Make things work by using the new solver

compiler/rustc_trait_selection/src/solve/assembly/mod.rs

@@ -37,6 +37,8 @@ pub(super) trait GoalKind<'tcx>:
 
     fn trait_ref(self, tcx: TyCtxt<'tcx>) -> ty::TraitRef<'tcx>;
 
+    fn polarity(self) -> ty::ImplPolarity;
+
     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self;
 
     fn trait_def_id(self, tcx: TyCtxt<'tcx>) -> DefId;

compiler/rustc_trait_selection/src/solve/project_goals/mod.rs

@@ -101,6 +101,10 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         self.projection_ty.trait_ref(tcx)
     }
 
+    fn polarity(self) -> ty::ImplPolarity {
+        ty::ImplPolarity::Positive
+    }
+
     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
         self.with_self_ty(tcx, self_ty)
     }

compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -22,6 +22,10 @@ impl<'tcx> assembly::GoalKind<'tcx> for TraitPredicate<'tcx> {
         self.trait_ref
     }
 
+    fn polarity(self) -> ty::ImplPolarity {
+        self.polarity
+    }
+
     fn with_self_ty(self, tcx: TyCtxt<'tcx>, self_ty: Ty<'tcx>) -> Self {
         self.with_self_ty(tcx, self_ty)
     }

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -398,8 +398,8 @@ fn impl_intersection_has_negative_obligation(
 ) -> bool {
     debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
 
-    let ref infcx = tcx.infer_ctxt().intercrate(true).build();
-    let universe = infcx.create_next_universe();
+    let ref infcx = tcx.infer_ctxt().intercrate(true).with_next_trait_solver(true).build();
+    let universe = infcx.universe();
 
     let impl1_header = fresh_impl_header(infcx, impl1_def_id);
     let param_env =

compiler/rustc_trait_selection/src/traits/engine.rs

@@ -37,7 +37,7 @@ impl<'tcx> TraitEngineExt<'tcx> for dyn TraitEngine<'tcx> {
             (TraitSolver::Classic, false) | (TraitSolver::NextCoherence, false) => {
                 Box::new(FulfillmentContext::new(infcx))
             }
-            (TraitSolver::Next | TraitSolver::NextCoherence, true) => {
+            (TraitSolver::Classic | TraitSolver::Next | TraitSolver::NextCoherence, true) => {
                 Box::new(NextFulfillmentCtxt::new(infcx))
             }
             _ => bug!(

tests/ui/coherence/coherence-overlap-trait-alias.rs

@@ -13,8 +13,6 @@ impl B for u32 {}
 trait C {}
 impl<T: AB> C for T {}
 impl C for u32 {}
-//~^ ERROR
-// FIXME it's giving an ungreat error but unsure if we care given that it's using an internal rustc
-// attribute and an artificial code path for testing purposes
+//~^ ERROR conflicting implementations of trait `C` for type `u32`
 
 fn main() {}

tests/ui/coherence/coherence-overlap-trait-alias.stderr

@@ -1,17 +1,11 @@
-error[E0283]: type annotations needed: cannot satisfy `u32: C`
-  --> $DIR/coherence-overlap-trait-alias.rs:15:12
-   |
-LL | impl C for u32 {}
-   |            ^^^
-   |
-note: multiple `impl`s satisfying `u32: C` found
-  --> $DIR/coherence-overlap-trait-alias.rs:14:1
+error[E0119]: conflicting implementations of trait `C` for type `u32`
+  --> $DIR/coherence-overlap-trait-alias.rs:15:1
    |
 LL | impl<T: AB> C for T {}
-   | ^^^^^^^^^^^^^^^^^^^
+   | ------------------- first implementation here
 LL | impl C for u32 {}
-   | ^^^^^^^^^^^^^^
+   | ^^^^^^^^^^^^^^ conflicting implementation for `u32`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0283`.
+For more information about this error, try `rustc --explain E0119`.

tests/ui/impl-trait/negative-reasoning.stderr

@@ -7,7 +7,7 @@ LL | impl<T: std::fmt::Debug> AnotherTrait for T {}
 LL | impl AnotherTrait for D<OpaqueType> {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `D<OpaqueType>`
    |
-   = note: upstream crates may add a new impl of trait `std::fmt::Debug` for type `OpaqueType` in future versions
+   = note: upstream crates may add a new impl of trait `std::marker::FnPtr` for type `OpaqueType` in future versions
 
 error: aborting due to previous error
 

tests/ui/type-alias-impl-trait/impl_trait_for_same_tait.stderr

@@ -15,6 +15,8 @@ LL | impl Bop for Bar<()> {}
 ...
 LL | impl Bop for Barr {}
    | ^^^^^^^^^^^^^^^^^ conflicting implementation for `Bar<()>`
+   |
+   = note: upstream crates may add a new impl of trait `std::marker::FnPtr` for type `Barr` in future versions
 
 error[E0119]: conflicting implementations of trait `Bop` for type `Bar<()>`
   --> $DIR/impl_trait_for_same_tait.rs:30:1