rust/README.rst

remotephase: fast path newheads computation in simple case (issue5964)

Changeset 88efb7d6bcb6 fixed the logic of `phases.newheads` but greatly
regressed its performance (up to many order of magnitude). The first step to
fix the regression is to exit early when there is no work to do. If there are
no heads to filter or not roots to filter them, we don't have to do any work.

This fixes the regression when talking to an all public changeset. The
performance is even better than before.

pypy, compared to an all public repo
------------------------------------

8eeed92475d5: 0.005758 seconds
88efb7d6bcb6: 0.602517 seconds (x104)
this code:    0.001508 seconds (-74% from base)

mercurial compared to an all public repo
----------------------------------------

8eeed92475d5: 0.000577 seconds
88efb7d6bcb6: 0.185316 seconds (x321)
this code:    0.000150 seconds (-74% from base)

The performance of newheads, when actual computations are required, is fixed
in the next changeset.

===================
Mercurial Rust Code
===================

This directory contains various Rust code for the Mercurial project.

The top-level ``Cargo.toml`` file defines a workspace containing
all primary Mercurial crates.

Building
========

To build the Rust components::

   $ cargo build

If you prefer a non-debug / release configuration::

   $ cargo build --release

Features
--------

The following Cargo features are available:

localdev (default)
   Produce files that work with an in-source-tree build.

   In this mode, the build finds and uses a ``python2.7`` binary from
   ``PATH``. The ``hg`` binary assumes it runs from ``rust/target/<target>hg``
   and it finds Mercurial files at ``dirname($0)/../../../``.

Build Mechanism
---------------

The produced ``hg`` binary is *bound* to a CPython installation. The
binary links against and loads a CPython library that is discovered
at build time (by a ``build.rs`` Cargo build script). The Python
standard library defined by this CPython installation is also used.

Finding the appropriate CPython installation to use is done by
the ``python27-sys`` crate's ``build.rs``. Its search order is::

1. ``PYTHON_SYS_EXECUTABLE`` environment variable.
2. ``python`` executable on ``PATH``
3. ``python2`` executable on ``PATH``
4. ``python2.7`` executable on ``PATH``

Additional verification of the found Python will be performed by our
``build.rs`` to ensure it meets Mercurial's requirements.

Details about the build-time configured Python are built into the
produced ``hg`` binary. This means that a built ``hg`` binary is only
suitable for a specific, well-defined role. These roles are controlled
by Cargo features (see above).

Running
=======

The ``hgcli`` crate produces an ``hg`` binary. You can run this binary
via ``cargo run``::

   $ cargo run --manifest-path hgcli/Cargo.toml

Or directly::

   $ target/debug/hg
   $ target/release/hg

You can also run the test harness with this binary::

   $ ./run-tests.py --with-hg ../rust/target/debug/hg

.. note::

   Integration with the test harness is still preliminary. Remember to
   ``cargo build`` after changes because the test harness doesn't yet
   automatically build Rust code.