Opt-in diagnostics reporting to avoid doing extra work in the new solver

compiler/rustc_traits/src/codegen.rs

@@ -50,7 +50,8 @@ pub fn codegen_select_candidate<'tcx>(
     // Currently, we use a fulfillment context to completely resolve
     // all nested obligations. This is because they can inform the
     // inference of the impl's type parameters.
-    let ocx = ObligationCtxt::new(&infcx);
+    // FIXME(-Znext-solver): Doesn't need diagnostics if new solver.
+    let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
     let impl_source = selection.map(|obligation| {
         ocx.register_obligation(obligation);
     });