Detect cyclic subtype goals in new solver

compiler/rustc_infer/src/infer/mod.rs

@@ -199,7 +199,7 @@ impl<'tcx> InferCtxtInner<'tcx> {
     }
 
     #[inline]
-    fn type_variables(&mut self) -> type_variable::TypeVariableTable<'_, 'tcx> {
+    pub fn type_variables(&mut self) -> type_variable::TypeVariableTable<'_, 'tcx> {
         self.type_variable_storage.with_log(&mut self.undo_log)
     }
 

compiler/rustc_middle/src/traits/solve.rs

@@ -125,6 +125,7 @@ pub struct ExternalConstraintsData<'tcx> {
     // FIXME: implement this.
     pub region_constraints: QueryRegionConstraints<'tcx>,
     pub opaque_types: Vec<(Ty<'tcx>, Ty<'tcx>)>,
+    pub stalled_subtypes: Vec<(Ty<'tcx>, Ty<'tcx>)>,
 }
 
 // FIXME: Having to clone `region_constraints` for folding feels bad and
@@ -144,13 +145,23 @@ impl<'tcx> TypeFoldable<TyCtxt<'tcx>> for ExternalConstraints<'tcx> {
                 .iter()
                 .map(|opaque| opaque.try_fold_with(folder))
                 .collect::<Result<_, F::Error>>()?,
+            stalled_subtypes: self
+                .stalled_subtypes
+                .iter()
+                .map(|subtype| subtype.try_fold_with(folder))
+                .collect::<Result<_, F::Error>>()?,
         }))
     }
 
     fn fold_with<F: TypeFolder<TyCtxt<'tcx>>>(self, folder: &mut F) -> Self {
         TypeFolder::interner(folder).mk_external_constraints(ExternalConstraintsData {
             region_constraints: self.region_constraints.clone().fold_with(folder),
             opaque_types: self.opaque_types.iter().map(|opaque| opaque.fold_with(folder)).collect(),
+            stalled_subtypes: self
+                .stalled_subtypes
+                .iter()
+                .map(|subtype| subtype.fold_with(folder))
+                .collect(),
         })
     }
 }
@@ -162,6 +173,7 @@ impl<'tcx> TypeVisitable<TyCtxt<'tcx>> for ExternalConstraints<'tcx> {
     ) -> std::ops::ControlFlow<V::BreakTy> {
         self.region_constraints.visit_with(visitor)?;
         self.opaque_types.visit_with(visitor)?;
+        self.stalled_subtypes.visit_with(visitor)?;
         ControlFlow::Continue(())
     }
 }

compiler/rustc_trait_selection/src/solve/eval_ctxt.rs

@@ -57,6 +57,11 @@ pub struct EvalCtxt<'a, 'tcx> {
     pub(super) search_graph: &'a mut SearchGraph<'tcx>,
 
     pub(super) nested_goals: NestedGoals<'tcx>,
+
+    // Subtyping predicates which have stalled. These are passed down in query
+    // responses to help detect cycle errors in cases where we're computing a
+    // goal like `?0 <: Box<?1>` when we already know that `?0 <: ?1`.
+    pub(super) stalled_subtypes: Vec<(Ty<'tcx>, Ty<'tcx>)>,
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
@@ -121,6 +126,7 @@ impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
             max_input_universe: ty::UniverseIndex::ROOT,
             var_values: CanonicalVarValues::dummy(),
             nested_goals: NestedGoals::new(),
+            stalled_subtypes: vec![],
         };
         let result = ecx.evaluate_goal(IsNormalizesToHack::No, goal);
 
@@ -172,6 +178,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 max_input_universe: canonical_goal.max_universe,
                 search_graph,
                 nested_goals: NestedGoals::new(),
+                stalled_subtypes: vec![],
             };
             ecx.compute_goal(goal)
         })
@@ -404,6 +411,7 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             max_input_universe: self.max_input_universe,
             search_graph: self.search_graph,
             nested_goals: self.nested_goals.clone(),
+            stalled_subtypes: self.stalled_subtypes.clone(),
         };
         self.infcx.probe(|_| f(&mut ecx))
     }

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

@@ -16,7 +16,7 @@ use rustc_infer::infer::canonical::CanonicalVarValues;
 use rustc_infer::infer::canonical::{CanonicalExt, QueryRegionConstraints};
 use rustc_middle::traits::query::NoSolution;
 use rustc_middle::traits::solve::{ExternalConstraints, ExternalConstraintsData};
-use rustc_middle::ty::{self, GenericArgKind};
+use rustc_middle::ty::{self, GenericArgKind, Ty};
 use rustc_span::DUMMY_SP;
 use std::iter;
 use std::ops::Deref;
@@ -79,9 +79,11 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             )
         });
         let opaque_types = self.infcx.clone_opaque_types_for_query_response();
-        Ok(self
-            .tcx()
-            .mk_external_constraints(ExternalConstraintsData { region_constraints, opaque_types }))
+        Ok(self.tcx().mk_external_constraints(ExternalConstraintsData {
+            region_constraints,
+            opaque_types,
+            stalled_subtypes: self.stalled_subtypes.clone(),
+        }))
     }
 
