Don't require intercrate mode for negative coherence

compiler/rustc_infer/src/infer/at.rs

@@ -65,8 +65,15 @@ impl<'tcx> InferCtxt<'tcx> {
 
     /// Forks the inference context, creating a new inference context with the same inference
     /// variables in the same state. This can be used to "branch off" many tests from the same
-    /// common state. Used in coherence.
+    /// common state.
     pub fn fork(&self) -> Self {
+        self.fork_with_intercrate(self.intercrate)
+    }
+
+    /// Forks the inference context, creating a new inference context with the same inference
+    /// variables in the same state, except possibly changing the intercrate mode. This can be
+    /// used to "branch off" many tests from the same common state. Used in negative coherence.
+    pub fn fork_with_intercrate(&self, intercrate: bool) -> Self {
         Self {
             tcx: self.tcx,
             defining_use_anchor: self.defining_use_anchor,
@@ -81,7 +88,7 @@ impl<'tcx> InferCtxt<'tcx> {
             tainted_by_errors: self.tainted_by_errors.clone(),
             err_count_on_creation: self.err_count_on_creation,
             universe: self.universe.clone(),
-            intercrate: self.intercrate,
+            intercrate,
             next_trait_solver: self.next_trait_solver,
         }
     }

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -397,6 +397,8 @@ fn impl_intersection_has_negative_obligation(
 ) -> bool {
     debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
 
+    // N.B. We need to unify impl headers *with* intercrate mode, even if proving negative predicates
+    // do not need intercrate mode enabled.
     let ref infcx = tcx.infer_ctxt().intercrate(true).with_next_trait_solver(true).build();
     let root_universe = infcx.universe();
     assert_eq!(root_universe, ty::UniverseIndex::ROOT);
@@ -554,7 +556,11 @@ fn try_prove_negated_where_clause<'tcx>(
         return false;
     };
 
-    let ref infcx = root_infcx.fork();
+    // N.B. We don't need to use intercrate mode here because we're trying to prove
+    // the *existence* of a negative goal, not the non-existence of a positive goal.
+    // Without this, we over-eagerly register coherence ambiguity candidates when
+    // impl candidates do exist.
+    let ref infcx = root_infcx.fork_with_intercrate(false);
     let ocx = ObligationCtxt::new(infcx);
 
     ocx.register_obligation(Obligation::new(

tests/ui/coherence/coherence-overlap-with-regions.rs

@@ -1,10 +1,4 @@
-// known-bug: unknown
-
-// This fails because we currently perform negative coherence in coherence mode.
-// This means that when looking for a negative predicate, we also assemble a
-// coherence-unknowable predicate. Since confirming the negative impl has region
-// obligations, we don't prefer the impl over the unknowable predicate
-// unconditionally and instead flounder.
+// check-pass
 
 #![feature(negative_impls)]
 #![feature(rustc_attrs)]

tests/ui/coherence/negative-coherence-considering-regions.any_lt.stderr

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `Bar` for type `&_`
-  --> $DIR/negative-coherence-considering-regions.rs:22:1
+  --> $DIR/negative-coherence-considering-regions.rs:16:1
    |
 LL | impl<T> Bar for T where T: Foo {}
    | ------------------------------ first implementation here

tests/ui/coherence/negative-coherence-considering-regions.rs

@@ -1,11 +1,5 @@
 // revisions: any_lt static_lt
-//[static_lt] known-bug: unknown
-
-// This fails because we currently perform negative coherence in coherence mode.
-// This means that when looking for a negative predicate, we also assemble a
-// coherence-unknowable predicate. Since confirming the negative impl has region
-// obligations, we don't prefer the impl over the unknowable predicate
-// unconditionally and instead flounder.
+//[static_lt] check-pass
 
 #![feature(negative_impls)]
 #![feature(with_negative_coherence)]