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+.. _rust:
+
+==============================
+Including Rust Code in Firefox
+==============================
+
+This page explains how to add, build, link, and vendor Rust crates.
+
+The `code documentation <../../writing-rust-code>`_ explains how to write and
+work with Rust code in Firefox. The
+`test documentation <../../testing-rust-code>`_ explains how to test and debug
+Rust code in Firefox.
+
+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://searchfox.org/mozilla-central/source/toolkit/library/rust/shared/Cargo.toml>`_.
+You must also add an ``extern crate`` reference to
+`toolkit/library/rust/shared/lib.rs <https://searchfox.org/mozilla-central/source/toolkit/library/rust/shared/lib.rs>`_.
+This ensures that the Rust code will be linked properly into libxul as well
+as the copy of libxul used for gtests. (Even though Rust 2018 mostly doesn't
+require ``extern crate`` declarations, these ones are necessary because the
+gkrust setup is non-typical.)
+
+After adding your crate, execute ``cargo update -p gkrust-shared`` to update
+the ``Cargo.lock`` file. You will also need to do this any time you change the
+dependencies in a ``Cargo.toml`` file. If you don't, you will get a build error
+saying **"error: the lock file /home/njn/moz/mc3/Cargo.lock needs to be updated
+but --frozen was passed to prevent this"**.
+
+By default, all Cargo packages in the mozilla-central repository are part of
+the same
+`workspace <https://searchfox.org/mozilla-central/source/toolkit/library/rust/shared/lib.rs>`_
+and will share the ``Cargo.lock`` file and ``target`` directory in the root of
+the repository.  You can change this behavior by adding a path to the
+``exclude`` list in the top-level ``Cargo.toml`` file.  You may want to do
+this if your package's development workflow includes dev-dependencies that
+aren't needed by general Firefox developers or test infrastructure.
+
+The actual build mechanism is as follows. The build system generates a special
+'Rust unified library' crate, compiles that to a static library
+(``libgkrust.a``), and links that into libxul, so all public symbols will be
+available to C++ code. Building a static library that is linked into a dynamic
+library is easier than building dynamic libraries directly, and it also avoids
+some subtle issues around how mozalloc works that make the Rust dynamic library
+path a little wonky.
+
+Linking Rust crates into something else
+=======================================
+
+To link Rust code into libraries other than libxul, 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://searchfox.org/mozilla-central/rev/3f4c3a3cabaf94958834d3a8935adfb4a887942d/toolkit/library/rust/moz.build#7>`_ and `the Cargo.toml file <https://searchfox.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 be
+    eventually 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`_.
+
+Conditional compilation
+========================
+
+Edit `tool/library/rust/gkrust-features.mozbuild
+<https://searchfox.org/mozilla-central/source/toolkit/library/rust/gkrust-features.mozbuild>`_
+to expose build flags as Cargo features.
+
+Standalone Rust programs
+========================
+
+It is also possible to build standalone Rust programs. First, put the Rust
+program (including the ``Cargo.toml`` file and the ``src`` directory) in its
+own directory, and add an empty ``moz.build`` file to the same directory.
+
+Then, if the standalone Rust program must run on the compile target (e.g.
+because it's shipped with Firefox) then add this rule to the ``moz.build``
+file:
+
+.. code-block:: python
+
+    RUST_PROGRAMS = ['prog_name']
+
+where *prog_name* is the name of the executable as specified in the
+``Cargo.toml`` (and probably also matches the name of the directory).
+
+Otherwise, if the standalone Rust program must run on the compile host (e.g.
+because it's used to build Firefox but not shipped with Firefox) then do the
+same thing, but use ``HOST_RUST_PROGRAMS`` instead of ``RUST_PROGRAMS``.
+
+Where should I put my crate?
+============================
+
+If your crate's canonical home is mozilla-central, you can put it next to the
+related code in the appropriate directory.
+
+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.
+
+Third-party crate dependencies
+==============================
+
+Third-party dependencies for in-tree Rust crates are *vendored* into the
+``third_party/rust`` directory of mozilla-central. This means that a copy of
+each third-party crate's code is committed into mozilla-central. As a result,
+building Firefox does not involve downloading any third-party crates.
+
+If you add a dependency on a new crate you must run ``mach vendor rust`` to
+vendor the dependencies into that directory. (Note that ``mach vendor rust``
+`may not work as well on Windows <https://bugzilla.mozilla.org/show_bug.cgi?id=1647582>`_
+as on other platforms.)
+
+When it comes to checking the suitability of third-party code for inclusion
+into mozilla-central, keep the following in mind.
+
+- ``mach vendor rust`` will check that the licenses of all crates are suitable.
+- You should review the crate code to some degree to check that it looks
+  reasonable (especially for unsafe code) and that it has reasonable tests.
+- Third-party crate tests aren't run, which means that large test fixtures will
+  bloat mozilla-central. Consider working with upstream to mark those test
+  fixtures with ``[package] exclude = ...`` as described
+  `here <https://doc.rust-lang.org/cargo/reference/manifest.html#the-exclude-and-include-fields>`_.
+- If you specify a dependency on a branch, pin it to a specific revision,
+  otherwise other people will get unexpected changes when they run ``./mach
+  vendor rust`` any time the branch gets updated. See `bug 1612619
+  <https://bugzil.la/1612619>`_ for a case where such a problem was fixed.
+- Other than that, there is no formal sign-off procedure, but one may be added
+  in the future.
+
+Note that all dependencies will be vendored, even ones that aren't used due to
+disabled features. It's possible that multiple versions of a crate will end up
+vendored into mozilla-central.
+
+Patching third-party crates
+===========================
+
+Sometimes you might want to temporarily patch a third-party crate, for local
+builds or for a try push.
+
+To do this, first add an entry to the ``[patch.crates-io]`` section of the
+top-level ``Cargo.toml`` that points to the crate within ``third_party``. For
+example
+
+.. code-block:: toml
+
+    bitflags = { path = "third_party/rust/bitflags" }
+
+Next, run ``cargo update -p $CRATE_NAME --precise $VERSION``, where
+``$CRATE_NAME`` is the name of the patched crate, and ``$VERSION`` is its
+version number. This will update the ``Cargo.lock`` file.
+
+Then, make the local changes to the crate.
+
+Finally, make sure you don't accidentally land the changes to the crate or the
+``Cargo.lock`` file.
+
+For an example of a more complex workflow involving a third-party crate, see
+`mp4parse-rust/README.md <https://searchfox.org/mozilla-central/source/media/mp4parse-rust/README.md>`_.
+It describes the workflow for a crate that is hosted on GitHub, and for which
+changes are made via GitHub pull requests, but all pull requests must also be
+tested within mozilla-central before being merged.