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localrepo: refactor `.hg/requires` reading logic in separate function...
localrepo: refactor `.hg/requires` reading logic in separate function In an upcoming patch, we will be reusing this to read `.hg/store/requires`, so let's separate it in a different function before. Differential Revision: https://phab.mercurial-scm.org/D8910
Pulkit Goyal -
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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.

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).

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).

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.