Consider negative polarity on overlap check

compiler/rustc_infer/src/traits/mod.rs

@@ -10,7 +10,7 @@
 
 use rustc_hir as hir;
 use rustc_middle::ty::error::{ExpectedFound, TypeError};
-use rustc_middle::ty::{self, Const, Ty};
+use rustc_middle::ty::{self, Const, Ty, TyCtxt};
 use rustc_span::Span;
 
 pub use self::FulfillmentErrorCode::*;
@@ -55,6 +55,17 @@ pub struct Obligation<'tcx, T> {
 pub type PredicateObligation<'tcx> = Obligation<'tcx, ty::Predicate<'tcx>>;
 pub type TraitObligation<'tcx> = Obligation<'tcx, ty::PolyTraitPredicate<'tcx>>;
 
+impl PredicateObligation<'tcx> {
+    pub fn flip_polarity(&self, tcx: TyCtxt<'tcx>) -> Option<PredicateObligation<'tcx>> {
+        Some(PredicateObligation {
+            cause: self.cause.clone(),
+            param_env: self.param_env,
+            predicate: self.predicate.flip_polarity(tcx)?,
+            recursion_depth: self.recursion_depth,
+        })
+    }
+}
+
 // `PredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
 static_assert_size!(PredicateObligation<'_>, 32);

compiler/rustc_middle/src/traits/select.rs

@@ -12,7 +12,7 @@
 use rustc_query_system::cache::Cache;
 
 pub type SelectionCache<'tcx> = Cache<
-    ty::ConstnessAnd<ty::ParamEnvAnd<'tcx, ty::TraitRef<'tcx>>>,
+    (ty::ConstnessAnd<ty::ParamEnvAnd<'tcx, ty::TraitRef<'tcx>>>, ty::ImplPolarity),
     SelectionResult<'tcx, SelectionCandidate<'tcx>>,
 >;
 
@@ -101,7 +101,7 @@ pub enum SelectionCandidate<'tcx> {
         /// `false` if there are no *further* obligations.
         has_nested: bool,
     },
-    ParamCandidate(ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>),
+    ParamCandidate((ty::ConstnessAnd<ty::PolyTraitRef<'tcx>>, ty::ImplPolarity)),
     ImplCandidate(DefId),
     AutoImplCandidate(DefId),
 

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -5,6 +5,7 @@
 //! [trait-specialization]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html
 
 use crate::infer::{CombinedSnapshot, InferOk, TyCtxtInferExt};
+use crate::traits::query::evaluate_obligation::InferCtxtExt;
 use crate::traits::select::IntercrateAmbiguityCause;
 use crate::traits::SkipLeakCheck;
 use crate::traits::{self, Normalized, Obligation, ObligationCause, SelectionContext};
@@ -186,6 +187,7 @@ fn overlap_within_probe(
 
     // Are any of the obligations unsatisfiable? If so, no overlap.
     let infcx = selcx.infcx();
+    let tcx = infcx.tcx;
     let opt_failing_obligation = a_impl_header
         .predicates
         .iter()
@@ -199,7 +201,13 @@ fn overlap_within_probe(
             predicate: p,
         })
         .chain(obligations)
-        .find(|o| !selcx.predicate_may_hold_fatal(o));
+        .find(|o| {
+            !selcx.predicate_may_hold_fatal(o)
+                || o.flip_polarity(tcx)
+                    .as_ref()
+                    .map(|o| selcx.infcx().predicate_must_hold_considering_regions(o))
+                    .unwrap_or(false)
+        });
     // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
     // to the canonical trait query form, `infcx.predicate_may_hold`, once
     // the new system supports intercrate mode (which coherence needs).

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

@@ -376,7 +376,10 @@ fn assemble_candidates_from_caller_bounds<'o>(
         for bound in matching_bounds {
             let wc = self.evaluate_where_clause(stack, bound.value)?;
             if wc.may_apply() {
-                candidates.vec.push(ParamCandidate(bound));
+                candidates.vec.push(ParamCandidate((
+                    bound,
+                    stack.obligation.predicate.skip_binder().polarity,
+                )));
             }
         }
 

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

@@ -58,8 +58,8 @@ pub(super) fn confirm_candidate(
             }
 
             ParamCandidate(param) => {
-                let obligations = self.confirm_param_candidate(obligation, param.value);
-                Ok(ImplSource::Param(obligations, param.constness))
+                let obligations = self.confirm_param_candidate(obligation, param.0.value);
+                Ok(ImplSource::Param(obligations, param.0.constness))
             }
 
