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 three independent rust projects: - chg. An implementation of chg, in rust instead of C. - hgcli. A project that provide 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.
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
You might want to check the features section in hg-cpython/Cargo.toml. It may contain features that might be interesting to try out.
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
The current version of Rust in use is 1.41.1, because it's what Debian stable has. You can use rustup override set 1.41.1 at the root of the repo to make it easier on you.
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.
To format the entire Rust workspace:
$ cargo +nightly fmt
This requires you to have the nightly toolchain installed.