selection failure: recompute applicable impls

Author: lcnr

compiler/rustc_middle/src/traits/mod.rs

@@ -576,9 +576,6 @@ pub enum SelectionError<'tcx> {
     /// Signaling that an error has already been emitted, to avoid
     /// multiple errors being shown.
     ErrorReporting,
-    /// Multiple applicable `impl`s where found. The `DefId`s correspond to
-    /// all the `impl`s' Items.
-    Ambiguous(Vec<DefId>),
 }
 
 /// When performing resolution, it is typically the case that there

compiler/rustc_middle/src/ty/mod.rs

@@ -2550,11 +2550,11 @@ impl<'tcx> TyCtxt<'tcx> {
 
     /// Looks up the span of `impl_did` if the impl is local; otherwise returns `Err`
     /// with the name of the crate containing the impl.
-    pub fn span_of_impl(self, impl_did: DefId) -> Result<Span, Symbol> {
-        if let Some(impl_did) = impl_did.as_local() {
-            Ok(self.def_span(impl_did))
+    pub fn span_of_impl(self, impl_def_id: DefId) -> Result<Span, Symbol> {
+        if let Some(impl_def_id) = impl_def_id.as_local() {
+            Ok(self.def_span(impl_def_id))
         } else {
-            Err(self.crate_name(impl_did.krate))
+            Err(self.crate_name(impl_def_id.krate))
         }
     }
 

compiler/rustc_trait_selection/src/traits/chalk_fulfill.rs

@@ -14,15 +14,22 @@ pub struct FulfillmentContext<'tcx> {
     obligations: FxIndexSet<PredicateObligation<'tcx>>,
 
     relationships: FxHashMap<ty::TyVid, ty::FoundRelationships>,
+
+    usable_in_snapshot: bool,
 }
 
 impl FulfillmentContext<'_> {
     pub(crate) fn new() -> Self {
         FulfillmentContext {
             obligations: FxIndexSet::default(),
             relationships: FxHashMap::default(),
+            usable_in_snapshot: false,
         }
     }
+
+    pub(crate) fn new_in_snapshot() -> Self {
+        FulfillmentContext { usable_in_snapshot: true, ..Self::new() }
+    }
 }
 
 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
@@ -41,7 +48,9 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
         infcx: &InferCtxt<'tcx>,
         obligation: PredicateObligation<'tcx>,
     ) {
-        assert!(!infcx.is_in_snapshot());
+        if !self.usable_in_snapshot {
+            assert!(!infcx.is_in_snapshot());
+        }
         let obligation = infcx.resolve_vars_if_possible(obligation);
 
         super::relationships::update(self, infcx, &obligation);
@@ -72,7 +81,9 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
     }
 
     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
-        assert!(!infcx.is_in_snapshot());
+        if !self.usable_in_snapshot {
+            assert!(!infcx.is_in_snapshot());
+        }
 
         let mut errors = Vec::new();
         let mut next_round = FxIndexSet::default();

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -38,7 +38,7 @@ impl<'tcx> TraitEngineExt<'tcx> for dyn TraitEngine<'tcx> {
 
     fn new_in_snapshot(tcx: TyCtxt<'tcx>) -> Box<Self> {
         if tcx.sess.opts.unstable_opts.chalk {
-            Box::new(ChalkFulfillmentContext::new())
+            Box::new(ChalkFulfillmentContext::new_in_snapshot())
         } else {
             Box::new(FulfillmentContext::new_in_snapshot())
         }
@@ -119,13 +119,10 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
         expected: T,
         actual: T,
     ) -> Result<(), TypeError<'tcx>> {
-        match self.infcx.at(cause, param_env).eq(expected, actual) {
-            Ok(InferOk { obligations, value: () }) => {
-                self.register_obligations(obligations);
-                Ok(())
-            }
-            Err(e) => Err(e),
-        }
+        self.infcx
+            .at(cause, param_env)
+            .eq(expected, actual)
+            .map(|infer_ok| self.register_infer_ok_obligations(infer_ok))
     }
 
     pub fn sup<T: ToTrace<'tcx>>(
@@ -144,6 +141,10 @@ impl<'a, 'tcx> ObligationCtxt<'a, 'tcx> {
         }
     }
 
