Enum CacheHit 

pub enum CacheHit<'a> {
    Primary(&'a mut File),
    Secondary(&'a mut File),
}

Where does a cache hit come from: the primary read-write cache, or one of the secondary read-only caches?

Variants

Primary(&'a mut File)

The file was found in the primary read-write cache; promoting it is a no-op.

Secondary(&'a mut File)

The file was found in one of the secondary read-only caches. Promoting it to the primary cache will require a full copy.