fix(flask): include cargo build-dep

charmcraft/extensions/gunicorn.py

@@ -223,6 +223,12 @@ def is_experimental(base: tuple[str, ...] | None) -> bool:  # noqa: ARG004
         """Check if the extension is in an experimental state."""
         return False
 
+    @override
+    def get_parts_snippet(self) -> dict[str, Any]:
+        """Return the parts to add to parts."""
+        # rust is needed to build pydantic-core, a dependency of flask.
+        return {"rust-deps": {"build-packages": ["cargo"]}}
+
 
 class DjangoFramework(_GunicornBase):
     """Extension for 12-factor Django applications."""