+    pub fn select_where_possible(&self) -> Vec<FulfillmentError<'tcx>> {
+        self.engine.borrow_mut().select_where_possible(self.infcx)
+    }
+
     pub fn select_all_or_error(&self) -> Vec<FulfillmentError<'tcx>> {
         self.engine.borrow_mut().select_all_or_error(self.infcx)
     }

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

@@ -0,0 +1,52 @@
+use rustc_hir::def_id::DefId;
+use rustc_infer::infer::InferCtxt;
+use rustc_infer::traits::{Obligation, ObligationCause, TraitObligation};
+use rustc_span::DUMMY_SP;
+
+use crate::traits::ObligationCtxt;
+
+pub fn recompute_applicable_impls<'tcx>(
+    infcx: &InferCtxt<'tcx>,
+    obligation: &TraitObligation<'tcx>,
+) -> Vec<DefId> {
+    let tcx = infcx.tcx;
+    let param_env = obligation.param_env;
+    let dummy_cause = ObligationCause::dummy();
+    let impl_may_apply = |impl_def_id| {
+        let ocx = ObligationCtxt::new_in_snapshot(infcx);
+        let placeholder_obligation =
+            infcx.replace_bound_vars_with_placeholders(obligation.predicate);
+        let obligation_trait_ref =
+            ocx.normalize(dummy_cause.clone(), param_env, placeholder_obligation.trait_ref);
+
+        let impl_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
+        let impl_trait_ref = tcx.bound_impl_trait_ref(impl_def_id).unwrap().subst(tcx, impl_substs);
+        let impl_trait_ref = ocx.normalize(ObligationCause::dummy(), param_env, impl_trait_ref);
+
+        if let Err(_) = ocx.eq(&dummy_cause, param_env, obligation_trait_ref, impl_trait_ref) {
+            return false;
+        }
+
+        let impl_predicates = tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs);
+        ocx.register_obligations(
+            impl_predicates
+                .predicates
+                .iter()
+                .map(|&predicate| Obligation::new(dummy_cause.clone(), param_env, predicate)),
+        );
+
+        ocx.select_where_possible().is_empty()
+    };
+
+    let mut impls = Vec::new();
+    tcx.for_each_relevant_impl(
+        obligation.predicate.def_id(),
+        obligation.predicate.skip_binder().trait_ref.self_ty(),
+        |impl_def_id| {
+            if infcx.probe(move |_snapshot| impl_may_apply(impl_def_id)) {
+                impls.push(impl_def_id)
+            }
+        },
+    );
+    impls
+}

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

@@ -1,3 +1,4 @@
+mod ambiguity;
 pub mod on_unimplemented;
 pub mod suggestions;
 
@@ -535,15 +536,6 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         let mut span = obligation.cause.span;
 
