Rebased and improved errors

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -125,7 +125,7 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
                         param_env,
                         Binder::dummy(TraitPredicate {
                             trait_ref,
-                            constness: BoundConstness::ConstIfConst,
+                            constness: BoundConstness::NotConst,
                             polarity: ImplPolarity::Positive,
                         }),
                     );

compiler/rustc_const_eval/src/util/call_kind.rs

@@ -71,9 +71,7 @@ pub fn call_kind<'tcx>(
         AssocItemContainer::TraitContainer(trait_did) => Some(trait_did),
     });
 
-    let fn_call = (!from_hir_call)
-        .then(|| parent)
-        .flatten()
+    let fn_call = parent
         .and_then(|p| tcx.lang_items().group(LangItemGroup::Fn).iter().find(|did| **did == p));
 
     let operator = (!from_hir_call)