Make TraitEngine::new use the right solver, add compare mode

compiler/rustc_traits/src/codegen.rs

@@ -55,7 +55,7 @@ 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 mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(tcx);
+    let mut fulfill_cx = <dyn TraitEngine<'tcx>>::new(&infcx);
     let impl_source = selection.map(|predicate| {
         fulfill_cx.register_predicate_obligation(&infcx, predicate);
     });