Macro declare_repository_format 

macro_rules! declare_repository_format {
    (
        $name:ident {
            format_string: $fmt:expr,
            description: $desc:expr,
            revision_serializer: $rev:expr,
            inventory_serializer: $inv:expr,
            $( $field:ident : $value:expr, )*
        }
    ) => { ... };
}

Declare a repository format: define a static RepositoryFormat and register it so [registry] can find it by marker string.

Usage names the static, then gives field: value pairs. Capability fields default to false and supported/deprecated likewise unless set, so a declaration only states what differs from a plain format.