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
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.
After adding your crate, execute
cargo update -p gkrust-shared
to update the
Cargo.lock file. You’ll also
need to add an
extern crate reference to
This ensures that the Rust code will be linked properly into libxul as well
as the copy of libxul used for gtests.
By default, all Cargo packages in the mozilla-central repository are part of
the same workspace
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
excludes 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.
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
Cargo.toml file for your crate, and a
moz.build file that contains:
Where crate_name matches the name from the
[package] section of your
Cargo.toml. You can refer to the moz.build file and the Cargo.toml file that are used for libxul.
You can then add
USE_LIBS += ['crate_name'] to the
that defines the binary as you would with any other library in the tree.
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
crates.io-style dependency with a version number, and let it be
vendored into the
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.
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.