Don't ICE on DiscriminantKind projection in new solver

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -485,9 +485,49 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
         ecx: &mut EvalCtxt<'_, 'tcx>,
         goal: Goal<'tcx, Self>,
     ) -> QueryResult<'tcx> {
-        let discriminant = goal.predicate.self_ty().discriminant_ty(ecx.tcx());
+        let self_ty = goal.predicate.self_ty();
+        let discriminant_ty = match *self_ty.kind() {
+            ty::Bool
+            | ty::Char
+            | ty::Int(..)
+            | ty::Uint(..)
+            | ty::Float(..)
+            | ty::Array(..)
+            | ty::RawPtr(..)
+            | ty::Ref(..)
+            | ty::FnDef(..)
+            | ty::FnPtr(..)
+            | ty::Closure(..)
+            | ty::Infer(ty::IntVar(..) | ty::FloatVar(..))
+            | ty::Generator(..)
+            | ty::GeneratorWitness(..)
+            | ty::GeneratorWitnessMIR(..)
+            | ty::Never
+            | ty::Foreign(..)
+            | ty::Adt(_, _)
+            | ty::Str
+            | ty::Slice(_)
+            | ty::Dynamic(_, _, _)
+            | ty::Tuple(_)
+            | ty::Error(_) => self_ty.discriminant_ty(ecx.tcx()),
+
+            // We do not call `Ty::discriminant_ty` on alias, param, or placeholder
+            // types, which return `<self_ty as DiscriminantKind>::Discriminant`
+            // (or ICE in the case of placeholders). Projecting a type to itself
+            // is never really productive.
+            ty::Alias(_, _) | ty::Param(_) | ty::Placeholder(..) => {
+                return Err(NoSolution);
+            }
+
+            ty::Infer(ty::TyVar(_) | ty::FreshTy(_) | ty::FreshIntTy(_) | ty::FreshFloatTy(_))
+            | ty::Bound(..) => bug!(
+                "unexpected self ty `{:?}` when normalizing `<T as DiscriminantKind>::Discriminant`",
+                goal.predicate.self_ty()
+            ),
+        };
+
         ecx.probe(|ecx| {
-            ecx.eq(goal.param_env, goal.predicate.term, discriminant.into())?;
+            ecx.eq(goal.param_env, goal.predicate.term, discriminant_ty.into())?;
             ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes)
         })
     }

tests/ui/traits/new-solver/param-discr-kind.rs

@@ -0,0 +1,8 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+fn foo<T>(x: T) {
+    std::mem::discriminant(&x);
+}
+
+fn main() {}

tests/ui/traits/new-solver/projection-discr-kind.rs

@@ -0,0 +1,18 @@
+// compile-flags: -Ztrait-solver=next
+
+// Check that `<T::Assoc as DiscriminantKind>::Discriminant` doesn't normalize
+// to itself and cause overflow/ambiguity.
+
+trait Foo {
+    type Assoc;
+}
+
+trait Bar {}
+fn needs_bar(_: impl Bar) {}
+
+fn foo<T: Foo>(x: T::Assoc) {
+    needs_bar(std::mem::discriminant(&x));
+    //~^ ERROR the trait bound `Discriminant<<T as Foo>::Assoc>: Bar` is not satisfied
+}
+
+fn main() {}

tests/ui/traits/new-solver/projection-discr-kind.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `Discriminant<<T as Foo>::Assoc>: Bar` is not satisfied
+  --> $DIR/projection-discr-kind.rs:14:15
+   |
+LL |     needs_bar(std::mem::discriminant(&x));
+   |     --------- ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `Discriminant<<T as Foo>::Assoc>`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `needs_bar`
+  --> $DIR/projection-discr-kind.rs:11:22
+   |
+LL | fn needs_bar(_: impl Bar) {}
+   |                      ^^^ required by this bound in `needs_bar`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.