         let mut err = match *error {
-            SelectionError::Ambiguous(ref impls) => {
-                let mut err = self.tcx.sess.struct_span_err(
-                    obligation.cause.span,
-                    &format!("multiple applicable `impl`s for `{}`", obligation.predicate),
-                );
-                self.annotate_source_of_ambiguity(&mut err, impls, obligation.predicate);
-                err.emit();
-                return;
-            }
             SelectionError::Unimplemented => {
                 // If this obligation was generated as a result of well-formedness checking, see if we
                 // can get a better error message by performing HIR-based well-formedness checking.
@@ -2144,8 +2136,21 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     crate::traits::TraitQueryMode::Standard,
                 );
                 match selcx.select_from_obligation(&obligation) {
-                    Err(SelectionError::Ambiguous(impls)) if impls.len() > 1 => {
-                        self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
+                    Ok(None) => {
+                        let impls = ambiguity::recompute_applicable_impls(self.infcx, &obligation);
+                        let has_non_region_infer =
+                            trait_ref.skip_binder().substs.types().any(|t| !t.is_ty_infer());
+                        // It doesn't make sense to talk about applicable impls if there are more
+                        // than a handful of them.
+                        if impls.len() > 1 && impls.len() < 5 && has_non_region_infer {
+                            self.annotate_source_of_ambiguity(&mut err, &impls, predicate);
+                        } else {
+                            if self.is_tainted_by_errors() {
+                                err.cancel();
+                                return;
+                            }
+                            err.note(&format!("cannot satisfy `{}`", predicate));
+                        }
                     }
                     _ => {
                         if self.is_tainted_by_errors() {
@@ -2441,7 +2446,6 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                 }
             }
         }
-        let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
         let mut crate_names: Vec<_> = crates.iter().map(|n| format!("`{}`", n)).collect();
         crate_names.sort();
         crate_names.dedup();
@@ -2462,13 +2466,9 @@ impl<'tcx> InferCtxtPrivExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             err.downgrade_to_delayed_bug();
             return;
         }
-        let post = if post.len() > 4 {
-            format!(
-                ":\n{}\nand {} more",
-                post.iter().map(|p| format!("- {}", p)).take(4).collect::<Vec<_>>().join("\n"),
-                post.len() - 4,
-            )
-        } else if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
+
+        let msg = format!("multiple `impl`s satisfying `{}` found", predicate);
+        let post = if post.len() > 1 || (post.len() == 1 && post[0].contains('\n')) {
             format!(":\n{}", post.iter().map(|p| format!("- {}", p)).collect::<Vec<_>>().join("\n"),)
         } else if post.len() == 1 {
             format!(": `{}`", post[0])

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -20,7 +20,7 @@ use crate::traits;
 use crate::traits::coherence::Conflict;
 use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::{util, SelectionResult};
-use crate::traits::{Ambiguous, ErrorReporting, Overflow, Unimplemented};
+use crate::traits::{ErrorReporting, Overflow, Unimplemented};
 
 use super::BuiltinImplConditions;
 use super::IntercrateAmbiguityCause;
@@ -200,15 +200,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     // and report ambiguity.
                     if i > 1 {
                         debug!("multiple matches, ambig");
-                        return Err(Ambiguous(
-                            candidates
-                                .into_iter()
-                                .filter_map(|c| match c.candidate {
-                                    SelectionCandidate::ImplCandidate(def_id) => Some(def_id),
-                                    _ => None,
-                                })
-                                .collect(),
-                        ));
+                        return Ok(None);
                     }
                 }
             }

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

@@ -294,9 +294,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 assert!(self.query_mode == TraitQueryMode::Canonical);
                 return Err(SelectionError::Overflow(OverflowError::Canonical));
             }
-            Err(SelectionError::Ambiguous(_)) => {
-                return Ok(None);
-            }
             Err(e) => {
                 return Err(e);
             }
@@ -931,7 +928,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         match self.candidate_from_obligation(stack) {
             Ok(Some(c)) => self.evaluate_candidate(stack, &c),
-            Err(SelectionError::Ambiguous(_)) => Ok(EvaluatedToAmbig),
             Ok(None) => Ok(EvaluatedToAmbig),
             Err(Overflow(OverflowError::Canonical)) => Err(OverflowError::Canonical),
             Err(ErrorReporting) => Err(OverflowError::ErrorReporting),

src/test/ui/error-codes/E0282.rs

@@ -1,3 +1,4 @@
 fn main() {
-    let x = "hello".chars().rev().collect(); //~ ERROR E0282
+    let x = "hello".chars().rev().collect();
+    //~^ ERROR E0282
 }

src/test/ui/error-codes/E0401.rs

@@ -8,7 +8,9 @@ fn foo<T>(x: T) {
            W: Fn()>
            (y: T) { //~ ERROR E0401
     }
-    bfnr(x); //~ ERROR type annotations needed
+    bfnr(x);
+    //~^ ERROR type annotations needed
+    //~| ERROR type annotations needed
 }