Fix up error message for debug_assert_args_compat for IATs

compiler/rustc_middle/src/ty/context.rs

@@ -2009,7 +2009,10 @@ impl<'tcx> TyCtxt<'tcx> {
         true
     }
 
+    // With `cfg(debug_assertions)`, assert that args are compatible with their generics,
+    // and print out the args if not.
     pub fn debug_assert_args_compatible(self, def_id: DefId, args: &'tcx [ty::GenericArg<'tcx>]) {
+        // Fast-path for non-debug, bail out early.
         #[cfg(not(debug_assertions))]
         {
             return;
@@ -2019,7 +2022,20 @@ impl<'tcx> TyCtxt<'tcx> {
             if let DefKind::AssocTy = self.def_kind(def_id)
                 && let DefKind::Impl { of_trait: false } = self.def_kind(self.parent(def_id))
             {
-                bug!()
+                bug!(
+                    "args not compatible with generics for {}: args={:#?}, generics={:#?}",
+                    self.def_path_str(def_id),
+                    args,
+                    // Make `[Self, GAT_ARGS...]` (this could be simplified)
+                    self.mk_args_from_iter(
+                        [self.types.self_param.into()].into_iter().chain(
+                            self.generics_of(def_id)
+                                .own_args(ty::GenericArgs::identity_for_item(self, def_id))
+                                .iter()
+                                .copied()
+                        )
+                    )
+                );
             } else {
                 bug!(
                     "args not compatible with generics for {}: args={:#?}, generics={:#?}",
@@ -2034,11 +2050,11 @@ impl<'tcx> TyCtxt<'tcx> {
     #[inline(always)]
     pub(crate) fn check_and_mk_args(
         self,
-        _def_id: DefId,
+        def_id: DefId,
         args: impl IntoIterator<Item: Into<GenericArg<'tcx>>>,
     ) -> GenericArgsRef<'tcx> {
         let args = self.mk_args_from_iter(args.into_iter().map(Into::into));
-        self.debug_assert_args_compatible(_def_id, args);
+        self.debug_assert_args_compatible(def_id, args);
         args
     }