Use error term in projection if missing associated item in new solver

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -124,10 +124,24 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
             };
 
             if !assoc_def.item.defaultness(tcx).has_value() {
-                tcx.sess.delay_span_bug(
+                let guar = tcx.sess.delay_span_bug(
                     tcx.def_span(assoc_def.item.def_id),
                     "missing value for assoc item in impl",
                 );
+                let error_term = match assoc_def.item.kind {
+                    ty::AssocKind::Const => tcx
+                        .const_error(
+                            tcx.type_of(goal.predicate.def_id())
+                                .subst(tcx, goal.predicate.projection_ty.substs),
+                            guar,
+                        )
+                        .into(),
+                    ty::AssocKind::Type => tcx.ty_error(guar).into(),
+                    ty::AssocKind::Fn => unreachable!(),
+                };
+                ecx.eq(goal.param_env, goal.predicate.term, error_term)
+                    .expect("expected goal term to be fully unconstrained");
+                return ecx.evaluate_added_goals_and_make_canonical_response(Certainty::Yes);
             }
 
             // Getting the right substitutions here is complex, e.g. given:

tests/ui/impl-trait/issue-103181-1.stderr → tests/ui/impl-trait/issue-103181-1.current.stderr

@@ -1,5 +1,5 @@
 error[E0046]: not all trait items implemented, missing: `Error`
-  --> $DIR/issue-103181-1.rs:9:5
+  --> $DIR/issue-103181-1.rs:11:5
    |
 LL |         type Error;
    |         ---------- `Error` from trait

tests/ui/impl-trait/issue-103181-1.next.stderr

@@ -0,0 +1,12 @@
+error[E0046]: not all trait items implemented, missing: `Error`
+  --> $DIR/issue-103181-1.rs:11:5
+   |
+LL |         type Error;
+   |         ---------- `Error` from trait
+LL |     }
+LL |     impl HttpBody for () {
+   |     ^^^^^^^^^^^^^^^^^^^^ missing `Error` in implementation
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0046`.

tests/ui/impl-trait/issue-103181-1.rs

@@ -1,3 +1,5 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
 // edition:2021
 
 mod hyper {