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 1.79.0 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.