rust: add `dirstate_tree` module...
rust: add `dirstate_tree` module
Mercurial needs to represent the filesystem hierarchy on which it operates, for
example in the dirstate. Its current on-disk representation is an unsorted, flat
structure that gets transformed in the current Rust code into a `HashMap`.
This loses the hierarchical information of the dirstate, leading to some
unfortunate performance and algorithmic compromises.
This module adds an implementation of a radix tree that is specialized for
representing the dirstate: its unit is the path component. I have made no
efforts to optimize either its memory footprint or its insertion speed: they're
pretty bad for now.
Following will be a few patches that modify the dirstate.status logic to use
that new hierarchical information, fixing issue 6335 in the same swing.
Differential Revision:
https://phab.mercurial-scm.org/D9085
