Macro item_if_cache_on_disk 

macro_rules! item_if_cache_on_disk {
    ([] $($item:tt)*) => { ... };
    ([(cache_on_disk) $($rest:tt)*] $($item:tt)*) => { ... };
    ([$other:tt $($modifiers:tt)*] $($item:tt)*) => { ... };
}

Conditionally expands to some token trees, if the current query has the cache_on_disk_if modifier.