Macro if_cache_on_disk 

macro_rules! if_cache_on_disk {
    ([] $yes:tt $no:tt) => { ... };
    ([(cache_on_disk) $($rest:tt)*] $yes:tt $no:tt) => { ... };
    ([$other:tt $($modifiers:tt)*] $yes:tt $no:tt) => { ... };
}

Expands to one of two token trees, depending on whether the current query has the cache_on_disk_if modifier.