only assemble alias bound candidates for rigid aliases

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

@@ -253,82 +253,39 @@ pub(super) trait GoalKind<'tcx>:
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> Vec<(CanonicalResponse<'tcx>, BuiltinImplSource)>;
-
-    /// Consider the `Unsize` candidate corresponding to coercing a sized type
-    /// into a `dyn Trait`.
-    ///
-    /// This is computed separately from the rest of the `Unsize` candidates
-    /// since it is only done once per self type, and not once per
-    /// *normalization step* (in `assemble_candidates_via_self_ty`).
-    fn consider_unsize_to_dyn_candidate(
-        ecx: &mut EvalCtxt<'_, 'tcx>,
-        goal: Goal<'tcx, Self>,
-    ) -> QueryResult<'tcx>;
 }
 
 impl<'tcx> EvalCtxt<'_, 'tcx> {
     pub(super) fn assemble_and_evaluate_candidates<G: GoalKind<'tcx>>(
         &mut self,
         goal: Goal<'tcx, G>,
     ) -> Vec<Candidate<'tcx>> {
-        debug_assert_eq!(goal, self.resolve_vars_if_possible(goal));
-        if let Some(ambig) = self.assemble_self_ty_infer_ambiguity_response(goal) {
-            return vec![ambig];
-        }
-
-        let mut candidates = self.assemble_candidates_via_self_ty(goal, 0);
-
-        self.assemble_unsize_to_dyn_candidate(goal, &mut candidates);
-
-        self.assemble_blanket_impl_candidates(goal, &mut candidates);
-
-        self.assemble_param_env_candidates(goal, &mut candidates);
-
-        self.assemble_coherence_unknowable_candidates(goal, &mut candidates);
-
-        candidates
-    }
-
-    /// `?0: Trait` is ambiguous, because it may be satisfied via a builtin rule,
-    /// object bound, alias bound, etc. We are unable to determine this until we can at
-    /// least structurally resolve the type one layer.
-    ///
-    /// It would also require us to consider all impls of the trait, which is both pretty
-    /// bad for perf and would also constrain the self type if there is just a single impl.
-    fn assemble_self_ty_infer_ambiguity_response<G: GoalKind<'tcx>>(
-        &mut self,
-        goal: Goal<'tcx, G>,
-    ) -> Option<Candidate<'tcx>> {
-        if goal.predicate.self_ty().is_ty_var() {
-            debug!("adding self_ty_infer_ambiguity_response");
+        let dummy_candidate = |this: &mut EvalCtxt<'_, 'tcx>, certainty| {
             let source = CandidateSource::BuiltinImpl(BuiltinImplSource::Misc);
-            let result = self
-                .evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-                .unwrap();
-            let mut dummy_probe = self.inspect.new_probe();
+            let result = this.evaluate_added_goals_and_make_canonical_response(certainty).unwrap();
+            let mut dummy_probe = this.inspect.new_probe();
             dummy_probe.probe_kind(ProbeKind::TraitCandidate { source, result: Ok(result) });
-            self.inspect.finish_probe(dummy_probe);
-            Some(Candidate { source, result })
-        } else {
-            None
+            this.inspect.finish_probe(dummy_probe);
+            vec![Candidate { source, result }]
+        };
+
+        let Some(normalized_self_ty) =
+            self.try_normalize_ty(goal.param_env, goal.predicate.self_ty())
+        else {
+            debug!("overflow while evaluating self type");
+            return dummy_candidate(self, Certainty::OVERFLOW);
+        };
+
+        if normalized_self_ty.is_ty_var() {
+            debug!("self type has been normalized to infer");
+            return dummy_candidate(self, Certainty::AMBIGUOUS);
         }
-    }
 
-    /// Assemble candidates which apply to the self type. This only looks at candidate which
-    /// apply to the specific self type and ignores all others.
-    ///
-    /// Returns `None` if the self type is still ambiguous.
-    fn assemble_candidates_via_self_ty<G: GoalKind<'tcx>>(
-        &mut self,
-        goal: Goal<'tcx, G>,
-        num_steps: usize,
-    ) -> Vec<Candidate<'tcx>> {
+        let goal =
+            goal.with(self.tcx(), goal.predicate.with_self_ty(self.tcx(), normalized_self_ty));
         debug_assert_eq!(goal, self.resolve_vars_if_possible(goal));
-        if let Some(ambig) = self.assemble_self_ty_infer_ambiguity_response(goal) {
-            return vec![ambig];
-        }
 
