Make select_* methods return Vec for TraitEngine

compiler/rustc_trait_selection/src/autoderef.rs

@@ -152,11 +152,12 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
             },
             cause,
         );
-        if let Err(e) = fulfillcx.select_where_possible(&self.infcx) {
+        let errors = fulfillcx.select_where_possible(&self.infcx);
+        if !errors.is_empty() {
             // This shouldn't happen, except for evaluate/fulfill mismatches,
             // but that's not a reason for an ICE (`predicate_may_hold` is conservative
             // by design).
-            debug!("overloaded_deref_ty: encountered errors {:?} while fulfilling", e);
+            debug!("overloaded_deref_ty: encountered errors {:?} while fulfilling", errors);
             return None;
         }
         let obligations = fulfillcx.pending_obligations();

compiler/rustc_trait_selection/src/traits/auto_trait.rs

@@ -187,9 +187,11 @@ impl<'tcx> AutoTraitFinder<'tcx> {
             // an additional sanity check.
             let mut fulfill = FulfillmentContext::new();
             fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
-            fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
-                panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, e)
-            });
+            let errors = fulfill.select_all_or_error(&infcx);
+            
+            if !errors.is_empty() {
+                panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
+            }
 
             let body_id_map: FxHashMap<_, _> = infcx
                 .inner

compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs

@@ -52,31 +52,33 @@ impl TraitEngine<'tcx> for FulfillmentContext<'tcx> {
     fn select_all_or_error(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
-    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
-        self.select_where_possible(infcx)?;
+    ) -> Vec<FulfillmentError<'tcx>> {
+        {
+            let errors = self.select_where_possible(infcx);
 
-        if self.obligations.is_empty() {
-            Ok(())
-        } else {
-            let errors = self
-                .obligations
-                .iter()
-                .map(|obligation| FulfillmentError {
-                    obligation: obligation.clone(),
-                    code: FulfillmentErrorCode::CodeAmbiguity,
-                    // 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(),
-                })
-                .collect();
-            Err(errors)
+            if !errors.is_empty() {
+                return errors;
+            }
         }
+
+        // any remaining obligations are errors
+        self
+            .obligations
+            .iter()
+            .map(|obligation| FulfillmentError {
+                obligation: obligation.clone(),
+                code: FulfillmentErrorCode::CodeAmbiguity,
+                // 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(),
+            })
+            .collect()
     }
 
     fn select_where_possible(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
-    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+    ) -> Vec<FulfillmentError<'tcx>> {
         assert!(!infcx.is_in_snapshot());
 
         let mut errors = Vec::new();
@@ -147,7 +149,7 @@ impl TraitEngine<'tcx> for FulfillmentContext<'tcx> {
             }
         }
 
-        if errors.is_empty() { Ok(()) } else { Err(errors) }
+        errors
     }
 
     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {

compiler/rustc_trait_selection/src/traits/codegen.rs

@@ -120,7 +120,8 @@ where
     // In principle, we only need to do this so long as `result`
     // contains unbound type parameters. It could be a slight
     // optimization to stop iterating early.
-    if let Err(errors) = fulfill_cx.select_all_or_error(infcx) {
+    let errors = fulfill_cx.select_all_or_error(infcx);
+    if !errors.is_empty() {
         infcx.tcx.sess.delay_span_bug(
             rustc_span::DUMMY_SP,
             &format!("Encountered errors `{:?}` resolving bounds after type-checking", errors),

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -129,7 +129,7 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
     fn select(
         &mut self,
         selcx: &mut SelectionContext<'a, 'tcx>,
-    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+    ) -> Vec<FulfillmentError<'tcx>> {
         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
         let _enter = span.enter();
 
@@ -163,7 +163,7 @@ impl<'a, 'tcx> FulfillmentContext<'tcx> {
             errors.len()
         );
 
-        if errors.is_empty() { Ok(()) } else { Err(errors) }
+        errors
     }
 }
 
@@ -226,38 +226,46 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
     fn select_all_or_error(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
-    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
-        self.select_where_possible(infcx)?;
+    ) -> Vec<FulfillmentError<'tcx>> {
+        {
+            let errors = self.select_where_possible(infcx);
+            if !errors.is_empty() {
+                return errors;
+            }
+        }
 
-        let errors: Vec<_> = self
+        self
             .predicates
             .to_errors(CodeAmbiguity)
             .into_iter()
             .map(to_fulfillment_error)
-            .collect();
-        if errors.is_empty() { Ok(()) } else { Err(errors) }
+            .collect()
     }
 
     fn select_all_with_constness_or_error(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
         constness: rustc_hir::Constness,
-    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
-        self.select_with_constness_where_possible(infcx, constness)?;
+    ) -> Vec<FulfillmentError<'tcx>> {
+        {
+            let errors = self.select_with_constness_where_possible(infcx, constness);
+            if !errors.is_empty() {
+                return errors;
+            }
+        }
 
