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 experiment for starting hg in rust rather than in python,
by linking with the python runtime. Probably meant to be replaced by PyOxidizer at some point.
- 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) checking "re2" regexp engine Rust bindings (installed)
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.
Using the full hg status extension
The code for hg status needs to conform to .hgignore rules, which are all translated into regex. For compatibility and ease of development reasons the Re2 regex engine is in use until we figure out if the regex crate has similar enough behavior. This implies that you need to install Re2 following Google's guidelines: https://github.com/google/re2/wiki/Install
Then, use HG_RUST_FEATURES=with-re2 when building hg to use the full status code.
Developing Rust
The current version of Rust in use is 1.34.2, because it's what Debian stable has. You can use rustup override set 1.34.2 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.
Additional features
As mentioned in the section about hg status, code paths using re2 are opt-in.
For example:
$ cargo check --features with-re2