The dirstatemap is one of the last building blocks needed to get to a
dirstate.walk Rust implementation.
Disclaimer: This change is part of a big (10) series of patches, all of which
started as one big changeset that took a long time to write.
This dirstatemap implementation is a compromise in terms of complexity both
for me and for the reviewers. I chose to submit this patch right now because
while it is not perfect, it works and is simple enough (IMHO) to be reviewed.
The Python implementation uses a lot of lazy propertycaches, breaks
encapsulation and is used as an iterator in a lot of places, all of which
dictated the somewhat unidiomatic patterns in this change.
Like written in the comments, rewriting this struct to use the typestate
pattern might be a good idea, but this is a good first step.