diff --git a/pyo3-build-config/src/impl_.rs b/pyo3-build-config/src/impl_.rs
index 22add7f0..04b8da33 100644
--- a/pyo3-build-config/src/impl_.rs
+++ b/pyo3-build-config/src/impl_.rs
@@ -628,7 +628,7 @@ impl FromStr for BuildFlag {
 ///
 /// see Misc/SpecialBuilds.txt in the python source for what these mean.
 #[cfg_attr(test, derive(Debug, PartialEq))]
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct BuildFlags(pub HashSet<BuildFlag>);
 
 impl BuildFlags {