Check that impl method args are WF given implied-bounds assumptions from trait

Author: compiler-errors

compiler/rustc_hir_analysis/src/check/compare_method.rs

@@ -251,14 +251,14 @@ fn compare_predicate_entailment<'tcx>(
 
     let mut wf_tys = FxIndexSet::default();
 
-    let impl_sig = infcx.replace_bound_vars_with_fresh_vars(
+    let unnormalized_impl_sig = infcx.replace_bound_vars_with_fresh_vars(
         impl_m_span,
         infer::HigherRankedType,
         tcx.fn_sig(impl_m.def_id),
     );
 
     let norm_cause = ObligationCause::misc(impl_m_span, impl_m_hir_id);
-    let impl_sig = ocx.normalize(&norm_cause, param_env, impl_sig);
+    let impl_sig = ocx.normalize(&norm_cause, param_env, unnormalized_impl_sig);
     let impl_fty = tcx.mk_fn_ptr(ty::Binder::dummy(impl_sig));
     debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 
@@ -300,6 +300,25 @@ fn compare_predicate_entailment<'tcx>(
         return Err(emitted);
     }
 
+    // Finally, we need to check that the impl's args are well-formed given
+    // the hybrid param-env (impl + trait method where-clauses).
+    for (unnormalized_arg, arg) in
+        std::iter::zip(unnormalized_impl_sig.inputs_and_output, impl_sig.inputs_and_output)
+    {
+        ocx.register_obligation(traits::Obligation::new(
+            tcx,
+            cause.clone(),
+            param_env,
+            ty::Binder::dummy(ty::PredicateKind::WellFormed(unnormalized_arg.into())),
+        ));
+        ocx.register_obligation(traits::Obligation::new(
+            tcx,
+            cause.clone(),
+            param_env,
+            ty::Binder::dummy(ty::PredicateKind::WellFormed(arg.into())),
+        ));
+    }
+
     // Check that all obligations are satisfied by the implementation's
     // version.
     let errors = ocx.select_all_or_error();

src/test/ui/implied-bounds/impl-implied-bounds-compatibility.rs

@@ -0,0 +1,18 @@
+use std::cell::RefCell;
+
+pub struct MessageListeners<'a> {
+    listeners: RefCell<Vec<Box<dyn FnMut(()) + 'a>>>,
+}
+
+pub trait MessageListenersInterface {
+    fn listeners<'c>(&'c self) -> &'c MessageListeners<'c>;
+}
+
+impl<'a> MessageListenersInterface for MessageListeners<'a> {
+    fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
+        //~^ ERROR cannot infer an appropriate lifetime for lifetime parameter 'b in generic type due to conflicting requirement
+        self
+    }
+}
+
+fn main() {}

src/test/ui/implied-bounds/impl-implied-bounds-compatibility.stderr

@@ -0,0 +1,32 @@
+error[E0495]: cannot infer an appropriate lifetime for lifetime parameter 'b in generic type due to conflicting requirements
+  --> $DIR/impl-implied-bounds-compatibility.rs:12:5
+   |
+LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: first, the lifetime cannot outlive the lifetime `'c` as defined here...
+  --> $DIR/impl-implied-bounds-compatibility.rs:12:5
+   |
+LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...so that the method type is compatible with trait
+  --> $DIR/impl-implied-bounds-compatibility.rs:12:5
+   |
+LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: expected `fn(&'c MessageListeners<'a>) -> &'c MessageListeners<'c>`
+              found `fn(&MessageListeners<'a>) -> &'a MessageListeners<'_>`
+note: but, the lifetime must be valid for the lifetime `'a` as defined here...
+  --> $DIR/impl-implied-bounds-compatibility.rs:11:6
+   |
+LL | impl<'a> MessageListenersInterface for MessageListeners<'a> {
+   |      ^^
+note: ...so that the reference type `&'a MessageListeners<'_>` does not outlive the data it points at
+  --> $DIR/impl-implied-bounds-compatibility.rs:12:5
+   |
+LL |     fn listeners<'b>(&'b self) -> &'a MessageListeners<'b> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0495`.