     /// After calling a canonical query, we apply the constraints returned
@@ -102,12 +104,17 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         let Response { var_values, external_constraints, certainty } =
             response.substitute(self.tcx(), &substitution);
 
-        let nested_goals = self.unify_query_var_values(param_env, &original_values, var_values)?;
-
         // FIXME: implement external constraints.
-        let ExternalConstraintsData { region_constraints, opaque_types: _ } =
+        let ExternalConstraintsData { region_constraints, stalled_subtypes, opaque_types: _ } =
             external_constraints.deref();
         self.register_region_constraints(region_constraints);
+        assert!(
+            stalled_subtypes.is_empty() || certainty != Certainty::Yes,
+            "stalled_subtypes is non-empty: {stalled_subtypes:?}, but certainty is Yes"
+        );
+        self.register_stalled_subtypes(stalled_subtypes);
+
+        let nested_goals = self.unify_query_var_values(param_env, &original_values, var_values)?;
 
         Ok((certainty, nested_goals))
     }
@@ -227,4 +234,14 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             let _ = member_constraint;
         }
     }
+
+    fn register_stalled_subtypes(&mut self, stalled_subtypes: &[(Ty<'tcx>, Ty<'tcx>)]) {
+        for (sub, sup) in stalled_subtypes {
+            if let ty::Infer(ty::TyVar(sub_vid)) = *self.infcx.shallow_resolve(*sub).kind()
+                && let ty::Infer(ty::TyVar(sup_vid)) = *self.infcx.shallow_resolve(*sup).kind()
+            {
+               self.infcx.inner.borrow_mut().type_variables().sub(sub_vid, sup_vid);
+            }
+        }
+    }
 }

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -105,6 +105,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         goal: Goal<'tcx, SubtypePredicate<'tcx>>,
     ) -> QueryResult<'tcx> {
         if goal.predicate.a.is_ty_var() && goal.predicate.b.is_ty_var() {
+            self.stalled_subtypes.push((goal.predicate.a, goal.predicate.b));
             self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
         } else {
             self.sub(goal.param_env, goal.predicate.a, goal.predicate.b)?;

tests/ui/traits/new-solver/cyclic-type.rs

@@ -0,0 +1,8 @@
+// compile-flags: -Ztrait-solver=next
+
+fn main() {
+    let x;
+    x = Box::new(x);
+    //~^ ERROR mismatched types
+    //~| NOTE cyclic type of infinite size
+}

tests/ui/traits/new-solver/cyclic-type.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/cyclic-type.rs:5:9
+   |
+LL |     x = Box::new(x);
+   |         ^^^^^^^^^^^ cyclic type of infinite size
+   |
+help: consider unboxing the value
+   |
+LL |     x = *Box::new(x);
+   |         +
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/traits/new-solver/equating-projection-cyclically.stderr

@@ -1,14 +1,32 @@
-error[E0308]: mismatched types
-  --> $DIR/equating-projection-cyclically.rs:22:19
+error[E0055]: reached the recursion limit while auto-dereferencing `<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<_ as Test>::Assoc as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target`
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`equating_projection_cyclically`)
+
+error[E0055]: reached the recursion limit while auto-dereferencing `<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<_ as Test>::Assoc as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target`
+  --> $DIR/equating-projection-cyclically.rs:22:9
    |
 LL |     x = transform(x);
-   |                   ^ expected inferred type, found associated type
+   |         ^^^^^^^^^^^^ deref recursion limit reached
    |
-   = note:         expected type `_`
-           found associated type `<_ as Test>::Assoc`
-   = help: consider constraining the associated type `<_ as Test>::Assoc` to `_`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`equating_projection_cyclically`)
+
+error[E0055]: reached the recursion limit while auto-dereferencing `<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<_ as Test>::Assoc as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target as Deref>::Target`
+  --> $DIR/equating-projection-cyclically.rs:22:9
+   |
+LL |     x = transform(x);
+   |         ^^^^^^^^^^^^ deref recursion limit reached
+   |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`equating_projection_cyclically`)
+
+error[E0308]: mismatched types
+  --> $DIR/equating-projection-cyclically.rs:22:9
+   |
+LL |     x = transform(x);
+   |         ^^^^^^^^^^^^- help: try using a conversion method: `.to_string()`
+   |         |
+   |         cyclic type of infinite size
 
-error: aborting due to previous error
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0055, E0308.
+For more information about an error, try `rustc --explain E0055`.

tests/ui/traits/new-solver/normalizes_to_ignores_unnormalizable_candidate.self_infer.stderr

@@ -1,13 +1,13 @@
 error[E0282]: type annotations needed
-  --> $DIR/normalizes_to_ignores_unnormalizable_candidate.rs:36:5
+  --> $DIR/normalizes_to_ignores_unnormalizable_candidate.rs:36:9
    |
 LL |     foo(unconstrained())
-   |     ^^^ cannot infer type of the type parameter `T` declared on the function `foo`
+   |         ^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `unconstrained`
    |
 help: consider specifying the generic argument
    |
-LL |     foo::<T>(unconstrained())
-   |        +++++
+LL |     foo(unconstrained::<T>())
+   |                      +++++
 
 error: aborting due to previous error