-        let mut candidates = Vec::new();
+        let mut candidates = vec![];
 
         self.assemble_non_blanket_impl_candidates(goal, &mut candidates);
 
@@ -338,61 +295,13 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
         self.assemble_object_bound_candidates(goal, &mut candidates);
 
-        self.assemble_candidates_after_normalizing_self_ty(goal, &mut candidates, num_steps);
-        candidates
-    }
+        self.assemble_blanket_impl_candidates(goal, &mut candidates);
 
-    /// If the self type of a goal is an alias we first try to normalize the self type
-    /// and compute the candidates for the normalized self type in case that succeeds.
-    ///
-    /// These candidates are used in addition to the ones with the alias as a self type.
-    /// We do this to simplify both builtin candidates and for better performance.
-    ///
-    /// We generate the builtin candidates on the fly by looking at the self type, e.g.
-    /// add `FnPtr` candidates if the self type is a function pointer. Handling builtin
-    /// candidates while the self type is still an alias seems difficult. This is similar
-    /// to `try_structurally_resolve_type` during hir typeck (FIXME once implemented).
-    ///
-    /// Looking at all impls for some trait goal is prohibitively expensive. We therefore
-    /// only look at implementations with a matching self type. Because of this function,
-    /// we can avoid looking at all existing impls if the self type is an alias.
-    #[instrument(level = "debug", skip_all)]
-    fn assemble_candidates_after_normalizing_self_ty<G: GoalKind<'tcx>>(
-        &mut self,
-        goal: Goal<'tcx, G>,
-        candidates: &mut Vec<Candidate<'tcx>>,
-        num_steps: usize,
-    ) {
-        let tcx = self.tcx();
-        let &ty::Alias(_, alias) = goal.predicate.self_ty().kind() else { return };
-
-        candidates.extend(self.probe(|_| ProbeKind::NormalizedSelfTyAssembly).enter(|ecx| {
-            if tcx.recursion_limit().value_within_limit(num_steps) {
-                let normalized_ty = ecx.next_ty_infer();
-                let normalizes_to_goal =
-                    goal.with(tcx, ty::NormalizesTo { alias, term: normalized_ty.into() });
-                ecx.add_goal(GoalSource::Misc, normalizes_to_goal);
-                if let Err(NoSolution) = ecx.try_evaluate_added_goals() {
-                    debug!("self type normalization failed");
-                    return vec![];
-                }
-                let normalized_ty = ecx.resolve_vars_if_possible(normalized_ty);
-                debug!(?normalized_ty, "self type normalized");
-                // NOTE: Alternatively we could call `evaluate_goal` here and only
-                // have a `Normalized` candidate. This doesn't work as long as we
-                // use `CandidateSource` in winnowing.
-                let goal = goal.with(tcx, goal.predicate.with_self_ty(tcx, normalized_ty));
-                ecx.assemble_candidates_via_self_ty(goal, num_steps + 1)
-            } else {
-                match ecx.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW) {
-                    Ok(result) => vec![Candidate {
-                        source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
-                        result,
-                    }],
-                    Err(NoSolution) => vec![],
-                }
-            }
-        }));
+        self.assemble_param_env_candidates(goal, &mut candidates);
+
+        self.assemble_coherence_unknowable_candidates(goal, &mut candidates);
+
+        candidates
     }
 
     #[instrument(level = "debug", skip_all)]
@@ -500,24 +409,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         }
     }
 
-    #[instrument(level = "debug", skip_all)]
-    fn assemble_unsize_to_dyn_candidate<G: GoalKind<'tcx>>(
-        &mut self,
-        goal: Goal<'tcx, G>,
-        candidates: &mut Vec<Candidate<'tcx>>,
-    ) {
-        let tcx = self.tcx();
-        if tcx.lang_items().unsize_trait() == Some(goal.predicate.trait_def_id(tcx)) {
-            match G::consider_unsize_to_dyn_candidate(self, goal) {
-                Ok(result) => candidates.push(Candidate {
-                    source: CandidateSource::BuiltinImpl(BuiltinImplSource::Misc),
-                    result,
-                }),
-                Err(NoSolution) => (),
-            }
-        }
-    }
-
     #[instrument(level = "debug", skip_all)]
     fn assemble_blanket_impl_candidates<G: GoalKind<'tcx>>(
         &mut self,

