add note suggesting that predicate is satisfied but is not const

compiler/rustc_middle/src/ty/mod.rs

@@ -803,6 +803,10 @@ impl<'tcx> PolyTraitPredicate<'tcx> {
             p
         });
     }
+
+    pub fn is_const(self) -> bool {
+        self.skip_binder().constness == BoundConstness::ConstIfConst
+    }
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
@@ -1388,6 +1392,10 @@ impl<'tcx> ParamEnv<'tcx> {
         self.packed.tag().constness
     }
 
+    pub fn is_const(self) -> bool {
+        self.packed.tag().constness == hir::Constness::Const
+    }
+
     /// Construct a trait environment with no where-clauses in scope
     /// where the values of all `impl Trait` and other hidden types
     /// are revealed. This is suitable for monomorphized, post-typeck
@@ -1503,6 +1511,7 @@ impl<'tcx> PolyTraitRef<'tcx> {
             polarity: ty::ImplPolarity::Positive,
         })
     }
+
     #[inline]
     pub fn without_const(self) -> PolyTraitPredicate<'tcx> {
         self.with_constness(BoundConstness::NotConst)