make check less conservative and add explanation

compiler/rustc_trait_selection/src/traits/query/evaluate_obligation.rs

@@ -1,6 +1,3 @@
-use rustc_hir as hir;
-use rustc_middle::ty::PredicateKind;
-
 use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::InferCtxt;
 use crate::traits::{
@@ -49,12 +46,6 @@ impl<'cx, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'cx, 'tcx> {
         &self,
         obligation: &PredicateObligation<'tcx>,
     ) -> bool {
-        if let PredicateKind::Trait(pred) = obligation.predicate.kind().skip_binder() {
-            if let hir::Constness::Const = pred.constness {
-                // do not evaluate to holds when we have a const predicate.
-                return false;
-            }
-        }
         self.evaluate_obligation_no_overflow(obligation).must_apply_considering_regions()
     }