Rygel stores unowned Structure in var which fails to compile, at least on valac 0.22:

rygel-playbin-player.vala:411.25-411.46: error: duplicating Structure
instance, use unowned variable or explicitly invoke copy method
        var structure = caps.get_structure (0);
                        ^^^^^^^^^^^^^^^^^^^^^
(...)
rygel-media-export-item-factory.vala:276.25-276.60: error: duplicating Structure instance, use unowned variable or explicitly invoke copy method
        var structure = sample.get_caps ().get_structure (0);
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^