             ImplCandidate(impl_def_id) => {

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

@@ -1107,10 +1107,10 @@ fn filter_impls(
                 // const impl
                 ImplCandidate(def_id) if tcx.impl_constness(def_id) == hir::Constness::Const => {}
                 // const param
-                ParamCandidate(ty::ConstnessAnd {
-                    constness: ty::BoundConstness::ConstIfConst,
-                    ..
-                }) => {}
+                ParamCandidate((
+                    ty::ConstnessAnd { constness: ty::BoundConstness::ConstIfConst, .. },
+                    _,
+                )) => {}
                 // auto trait impl
                 AutoImplCandidate(..) => {}
                 // generator, this will raise error in other places
@@ -1219,14 +1219,14 @@ fn check_candidate_cache(
         if self.can_use_global_caches(param_env) {
             if let Some(res) = tcx
                 .selection_cache
-                .get(&param_env.and(trait_ref).with_constness(pred.constness), tcx)
+                .get(&(param_env.and(trait_ref).with_constness(pred.constness), pred.polarity), tcx)
             {
                 return Some(res);
             }
         }
         self.infcx
             .selection_cache
-            .get(&param_env.and(trait_ref).with_constness(pred.constness), tcx)
+            .get(&(param_env.and(trait_ref).with_constness(pred.constness), pred.polarity), tcx)
     }
 
     /// Determines whether can we safely cache the result
@@ -1286,7 +1286,7 @@ fn insert_candidate_cache(
                     debug!(?trait_ref, ?candidate, "insert_candidate_cache global");
                     // This may overwrite the cache with the same value.
                     tcx.selection_cache.insert(
-                        param_env.and(trait_ref).with_constness(pred.constness),
+                        (param_env.and(trait_ref).with_constness(pred.constness), pred.polarity),
                         dep_node,
                         candidate,
                     );
@@ -1297,7 +1297,7 @@ fn insert_candidate_cache(
 
         debug!(?trait_ref, ?candidate, "insert_candidate_cache local");
         self.infcx.selection_cache.insert(
-            param_env.and(trait_ref).with_constness(pred.constness),
+            (param_env.and(trait_ref).with_constness(pred.constness), pred.polarity),
             dep_node,
             candidate,
         );
@@ -1523,10 +1523,14 @@ fn candidate_should_be_dropped_in_favor_of(
                 | ConstDropCandidate,
             ) => false,
 
-            (ParamCandidate(other), ParamCandidate(victim)) => {
+            (
+                ParamCandidate((other, other_polarity)),
+                ParamCandidate((victim, victim_polarity)),
+            ) => {
                 let same_except_bound_vars = other.value.skip_binder()
                     == victim.value.skip_binder()
                     && other.constness == victim.constness
+                    && other_polarity == victim_polarity
                     && !other.value.skip_binder().has_escaping_bound_vars();
                 if same_except_bound_vars {
                     // See issue #84398. In short, we can generate multiple ParamCandidates which are
@@ -1537,6 +1541,7 @@ fn candidate_should_be_dropped_in_favor_of(
                     other.value.bound_vars().len() <= victim.value.bound_vars().len()
                 } else if other.value == victim.value
                     && victim.constness == ty::BoundConstness::NotConst
+                    && other_polarity == victim_polarity
                 {
                     // Drop otherwise equivalent non-const candidates in favor of const candidates.
                     true
@@ -1566,11 +1571,11 @@ fn candidate_should_be_dropped_in_favor_of(
                 | TraitAliasCandidate(..)
                 | ObjectCandidate(_)
                 | ProjectionCandidate(_),
-            ) => !is_global(&cand.value),
+            ) => !is_global(&cand.0.value),
             (ObjectCandidate(_) | ProjectionCandidate(_), ParamCandidate(ref cand)) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
-                is_global(&cand.value)
+                is_global(&cand.0.value)
             }
             (
                 ImplCandidate(_)
@@ -1586,7 +1591,7 @@ fn candidate_should_be_dropped_in_favor_of(
             ) => {
                 // Prefer these to a global where-clause bound
                 // (see issue #50825).
-                is_global(&cand.value) && other.evaluation.must_apply_modulo_regions()
+                is_global(&cand.0.value) && other.evaluation.must_apply_modulo_regions()
             }
 
             (ProjectionCandidate(i), ProjectionCandidate(j))