Report overflows gracefully with new solver

Author: compiler-errors

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -578,7 +578,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     #[instrument(skip(self), level = "debug")]
     pub(in super::super) fn report_ambiguity_errors(&self) {
-        let mut errors = self.fulfillment_cx.borrow_mut().collect_remaining_errors();
+        let mut errors = self.fulfillment_cx.borrow_mut().collect_remaining_errors(self);
 
         if !errors.is_empty() {
             self.adjust_fulfillment_errors_for_expr_obligation(&mut errors);

compiler/rustc_hir_typeck/src/fn_ctxt/adjust_fulfillment_errors.rs

@@ -78,7 +78,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // Finally, for ambiguity-related errors, we actually want to look
         // for a parameter that is the source of the inference type left
         // over in this predicate.
-        if let traits::FulfillmentErrorCode::CodeAmbiguity = error.code {
+        if let traits::FulfillmentErrorCode::CodeAmbiguity { .. } = error.code {
             fallback_param_to_point_at = None;
             self_param_to_point_at = None;
             param_to_point_at =

compiler/rustc_infer/src/traits/engine.rs

@@ -38,7 +38,7 @@ pub trait TraitEngine<'tcx>: 'tcx {
 
     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>>;
 
-    fn collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>>;
+    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>>;
 
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>>;
 
@@ -78,6 +78,6 @@ impl<'tcx, T: ?Sized + TraitEngine<'tcx>> TraitEngineExt<'tcx> for T {
             return errors;
         }
 
-        self.collect_remaining_errors()
+        self.collect_remaining_errors(infcx)
     }
 }

compiler/rustc_infer/src/traits/mod.rs

@@ -128,7 +128,11 @@ pub enum FulfillmentErrorCode<'tcx> {
     CodeProjectionError(MismatchedProjectionTypes<'tcx>),
     CodeSubtypeError(ExpectedFound<Ty<'tcx>>, TypeError<'tcx>), // always comes from a SubtypePredicate
     CodeConstEquateError(ExpectedFound<Const<'tcx>>, TypeError<'tcx>),
-    CodeAmbiguity,
+    CodeAmbiguity {
+        /// Overflow reported from the new solver `-Ztrait-solver=next`, which will
+        /// be reported as an regular error as opposed to a fatal error.
+        overflow: bool,
+    },
 }
 
 impl<'tcx, O> Obligation<'tcx, O> {

compiler/rustc_infer/src/traits/structural_impls.rs

@@ -46,7 +46,8 @@ impl<'tcx> fmt::Debug for traits::FulfillmentErrorCode<'tcx> {
             super::CodeConstEquateError(ref a, ref b) => {
                 write!(f, "CodeConstEquateError({:?}, {:?})", a, b)
             }
-            super::CodeAmbiguity => write!(f, "Ambiguity"),
+            super::CodeAmbiguity { overflow: false } => write!(f, "Ambiguity"),
+            super::CodeAmbiguity { overflow: true } => write!(f, "Overflow"),
             super::CodeCycle(ref cycle) => write!(f, "Cycle({:?})", cycle),
         }
     }

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -1,6 +1,7 @@
 use std::mem;
 
 use rustc_infer::infer::InferCtxt;
+use rustc_infer::traits::solve::MaybeCause;
 use rustc_infer::traits::Obligation;
 use rustc_infer::traits::{
     query::NoSolution, FulfillmentError, FulfillmentErrorCode, MismatchedProjectionTypes,
@@ -41,13 +42,31 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
         self.obligations.push(obligation);
     }
 
-    fn collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>> {
+    fn collect_remaining_errors(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
         self.obligations
             .drain(..)
-            .map(|obligation| FulfillmentError {
-                obligation: obligation.clone(),
-                code: FulfillmentErrorCode::CodeAmbiguity,
-                root_obligation: obligation,
+            .map(|obligation| {
+                let code =
+                    infcx.probe(|_| match infcx.evaluate_root_goal(obligation.clone().into()) {
+                        Ok((_, Certainty::Maybe(MaybeCause::Ambiguity), _)) => {
+                            FulfillmentErrorCode::CodeAmbiguity { overflow: false }
+                        }
+                        Ok((_, Certainty::Maybe(MaybeCause::Overflow), _)) => {
+                            FulfillmentErrorCode::CodeAmbiguity { overflow: true }
+                        }
+                        Ok((_, Certainty::Yes, _)) => {
+                            bug!("did not expect successful goal when collecting ambiguity errors")
+                        }
+                        Err(_) => {
+                            bug!("did not expect selection error when collecting ambiguity errors")
+                        }
+                    });
+
+                FulfillmentError {
+                    obligation: obligation.clone(),
+                    code,
+                    root_obligation: obligation,
+                }
             })
             .collect()
     }

compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs

@@ -40,13 +40,16 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         self.obligations.insert(obligation);
     }
 
