rust/README.rst

hgweb: always use "?" when writing session vars

This code resolves a string to insert in URLs as part of a
query string. Essentially, it resolves the {sessionvars}
template keyword, which is used by hgweb templates to build
a URL as a string.

The whole approach here feels wrong because there's no way of
knowing when this code runs how the final URL will look. There
could be additional URL fragments added before this template
keyword that add a query string component.

Furthermore, I don't think there's *any* for req.url to have
a query string. That's because the code that populates this
variable only takes SCRIPT_NAME and REPO_NAME into account. The
"?" character it is searching for would only be added if some
code attempted to add QUERY_STRING to the URL. Hacking the code
up to raise if "?" is present in the URL yields a clean test
suite run. I'm not sure if we broke this code or if it has
always been broken.

Anyway, this commit removes support for emitting "&" as the
first character in {sessionvars} and makes it always emit "?",
which is what it was always doing before AFAICT.

Differential Revision: https://phab.mercurial-scm.org/D2733

===================
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.