mercurial/helptext/rust.txt

emitrevision: if we need to compute a delta on the fly, try p1 or p2 first

Falling back to `prev` does not yield any real value on modern storage and
result in pathological changes to be created on the other side. Doing a delta
against a parent will likely be smaller (helping the network) and will be safer
to apply on the client (helping future pulls by Triggering intermediate
snapshop where they will be needed by later deltas).

Mercurial can be augmented with Rust extensions for speeding up certain
operations.

Compatibility
=============

Though the Rust extensions are only tested by the project under Linux, users of
MacOS, FreeBSD and other UNIX-likes have been using the Rust extensions. Your
mileage may vary, but by all means do give us feedback or signal your interest
for better support.

No Rust extensions are available for Windows at this time.

Features
========

The following operations are sped up when using Rust:

    - discovery of differences between repositories (pull/push)
    - nodemap (see :hg:`help config.format.use-persistent-nodemap`)
    - all commands using the dirstate (status, commit, diff, add, update, etc.)
    - dirstate-v2 (see :hg:`help config.format.use-dirstate-v2`)
    - iteration over ancestors in a graph

More features are in the works, and improvements on the above listed are still
in progress. For more experimental work see the "rhg" section.

Checking for Rust
=================

You may already have the Rust extensions depending on how you install
Mercurial::

  $ hg debuginstall | grep -i rust
  checking Rust extensions (installed)
  checking module policy (rust+c-allow)

If those lines don't even exist, you're using an old version of `hg` which does
not have any Rust extensions yet.

Installing
==========

You will need `cargo` to be in your `$PATH`. See the "MSRV" section for which
version to use.

Using pip
---------

Users of `pip` can install the Rust extensions with the following command::

  $ pip install mercurial --global-option --rust --no-use-pep517

`--no-use-pep517` is here to tell `pip` to preserve backwards compatibility with
the legacy `setup.py` system. Mercurial has not yet migrated its complex setup
to the new system, so we still need this to add compiled extensions.

This might take a couple of minutes because you're compiling everything.

See the "Checking for Rust" section to see if the install succeeded.

From your distribution
----------------------

Some distributions are shipping Mercurial with Rust extensions enabled and
pre-compiled (meaning you won't have to install `cargo`), or allow you to
specify an install flag. Check with your specific distribution for how to do
that, or ask their team to add support for hg+Rust!

From source
-----------

Please refer to the `rust/README.rst` file in the Mercurial repository for
instructions on how to install from source.

MSRV
====

The minimum supported Rust version is currently 1.48.0. The project's policy is
to follow the version from Debian stable, to make the distributions' job easier.

rhg
===

There exists an experimental pure-Rust version of Mercurial called `rhg` with a
fallback mechanism for unsupported invocations. It allows for much faster
execution of certain commands while adding no discernable overhead for the rest.

The only way of trying it out is by building it from source. Please refer to
`rust/README.rst` in the Mercurial repository.

See `hg help config.rhg` for configuration options.

Contributing
============

If you would like to help the Rust endeavor, please refer to `rust/README.rst`
in the Mercurial repository.