review + some small stuff

compiler/rustc_middle/src/traits/solve.rs

@@ -56,9 +56,19 @@ pub enum Certainty {
 impl Certainty {
     pub const AMBIGUOUS: Certainty = Certainty::Maybe(MaybeCause::Ambiguity);
 
-    /// When proving multiple goals using **AND**, e.g. nested obligations for an impl,
-    /// use this function to unify the certainty of these goals
-    pub fn unify_and(self, other: Certainty) -> Certainty {
+    /// Use this function to merge the certainty of multiple nested subgoals.
+    ///
+    /// Given an impl like `impl<T: Foo + Bar> Baz for T {}`, we have 2 nested
+    /// subgoals whenever we use the impl as a candidate: `T: Foo` and `T: Bar`.
+    /// If evaluating `T: Foo` results in ambiguity and `T: Bar` results in
+    /// success, we merge these two responses. This results in ambiguity.
+    ///
+    /// If we unify ambiguity with overflow, we return overflow. This doesn't matter
+    /// inside of the solver as we distinguish ambiguity from overflow. It does
+    /// however matter for diagnostics. If `T: Foo` resulted in overflow and `T: Bar`
+    /// in ambiguity without changing the inference state, we still want to tell the
+    /// user that `T: Baz` results in overflow.
+    pub fn unify_with(self, other: Certainty) -> Certainty {
         match (self, other) {
             (Certainty::Yes, Certainty::Yes) => Certainty::Yes,
             (Certainty::Yes, Certainty::Maybe(_)) => other,

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

@@ -11,7 +11,7 @@
 //
 // For types with an "existential" binder, i.e. generator witnesses, we also
 // instantiate the binder with placeholders eagerly.
-pub(crate) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
+pub(in crate::solve) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
     ecx: &EvalCtxt<'_, 'tcx>,
     ty: Ty<'tcx>,
 ) -> Result<Vec<Ty<'tcx>>, NoSolution> {
@@ -87,7 +87,7 @@ pub(crate) fn instantiate_constituent_tys_for_auto_trait<'tcx>(
     }
 }
 
-pub(crate) fn replace_erased_lifetimes_with_bound_vars<'tcx>(
+pub(in crate::solve) fn replace_erased_lifetimes_with_bound_vars<'tcx>(
     tcx: TyCtxt<'tcx>,
     ty: Ty<'tcx>,
 ) -> ty::Binder<'tcx, Ty<'tcx>> {
@@ -108,7 +108,7 @@ pub(crate) fn replace_erased_lifetimes_with_bound_vars<'tcx>(
     ty::Binder::bind_with_vars(ty, bound_vars)
 }
 
-pub(crate) fn instantiate_constituent_tys_for_sized_trait<'tcx>(
+pub(in crate::solve) fn instantiate_constituent_tys_for_sized_trait<'tcx>(
     ecx: &EvalCtxt<'_, 'tcx>,
     ty: Ty<'tcx>,
 ) -> Result<Vec<Ty<'tcx>>, NoSolution> {
@@ -158,7 +158,7 @@ pub(crate) fn instantiate_constituent_tys_for_sized_trait<'tcx>(
     }
 }
 
-pub(crate) fn instantiate_constituent_tys_for_copy_clone_trait<'tcx>(
+pub(in crate::solve) fn instantiate_constituent_tys_for_copy_clone_trait<'tcx>(
     ecx: &EvalCtxt<'_, 'tcx>,
     ty: Ty<'tcx>,
 ) -> Result<Vec<Ty<'tcx>>, NoSolution> {
@@ -224,7 +224,7 @@ pub(crate) fn instantiate_constituent_tys_for_copy_clone_trait<'tcx>(
 }
 
