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

The current version of Rust in use is 1.48.0, because it's what Debian stable has. You can use rustup override set 1.48.0 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

$ cargo +nightly fmt