Add #![feature(const_fn_impl)]

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

@@ -559,11 +559,20 @@ pub mod ty {
     pub struct ImplTrait;
     impl NonConstOp for ImplTrait {
         fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-            mcf_status_in_item(ccx)
+            if ccx.const_kind() != hir::ConstContext::ConstFn {
+                Status::Allowed
+            } else {
+                Status::Unstable(sym::const_fn_impl_trait)
+            }
         }
 
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-            mcf_build_error(ccx, span, "`impl Trait` in const fn is unstable")
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_fn_impl_trait,
+                span,
+                &format!("`impl Trait` is not allowed in {}s", ccx.const_kind()),
+            )
         }
     }