diff --git a/serde_derive/src/internals/attr.rs b/serde_derive/src/internals/attr.rs
index 918d47bf..e53f5891 100644
--- a/serde_derive/src/internals/attr.rs
+++ b/serde_derive/src/internals/attr.rs
@@ -1909,8 +1909,12 @@ fn collect_lifetimes(ty: &syn::Type, out: &mut BTreeSet<syn::Lifetime>) {
         | syn::Type::TraitObject(_)
         | syn::Type::ImplTrait(_)
         | syn::Type::Infer(_)
-        | syn::Type::Verbatim(_)
-        | _ => {}
+        | syn::Type::Verbatim(_) => {}
+
+        #[cfg(test)]
+        syn::Type::__TestExhaustive(_) => unimplemented!(),
+        #[cfg(not(test))]
+        _ => {}
     }
 }