make check less conservative and add explanation

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -316,6 +316,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         self.infcx.tcx
     }
 
+    /// returns `true` if the predicate is considered `const` to
+    /// this selection context.
+    pub fn is_predicate_const(&self, pred: ty::Predicate<'_>) -> bool {
+        match pred.kind().skip_binder() {
+            ty::PredicateKind::Trait(ty::TraitPredicate {
+                constness: hir::Constness::Const,
+                ..
+            }) if self.const_impls_required => true,
+            _ => false,
+        }
+    }
+
     ///////////////////////////////////////////////////////////////////////////
     // Selection
     //