bug 1313448 - Add documentation for adding Rust code to the build system. r=froydnj

MozReview-Commit-ID: IpblCfAKwrL

--HG--
extra : rebase_source : cc9ccadc77da20c4043a26bec297d9764a807015

build/docs/index.rst

@@ -26,6 +26,7 @@ Important Concepts
    defining-binaries
    toolchains
    locales
+   rust
 
 integrated development environment (IDE)
 ========================================

build/docs/rust.rst

@@ -0,0 +1,78 @@
+.. _rust:
+
+==============================
+Including Rust Code in Firefox
+==============================
+
+The build system has support for building and linking Rust crates.
+Rust code is built using ``cargo`` in the typical way, so it is
+straightforward to take an existing Rust crate and integrate it
+into Firefox.
+
+.. important::
+
+   Rust code is not currently enabled by default in Firefox builds.
+   This should change soon (`bug 1283898 <https://bugzilla.mozilla.org/show_bug.cgi?id=1283898>`_),
+   but the option to build without Rust code will likely last a little longer
+   (`bug 1284816 <https://bugzilla.mozilla.org/show_bug.cgi?id=1284816>`_),
+   so Rust code cannot currently be used for required components.
+
+
+Linking Rust Crates into libxul
+===============================
+
+Rust crates that you want to link into libxul should be listed in the
+``dependencies`` section of `toolkit/library/rust/shared/Cargo.toml <https://dxr.mozilla.org/mozilla-central/source/toolkit/library/rust/shared/Cargo.toml>`_.
+This ensures that the Rust code will be linked properly into libxul as well
+as the copy of libxul used for gtests.
+
+Linking Rust Crates into something else
+=======================================
+
+There currently is not any Rust code being linked into binaries other than
+libxul. If you would like to do so, you'll need to create a directory with
+a ``Cargo.toml`` file for your crate, and a ``moz.build`` file that contains:
+
+.. code-block:: python
+
+    RustLibrary('crate_name')
+
+Where *crate_name* matches the name from the ``[package]`` section of your
+``Cargo.toml``. You can refer to `the moz.build file <https://dxr.mozilla.org/mozilla-central/rev/3f4c3a3cabaf94958834d3a8935adfb4a887942d/toolkit/library/rust/moz.build#7>`_ and `the Cargo.toml file <https://dxr.mozilla.org/mozilla-central/rev/3f4c3a3cabaf94958834d3a8935adfb4a887942d/toolkit/library/rust/Cargo.toml>`_ that are used for libxul.
+
+You can then add ``USE_LIBS += ['crate_name']`` to the ``moz.build`` file
+that defines the binary as you would with any other library in the tree.
+
+.. important::
+
+    You cannot link a Rust crate into an intermediate library that will wind
+    up being linked into libxul. The build system enforces that only a single
+    ``RustLibrary`` may be linked into a binary. If you need to do this, you
+    will have to add a ``RustLibrary`` to link to any standalone binaries that
+    link the intermediate library, and also add the Rust crate to the libxul
+    dependencies as in `linking Rust Crates into libxul`_.
+
+Where Should I put my Crate?
+============================
+
+If your crate's canonical home is mozilla-central, you can put it next to the
+other code in the module it belongs to.
+
+If your crate is mirrored into mozilla-central from another repository, and
+will not be actively developed in mozilla-central, you can simply list it
+as a ``crates.io``-style dependency with a version number, and let it be
+vendored into the ``third_party/rust`` directory.
+
+If your crate is mirrored into mozilla-central from another repository, but
+will be actively developed in both locations, you should send mail to the
+dev-builds mailing list to start a discussion on how to meet your needs.
+
+
+Crate dependencies
+==================
+
+All dependencies for in-tree Rust crates are vendored into the
+``third_party/rust`` directory. Currently if you add a dependency on a new
+crate you must run ``mach vendor rust`` to vendor the dependencies into
+that directory. In the future we hope to make it so that you only need to
+vendor the dependencies in order to build your changes in a CI push.