diff --git a/crates/ra_hir_ty/src/diagnostics.rs b/crates/ra_hir_ty/src/diagnostics.rs
index a9877d867ff..885abbaf2d6 100644
--- a/crates/ra_hir_ty/src/diagnostics.rs
+++ b/crates/ra_hir_ty/src/diagnostics.rs
@@ -234,6 +234,9 @@ impl Diagnostic for MismatchedArgCount {
     fn as_any(&self) -> &(dyn Any + Send + 'static) {
         self
     }
+    fn is_experimental(&self) -> bool {
+        true
+    }
 }
 
 impl AstDiagnostic for MismatchedArgCount {