rust-node: binary Node ID and conversion utilities
The choice of type makes sure that a Node has the exact
wanted size. We'll use a different type for prefixes.
Added dependency: hexadecimal conversion relies on the
The fact that sooner or later Mercurial is going to need
to change its hash sizes has been taken strongly in
- the hash length is a constant, but that is not directly exposed to callers. Changing the value of that constant is the only thing to do to change the hash length (even in unit tests)
- the code could be adapted to support several sizes of hashes, if that turned out to be useful. To that effect, only the size of a given Node is exposed in the public API.
- callers not involved in initial computation, I/O and FFI are able to operate without a priori assumptions on the hash size. The traits FromHex and ToHex have not been directly implemented, so that the doc-comments explaining these restrictions would stay really visible in cargo doc
Differential Revision: https://phab.mercurial-scm.org/D7788