rust/hgcli/README.rst

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+++ b/rust/hgcli/README.rst	Mon Jan 20 17:44:03 2020 -0500
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+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.