-    fn collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>> {
+    fn collect_remaining_errors(
+        &mut self,
+        _infcx: &InferCtxt<'tcx>,
+    ) -> Vec<FulfillmentError<'tcx>> {
         // any remaining obligations are errors
         self.obligations
             .iter()
             .map(|obligation| FulfillmentError {
                 obligation: obligation.clone(),
-                code: FulfillmentErrorCode::CodeAmbiguity,
+                code: FulfillmentErrorCode::CodeAmbiguity { overflow: false },
                 // FIXME - does Chalk have a notation of 'root obligation'?
                 // This is just for diagnostics, so it's okay if this is wrong
                 root_obligation: obligation.clone(),

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -125,6 +125,8 @@ pub trait TypeErrCtxtExt<'tcx> {
             + Print<'tcx, FmtPrinter<'tcx, 'tcx>, Output = FmtPrinter<'tcx, 'tcx>>,
         <T as Print<'tcx, FmtPrinter<'tcx, 'tcx>>>::Error: std::fmt::Debug;
 
+    fn report_overflow_no_abort(&self, obligation: PredicateObligation<'tcx>) -> ErrorGuaranteed;
+
     fn report_fulfillment_errors(&self, errors: &[FulfillmentError<'tcx>]) -> ErrorGuaranteed;
 
     fn report_overflow_obligation<T>(
@@ -602,6 +604,14 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         );
     }
 
+    fn report_overflow_no_abort(&self, obligation: PredicateObligation<'tcx>) -> ErrorGuaranteed {
+        let obligation = self.resolve_vars_if_possible(obligation);
+        let mut err = self.build_overflow_error(&obligation.predicate, obligation.cause.span, true);
+        self.note_obligation_cause(&mut err, &obligation);
+        self.point_at_returns_when_relevant(&mut err, &obligation);
+        err.emit()
+    }
+
     fn report_selection_error(
         &self,
         mut obligation: PredicateObligation<'tcx>,
@@ -1658,9 +1668,12 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             FulfillmentErrorCode::CodeProjectionError(ref e) => {
                 self.report_projection_error(&error.obligation, e);
             }
-            FulfillmentErrorCode::CodeAmbiguity => {
+            FulfillmentErrorCode::CodeAmbiguity { overflow: false } => {
                 self.maybe_report_ambiguity(&error.obligation);
             }
+            FulfillmentErrorCode::CodeAmbiguity { overflow: true } => {
+                self.report_overflow_no_abort(error.obligation.clone());
+            }
             FulfillmentErrorCode::CodeSubtypeError(ref expected_found, ref err) => {
                 self.report_mismatched_types(
                     &error.obligation.cause,

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -133,8 +133,15 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
     }
 
-    fn collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>> {
-        self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
+    fn collect_remaining_errors(
+        &mut self,
+        _infcx: &InferCtxt<'tcx>,
+    ) -> Vec<FulfillmentError<'tcx>> {
+        self.predicates
+            .to_errors(CodeAmbiguity { overflow: false })
+            .into_iter()
+            .map(to_fulfillment_error)
+            .collect()
     }
 
     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {

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

@@ -1,9 +1,10 @@
-error[E0282]: type annotations needed
+error[E0275]: overflow evaluating the requirement `for<'a> &'a mut Bar well-formed`
   --> $DIR/issue-95230.rs:9:13
    |
 LL |     for<'a> &'a mut Self:;
-   |             ^^^^^^^^^^^^ cannot infer type for mutable reference `&'a mut Bar`
+   |             ^^^^^^^^^^^^
    |
+   = help: consider increasing the recursion limit by adding a `#![recursion_limit = "256"]` attribute to your crate (`issue_95230`)
 note: required by a bound in `Bar`
   --> $DIR/issue-95230.rs:9:13
    |
@@ -15,4 +16,4 @@ LL |     for<'a> &'a mut Self:;
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0282`.
+For more information about this error, try `rustc --explain E0275`.