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -288,11 +288,9 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
 
     /// Normalize a type when it is structually matched on.
     ///
-    /// For self types this is generally already handled through
-    /// `assemble_candidates_after_normalizing_self_ty`, so anything happening
-    /// in [`EvalCtxt::assemble_candidates_via_self_ty`] does not have to normalize
-    /// the self type. It is required when structurally matching on any other
-    /// arguments of a trait goal, e.g. when assembling builtin unsize candidates.
+    /// In nearly all cases this function must be used before matching on a type.
+    /// Not doing so is likely to be incomplete and therefore unsound during
+    /// coherence.
     #[instrument(level = "debug", skip(self), ret)]
     fn try_normalize_ty(
         &mut self,

compiler/rustc_trait_selection/src/solve/normalizes_to/mod.rs

@@ -603,13 +603,6 @@ impl<'tcx> assembly::GoalKind<'tcx> for NormalizesTo<'tcx> {
         )
     }
 
-    fn consider_unsize_to_dyn_candidate(
-        _ecx: &mut EvalCtxt<'_, 'tcx>,
-        goal: Goal<'tcx, Self>,
-    ) -> QueryResult<'tcx> {
-        bug!("`Unsize` does not have an associated type: {:?}", goal)
-    }
-
     fn consider_structural_builtin_unsize_candidates(
         _ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -490,53 +490,6 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         ecx.evaluate_added_goals_and_make_canonical_response(certainty)
     }
 
-    fn consider_unsize_to_dyn_candidate(
-        ecx: &mut EvalCtxt<'_, 'tcx>,
-        goal: Goal<'tcx, Self>,
-    ) -> QueryResult<'tcx> {
-        ecx.probe(|_| ProbeKind::UnsizeAssembly).enter(|ecx| {
-            let a_ty = goal.predicate.self_ty();
-            // We need to normalize the b_ty since it's destructured as a `dyn Trait`.
-            let Some(b_ty) =
-                ecx.try_normalize_ty(goal.param_env, goal.predicate.trait_ref.args.type_at(1))
-            else {
-                return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::OVERFLOW);
-            };
-
-            let ty::Dynamic(b_data, b_region, ty::Dyn) = *b_ty.kind() else {
-                return Err(NoSolution);
-            };
-
-            let tcx = ecx.tcx();
-
-            // Can only unsize to an object-safe trait.
-            if b_data.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
-                return Err(NoSolution);
-            }
-
-            // Check that the type implements all of the predicates of the trait object.
-            // (i.e. the principal, all of the associated types match, and any auto traits)
-            ecx.add_goals(
-                GoalSource::ImplWhereBound,
-                b_data.iter().map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))),
-            );
-
-            // The type must be `Sized` to be unsized.
-            if let Some(sized_def_id) = tcx.lang_items().sized_trait() {
-                ecx.add_goal(
-                    GoalSource::ImplWhereBound,
-                    goal.with(tcx, ty::TraitRef::new(tcx, sized_def_id, [a_ty])),
-                );
-            } else {
-                return Err(NoSolution);
-            }
-
-            // The type must outlive the lifetime of the `dyn` we're unsizing into.
-            ecx.add_goal(GoalSource::Misc, goal.with(tcx, ty::OutlivesPredicate(a_ty, b_region)));
-            ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
-        })
-    }
-
     /// ```ignore (builtin impl example)
     /// trait Trait {
     ///     fn foo(&self);
@@ -588,8 +541,11 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
                     goal, a_data, a_region, b_data, b_region,
                 ),
 
-                // `T` -> `dyn Trait` unsizing is handled separately in `consider_unsize_to_dyn_candidate`
-                (_, &ty::Dynamic(..)) => vec![],
+                // `T` -> `dyn Trait` unsizing.
+                (_, &ty::Dynamic(b_region, b_data, ty::Dyn)) => result_to_single(
+                    ecx.consider_builtin_unsize_to_dyn_candidate(goal, b_region, b_data),
+                    BuiltinImplSource::Misc,
+                ),
 
