Enum EntryOf 

pub enum EntryOf<K> {
    Took(K),
    ThePathItself,
    NameWillNotRead,
    NotThisMaps,
}

What a map may be keyed by: a key that already is the name its entry sits at.

An entry lives at one level under the map, and that level is named by the key. A key that borrows its own name needs no spelling and no parsing: the store is addressed with what the caller already holds, a listing hands back what is already there, and neither can disagree with the other, because there is only one of them.

A key that is not a string is spelled by Id, which renders it once and holds the rendering. What one scanned key turned out to be, to the map it was scanned from.

A map is loaded from two places - a store that reads now and a backend that reads later - and each has its own vocabulary for what went wrong. What they may not have is their own opinion about which of these four a key is, and each working it out from the path by hand is how they came to differ.

Variants

Took(K)

One level below the map, and the name reads as K.

ThePathItself

The map’s own path. Not an entry, and not a fault: a scan of a prefix hands the prefix back.

NameWillNotRead

One level below, and the name does not read as K.

NotThisMaps

Deeper than the level the map owns, or not under it at all. A map owns the level below it and nothing further, so a key past that belongs to whatever claimed that level.