20 @@ 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 +877,6 @@ 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(

tests/ui/impl-trait/in-trait/rpitit-hidden-types-self-implied-wf-via-param.rs

@@ -0,0 +1,26 @@
+#![feature(return_position_impl_trait_in_trait)]
+
+trait Extend {
+    fn extend<'a: 'a>(_: &'a str) -> (impl Sized + 'a, &'static str);
+}
+
+impl Extend for () {
+    fn extend<'a: 'a>(s: &'a str) -> (Option<&'static &'a ()>, &'static str)
+    //~^ ERROR in type `&'static &'a ()`, reference has a longer lifetime than the data it references
+    where
+        'a: 'static,
+    {
+        (None, s)
+    }
+}
+
+// This indirection is not necessary for reproduction,
+// but it makes this test future-proof against #114936.
+fn extend<T: Extend>(s: &str) -> &'static str {
+    <T as Extend>::extend(s).1
+}
+
+fn main() {
+    let use_after_free = extend::<()>(&String::from("temporary"));
+    println!("{}", use_after_free);
+}

tests/ui/impl-trait/in-trait/rpitit-hidden-types-self-implied-wf-via-param.stderr

@@ -0,0 +1,16 @@
+error[E0491]: in type `&'static &'a ()`, reference has a longer lifetime than the data it references
+  --> $DIR/rpitit-hidden-types-self-implied-wf-via-param.rs:8:38
+   |
+LL |     fn extend<'a: 'a>(s: &'a str) -> (Option<&'static &'a ()>, &'static str)
+   |                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the pointer is valid for the static lifetime
+note: but the referenced data is only valid for the lifetime `'a` as defined here
+  --> $DIR/rpitit-hidden-types-self-implied-wf-via-param.rs:8:15
+   |
+LL |     fn extend<'a: 'a>(s: &'a str) -> (Option<&'static &'a ()>, &'static str)
+   |               ^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0491`.

tests/ui/impl-trait/in-trait/rpitit-hidden-types-self-implied-wf.rs

@@ -0,0 +1,23 @@
+#![feature(return_position_impl_trait_in_trait)]
+
+trait Extend {
+    fn extend(_: &str) -> (impl Sized + '_, &'static str);
+}
+
+impl Extend for () {
+    fn extend(s: &str) -> (Option<&'static &'_ ()>, &'static str) {
+        //~^ ERROR in type `&'static &()`, reference has a longer lifetime than the data it references
+        (None, s)
+    }
+}
+
+// This indirection is not necessary for reproduction,
+// but it makes this test future-proof against #114936.
+fn extend<T: Extend>(s: &str) -> &'static str {
+    <T as Extend>::extend(s).1
+}
+
+fn main() {
+    let use_after_free = extend::<()>(&String::from("temporary"));
+    println!("{}", use_after_free);
+}

tests/ui/impl-trait/in-trait/rpitit-hidden-types-self-implied-wf.stderr

@@ -0,0 +1,16 @@
+error[E0491]: in type `&'static &()`, reference has a longer lifetime than the data it references
+  --> $DIR/rpitit-hidden-types-self-implied-wf.rs:8:27
+   |
+LL |     fn extend(s: &str) -> (Option<&'static &'_ ()>, &'static str) {
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the pointer is valid for the static lifetime
+note: but the referenced data is only valid for the anonymous lifetime defined here
+  --> $DIR/rpitit-hidden-types-self-implied-wf.rs:8:18
+   |
+LL |     fn extend(s: &str) -> (Option<&'static &'_ ()>, &'static str) {
+   |                  ^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0491`.