===================

Mercurial Rust Code

===================

This directory contains various Rust code for the Mercurial project.

Rust is not required to use (or build) Mercurial, but using it

improves performance in some areas.

There are currently four independent Rust projects:

- chg. An implementation of chg, in Rust instead of C.

- hgcli. A project that provides a (mostly) self-contained "hg" binary,

  for ease of deployment and a bit of speed, using PyOxidizer. See

  ``hgcli/README.md``.

- hg-core (and hg-cpython): implementation of some

  functionality of mercurial in Rust, e.g. ancestry computations in

  revision graphs, status or pull discovery. The top-level ``Cargo.toml`` file

  defines a workspace containing these crates.

- rhg: a pure Rust implementation of Mercurial, with a fallback mechanism for

  unsupported invocations. It reuses the logic ``hg-core`` but

  completely forgoes interaction with Python. See

  ``rust/rhg/README.md`` for more details.

Using Rust code

===============

Local use (you need to clean previous build artifacts if you have

built without rust previously)::

  $ make PURE=--rust local # to use ./hg

  $ ./tests/run-tests.py --rust # to run all tests

  $ ./hg debuginstall | grep -i rust # to validate rust is in use

  checking Rust extensions (installed)

  checking module policy (rust+c-allow)

If the environment variable ``HGWITHRUSTEXT=cpython`` is set, the Rust

extension will be used by default unless ``--no-rust``.

One day we may use this environment variable to switch to new experimental

binding crates like a hypothetical ``HGWITHRUSTEXT=hpy``.

Special features

================

In the future, compile-time opt-ins may be added

to the ``features`` section in ``hg-cpython/Cargo.toml``.

To use features from the Makefile, use the ``HG_RUST_FEATURES`` environment

variable: for instance ``HG_RUST_FEATURES="some-feature other-feature"``.

Profiling

=========

Setting the environment variable ``RUST_LOG=trace`` will make hg print

a few high level rust-related performance numbers. It can also

indicate why the rust code cannot be used (say, using lookarounds in

hgignore).

Creating a ``.cargo/config`` file with the following content enables

debug information in optimized builds. This make profiles more informative

with source file name and line number for Rust stack frames and

(in some cases) stack frames for Rust functions that have been inlined::

  [profile.release]

  debug = true

``py-spy`` (https://github.com/benfred/py-spy) can be used to

construct a single profile with rust functions and python functions

(as opposed to ``hg --profile``, which attributes time spent in rust

to some unlucky python code running shortly after the rust code, and

as opposed to tools for native code like ``perf``, which attribute

time to the python interpreter instead of python functions).

Example usage::

  $ make PURE=--rust local # Don't forget to recompile after a code change

  $ py-spy record --native --output /tmp/profile.svg -- ./hg ...

Developing Rust

===============

Minimum Supported Rust Version

------------------------------

The minimum supported rust version (MSRV) is specified in the `Clippy`_

configuration file at ``rust/clippy.toml``. It is set to be ``1.61.0`` as of

this writing, but keep in mind that the authoritative value is the one

from the configuration file.

We bump it from time to time, with the general rule being that our

MSRV should not be greater that the version of the Rust toolchain

shipping with Debian testing, so that the Rust enhanced Mercurial can

be eventually packaged in Debian.

To ensure that you are not depending on features introduced in later

versions, you can issue ``rustup override set x.y.z`` at the root of

the repository.

Build and development

---------------------

Go to the ``hg-cpython`` folder::

  $ cd rust/hg-cpython

Or, only the ``hg-core`` folder. Be careful not to break compatibility::

  $ cd rust/hg-core

Simply run::

   $ cargo build --release

It is possible to build without ``--release``, but it is not

recommended if performance is of any interest: there can be an order

of magnitude of degradation when removing ``--release``.

For faster builds, you may want to skip code generation::

  $ cargo check

For even faster typing::

  $ cargo c

You can run only the rust-specific tests (as opposed to tests of

mercurial as a whole) with::

  $ cargo test --all

Formatting the code

-------------------

We use ``rustfmt`` to keep the code formatted at all times. For now, we are

using the nightly version because it has been stable enough and provides

comment folding.

Our CI enforces that the code does not need reformatting. Before

submitting your changes, please format the entire Rust workspace by running::

  $ cargo +nightly fmt

This requires you to have the nightly toolchain installed.

Linting: code sanity

--------------------

We're using `Clippy`_, the standard code diagnosis tool of the Rust

community.

Our CI enforces that the code is free of Clippy warnings, so you might

want to run it on your side before submitting your changes. Simply do::

  % cargo clippy

from the top of the Rust workspace. Clippy is part of the default

``rustup`` install, so it should work right away. In case it would

not, you can install it with ``rustup component add``.

.. _Clippy: https://doc.rust-lang.org/stable/clippy/