                 // `[T; N]` -> `[T]` unsizing
                 (&ty::Array(a_elem_ty, ..), &ty::Slice(b_elem_ty)) => result_to_single(
@@ -691,6 +647,42 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         responses
     }
 
+    fn consider_builtin_unsize_to_dyn_candidate(
+        &mut self,
+        goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,
+        b_data: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
+        b_region: ty::Region<'tcx>,
+    ) -> QueryResult<'tcx> {
+        let tcx = self.tcx();
+        let Goal { predicate: (a_ty, _), .. } = goal;
+
+        // Can only unsize to an object-safe trait.
+        if b_data.principal_def_id().is_some_and(|def_id| !tcx.check_is_object_safe(def_id)) {
+            return Err(NoSolution);
+        }
+
+        // Check that the type implements all of the predicates of the trait object.
+        // (i.e. the principal, all of the associated types match, and any auto traits)
+        self.add_goals(
+            GoalSource::ImplWhereBound,
+            b_data.iter().map(|pred| goal.with(tcx, pred.with_self_ty(tcx, a_ty))),
+        );
+
+        // The type must be `Sized` to be unsized.
+        if let Some(sized_def_id) = tcx.lang_items().sized_trait() {
+            self.add_goal(
+                GoalSource::ImplWhereBound,
+                goal.with(tcx, ty::TraitRef::new(tcx, sized_def_id, [a_ty])),
+            );
+        } else {
+            return Err(NoSolution);
+        }
+
+        // The type must outlive the lifetime of the `dyn` we're unsizing into.
+        self.add_goal(GoalSource::Misc, goal.with(tcx, ty::OutlivesPredicate(a_ty, b_region)));
+        self.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
+    }
+
     fn consider_builtin_upcast_to_principal(
         &mut self,
         goal: Goal<'tcx, (Ty<'tcx>, Ty<'tcx>)>,

tests/ui/traits/next-solver/alias-bound-unsound.rs

@@ -16,7 +16,7 @@ trait Foo {
 
 impl Foo for () {
     type Item = String where String: Copy;
-    //~^ ERROR overflow evaluating the requirement `<() as Foo>::Item: Copy`
+    //~^ ERROR overflow evaluating the requirement `String: Copy`
 }
 
 fn main() {

tests/ui/traits/next-solver/alias-bound-unsound.stderr

@@ -1,15 +1,17 @@
-error[E0275]: overflow evaluating the requirement `<() as Foo>::Item: Copy`
-  --> $DIR/alias-bound-unsound.rs:18:17
+error[E0275]: overflow evaluating the requirement `String: Copy`
+  --> $DIR/alias-bound-unsound.rs:18:38
    |
 LL |     type Item = String where String: Copy;
-   |                 ^^^^^^
+   |                                      ^^^^
    |
    = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`alias_bound_unsound`)
-note: required by a bound in `Foo::Item`
-  --> $DIR/alias-bound-unsound.rs:8:16
+note: the requirement `String: Copy` appears on the `impl`'s associated type `Item` but not on the corresponding trait's associated type
+  --> $DIR/alias-bound-unsound.rs:8:10
    |
+LL | trait Foo {
+   |       --- in this trait
 LL |     type Item: Copy
-   |                ^^^^ required by this bound in `Foo::Item`
+   |          ^^^^ this trait's associated type doesn't have the requirement `String: Copy`
 
 error[E0275]: overflow evaluating the requirement `String <: <() as Foo>::Item`
   --> $DIR/alias-bound-unsound.rs:24:31

tests/ui/traits/next-solver/assembly/ambig-projection-self-is-ambig.rs

@@ -0,0 +1,19 @@
+// check-pass
+// compile-flags: -Znext-solver
+
+trait Reader: Default {
+    fn read_u8_array<A>(&self) -> Result<A, ()> {
+        todo!()
+    }
+
+    fn read_u8(&self) -> Result<u8, ()> {
+        let a: [u8; 1] = self.read_u8_array::<_>()?;
+        // This results in a nested `<Result<?0, ()> as Try>::Residual: Sized` goal.
+        // The self type normalizes to `?0`. We previously did not force that to be
+        // ambiguous but instead incompletely applied the `Self: Sized` candidate
+        // from the `ParamEnv`, resulting in a type error.
+        Ok(a[0])
+    }
+}
+
+fn main() {}