 // Returns a binder of the tupled inputs types and output type from a builtin callable type.
-pub(crate) fn extract_tupled_inputs_and_output_from_callable<'tcx>(
+pub(in crate::solve) fn extract_tupled_inputs_and_output_from_callable<'tcx>(
     tcx: TyCtxt<'tcx>,
     self_ty: Ty<'tcx>,
     goal_kind: ty::ClosureKind,
@@ -337,7 +337,13 @@ pub(crate) fn extract_tupled_inputs_and_output_from_callable<'tcx>(
 /// additional step of eagerly folding the associated types in the where
 /// clauses of the impl. In this example, that means replacing
 /// `<Self as Foo>::Bar` with `Ty` in the first impl.
-pub(crate) fn predicates_for_object_candidate<'tcx>(
+///
+// FIXME: This is only necessary as `<Self as Trait>::Assoc: ItemBound`
+// bounds in impls are trivially proven using the item bound candidates.
+// This is unsound in general and once that is fixed, we don't need to
+// normalize eagerly here. See https://github.com/lcnr/solver-woes/issues/9
+// for more details.
+pub(in crate::solve) fn predicates_for_object_candidate<'tcx>(
     ecx: &EvalCtxt<'_, 'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     trait_ref: ty::TraitRef<'tcx>,

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -357,7 +357,7 @@ pub(super) fn try_evaluate_added_goals(&mut self) -> Result<Certainty, NoSolutio
                             // deal with `has_changed` in the next iteration.
                             new_goals.normalizes_to_hack_goal =
                                 Some(this.resolve_vars_if_possible(goal));
-                            has_changed = has_changed.map_err(|c| c.unify_and(certainty));
+                            has_changed = has_changed.map_err(|c| c.unify_with(certainty));
                         }
                     }
                 }
@@ -378,7 +378,7 @@ pub(super) fn try_evaluate_added_goals(&mut self) -> Result<Certainty, NoSolutio
                         Certainty::Yes => {}
                         Certainty::Maybe(_) => {
                             new_goals.goals.push(goal);
-                            has_changed = has_changed.map_err(|c| c.unify_and(certainty));
+                            has_changed = has_changed.map_err(|c| c.unify_with(certainty));
                         }
                     }
                 }

compiler/rustc_trait_selection/src/solve/eval_ctxt/canonical.rs

@@ -50,7 +50,7 @@ pub(in crate::solve) fn evaluate_added_goals_and_make_canonical_response(
         certainty: Certainty,
     ) -> QueryResult<'tcx> {
         let goals_certainty = self.try_evaluate_added_goals()?;
-        let certainty = certainty.unify_and(goals_certainty);
+        let certainty = certainty.unify_with(goals_certainty);
 
         let external_constraints = self.compute_external_query_constraints()?;
 

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -341,7 +341,7 @@ fn flounder(&mut self, responses: &[CanonicalResponse<'tcx>]) -> QueryResult<'tc
             return Err(NoSolution);
         }
         let certainty = responses.iter().fold(Certainty::AMBIGUOUS, |certainty, response| {
-            certainty.unify_and(response.value.certainty)
+            certainty.unify_with(response.value.certainty)
         });
 
         let response = self.evaluate_added_goals_and_make_canonical_response(certainty);

tests/ui/higher-rank-trait-bounds/issue-95230.new.stderr

@@ -0,0 +1,18 @@
+error[E0282]: type annotations needed
+  --> $DIR/issue-95230.rs:9:13
+   |
+LL |     for<'a> &'a mut Self:;
+   |             ^^^^^^^^^^^^ cannot infer type for mutable reference `&'a mut Bar`
+   |
+note: required by a bound in `Bar`
+  --> $DIR/issue-95230.rs:9:13
+   |
+LL | pub struct Bar
+   |            --- required by a bound in this struct
+LL | where
+LL |     for<'a> &'a mut Self:;
+   |             ^^^^^^^^^^^^ required by this bound in `Bar`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0282`.

tests/ui/higher-rank-trait-bounds/issue-95230.rs

@@ -1,4 +1,8 @@
-// check-pass
+// revisions: old new
+//[new] compile-flags: -Ztrait-solver=next
+//[old] check-pass
+//[new] known-bug: #109764
+
 
 pub struct Bar
 where