Turn assert_eq into a delay_span_bug

compiler/rustc_hir_analysis/src/coherence/orphan.rs

@@ -452,7 +452,13 @@ fn lint_auto_trait_impl<'tcx>(
     trait_ref: ty::TraitRef<'tcx>,
     impl_def_id: LocalDefId,
 ) {
-    assert_eq!(trait_ref.args.len(), 1);
+    if trait_ref.args.len() != 1 {
+        tcx.sess.diagnostic().delay_span_bug(
+            tcx.def_span(impl_def_id),
+            "auto traits cannot have generic parameters",
+        );
+        return;
+    }
     let self_ty = trait_ref.self_ty();
     let (self_type_did, args) = match self_ty.kind() {
         ty::Adt(def, args) => (def.did(), args),

tests/ui/auto-traits/issue-117789.rs

@@ -0,0 +1,7 @@
+#![deny(suspicious_auto_trait_impls)]
+
+auto trait Trait<P> {} //~ ERROR auto traits cannot have generic parameters
+//~^ ERROR auto traits are experimental and possibly buggy
+impl<P> Trait<P> for () {}
+
+fn main() {}

tests/ui/auto-traits/issue-117789.stderr

@@ -0,0 +1,21 @@
+error[E0567]: auto traits cannot have generic parameters
+  --> $DIR/issue-117789.rs:3:17
+   |
+LL | auto trait Trait<P> {}
+   |            -----^^^ help: remove the parameters
+   |            |
+   |            auto trait cannot have generic parameters
+
+error[E0658]: auto traits are experimental and possibly buggy
+  --> $DIR/issue-117789.rs:3:1
+   |
+LL | auto trait Trait<P> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #13231 <https://github.com/rust-lang/rust/issues/13231> for more information
+   = help: add `#![feature(auto_traits)]` to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0567, E0658.
+For more information about an error, try `rustc --explain E0567`.