-        let errors: Vec<_> = self
+        self
             .predicates
             .to_errors(CodeAmbiguity)
             .into_iter()
             .map(to_fulfillment_error)
-            .collect();
-        if errors.is_empty() { Ok(()) } else { Err(errors) }
+            .collect()
     }
 
     fn select_where_possible(
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
-    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+    ) -> Vec<FulfillmentError<'tcx>> {
         let mut selcx = SelectionContext::new(infcx);
         self.select(&mut selcx)
     }
@@ -266,7 +274,7 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         &mut self,
         infcx: &InferCtxt<'_, 'tcx>,
         constness: hir::Constness,
-    ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
+    ) -> Vec<FulfillmentError<'tcx>> {
         let mut selcx = SelectionContext::with_constness(infcx, constness);
         self.select(&mut selcx)
     }

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -180,8 +180,8 @@ pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
         // Note: we only assume something is `Copy` if we can
         // *definitively* show that it implements `Copy`. Otherwise,
         // assume it is move; linear is always ok.
-        match fulfill_cx.select_all_or_error(infcx) {
-            Ok(()) => {
+        match fulfill_cx.select_all_or_error(infcx).as_slice() {
+            [] => {
                 debug!(
                     "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} success",
                     ty,
@@ -189,12 +189,12 @@ pub fn type_known_to_meet_bound_modulo_regions<'a, 'tcx>(
                 );
                 true
             }
-            Err(e) => {
+            errors => {
                 debug!(
-                    "type_known_to_meet_bound_modulo_regions: ty={:?} bound={} errors={:?}",
-                    ty,
-                    infcx.tcx.def_path_str(def_id),
-                    e
+                    ?ty,
+                    bound = %infcx.tcx.def_path_str(def_id),
+                    ?errors,
+                    "type_known_to_meet_bound_modulo_regions"
                 );
                 false
             }
@@ -410,7 +410,10 @@ where
     }
 
     debug!("fully_normalize: select_all_or_error start");
-    fulfill_cx.select_all_or_error(infcx)?;
+    let errors = fulfill_cx.select_all_or_error(infcx);
+    if !errors.is_empty() {
+        return Err(errors);
+    }
     debug!("fully_normalize: select_all_or_error complete");
     let resolved_value = infcx.resolve_vars_if_possible(normalized_value);
     debug!("fully_normalize: resolved_value={:?}", resolved_value);
@@ -441,7 +444,9 @@ pub fn impossible_predicates<'tcx>(
             fulfill_cx.register_predicate_obligation(&infcx, obligation);
         }
 
-        fulfill_cx.select_all_or_error(&infcx).is_err()
+        let errors = fulfill_cx.select_all_or_error(&infcx);
+
+        !errors.is_empty()
     });
     debug!("impossible_predicates = {:?}", result);
     result

compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs

@@ -77,10 +77,11 @@ fn scrape_region_constraints<'tcx, Op: super::TypeOp<'tcx, Output = R>, R>(
     let InferOk { value, obligations } = infcx.commit_if_ok(|_| op())?;
     debug_assert!(obligations.iter().all(|o| o.cause.body_id == dummy_body_id));
     fulfill_cx.register_predicate_obligations(infcx, obligations);
-    if let Err(e) = fulfill_cx.select_all_or_error(infcx) {
+    let errors = fulfill_cx.select_all_or_error(infcx);
+    if !errors.is_empty()  {
         infcx.tcx.sess.diagnostic().delay_span_bug(
             DUMMY_SP,
-            &format!("errors selecting obligation during MIR typeck: {:?}", e),
+            &format!("errors selecting obligation during MIR typeck: {:?}", errors),
         );
     }
 

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

@@ -225,8 +225,18 @@ fn fulfill_implication<'a, 'tcx>(
         for oblig in obligations.chain(more_obligations) {
             fulfill_cx.register_predicate_obligation(&infcx, oblig);
         }
-        match fulfill_cx.select_all_or_error(infcx) {
-            Err(errors) => {
+        match fulfill_cx.select_all_or_error(infcx).as_slice() {
+            [] => {
+                debug!(
+                    "fulfill_implication: an impl for {:?} specializes {:?}",
+                    source_trait_ref, target_trait_ref
+                );
+
+                // Now resolve the *substitution* we built for the target earlier, replacing
+                // the inference variables inside with whatever we got from fulfillment.
+                Ok(infcx.resolve_vars_if_possible(target_substs))
+            }
+            errors => {
                 // no dice!
                 debug!(
                     "fulfill_implication: for impls on {:?} and {:?}, \
@@ -238,17 +248,6 @@ fn fulfill_implication<'a, 'tcx>(
                 );
                 Err(())
             }
-
-            Ok(()) => {
-                debug!(
-                    "fulfill_implication: an impl for {:?} specializes {:?}",
-                    source_trait_ref, target_trait_ref
-                );
-
-                // Now resolve the *substitution* we built for the target earlier, replacing
-                // the inference variables inside with whatever we got from fulfillment.
-                Ok(infcx.resolve_vars_if_possible(target_substs))
-            }
         }
     })
 }

compiler/rustc_trait_selection/src/traits/structural_match.rs

@@ -103,7 +103,7 @@ fn type_marked_structural(
     //
     // 2. We are sometimes doing future-incompatibility lints for
     //    now, so we do not want unconditional errors here.
-    fulfillment_cx.select_all_or_error(infcx).is_ok()
+    fulfillment_cx.select_all_or_error(infcx).is_empty()
 }
 
 /// This implements the traversal over the structure of a given type to try to