This repository contains Rust bindings for SpiderMonkey that are battle-tested in Servo, split in two crates:
mozjs-sys
: SpiderMonkey and low-level Rust bindings to its C++ API.mozjs
: Higher-level bindings to the SpiderMonkey API.Mozjs is currently tracking SpiderMonkey on ESR-128 branch (currently version 128.0).
SpiderMonkey is very large and can take a long time to compile. If building with default
features, mozjs
provides a pre-built archive which can be linked against. You can also create
your own archive and link to it. mozjs
currently offers two environment variables to enable
this feature:
MOZJS_CREATE_ARCHIVE=1
will create a SpiderMonkey binary archive for release usage. It will
be created in the target
directory.MOZJS_ARCHIVE
can be used to build against a pre-built archive. Using this flag, compiling
SpiderMonkey and the bindgen wrappers is unnecessary. There are two ways to use it:
MOZJS_ARCHIVE=path/to/libmozjs.tar.gz
: This option will look for the archive at the local
path, extract it, and then link against the static libraries included in the archive.MOZJS_ARCHIVE=https://url/to/release/page
: This option will download the archive from
the provided base URL, extract it, and then link against the static libraries included in the
archive. The base URL should be similar to https://github.com/servo/mozjs/releases
.
The build script will append the version and target accordingly. See the files at the example
URL for more details.If MOZJS_FROM_SOURCE=1
or MOZJS_CREATE_ARCHIVE
are enabled or linking against a
pre-built archive fails, mozjs
will build SpiderMonkey from source.
Install Python, Clang and build-essential
, for example on a Debian-based Linux:
sudo apt-get install build-essential python3 python3-distutils llvm libclang-dev clang curl
If you have more than one version of Clang installed, you can set the LIBCLANG_PATH
environment variable, for example:
export LIBCLANG_PATH=/usr/lib/clang/4.0/lib
Download and unzip MozTools 4.0.
Download and install Clang (LLVM version 14 or greater) for Windows (64 bit) from https://releases.llvm.org/download.html.
Download and install Visual Studio 2019
or Visual Studio 2022
with the C++ desktop development
component and the following features:
To install these dependencies from the command line, you can download vs_buildtools.exe and run the following command:
vs_BuildTools.exe^
--add Microsoft.Component.MSBuild^
--add Microsoft.VisualStudio.Component.CoreBuildTools^
--add Microsoft.VisualStudio.Workload.MSBuildTools^
--add Microsoft.VisualStudio.Component.Windows11SDK^
--add Microsoft.VisualStudio.Component.VC.CoreBuildTools^
--add Microsoft.VisualStudio.Component.VC.Tools.x86.x64^
--add Microsoft.VisualStudio.Component.VC.Redist.14.Latest^
--add Microsoft.VisualStudio.Component.VC.ATL^
--add Microsoft.VisualStudio.Component.VC.ATLMFC^
--add Microsoft.VisualStudio.Component.VC.CoreIde^
--add Microsoft.VisualStudio.ComponentGroup.NativeDesktop.Core^
--add Microsoft.VisualStudio.Workload.VCTools
Install Python 3.11.
PATH
PYTHON
and PYTHON3
environment variable point to the Python binary (ie C:\Python311\python.exe
Set the following environment variables according to where you installed the dependencies above:
$env:LIBCLANG_PATH="C:\Program Files\LLVM\lib"
$env:MOZTOOLS_PATH="C:\path\to\moztools-4.0"
$env:CC="clang-cl"
$env:CXX="clang-cl"
$env:LD="lld-link"
You can now build and test the crate using cargo:
cargo build
cargo test
cargo build --features debugmozjs
cargo test --features debugmozjs
Mozjs is currently not published to crates.io, but it can be used from git (binaries should use lockfile instead of rev
):
mozjs = { git = "https://github.com/servo/mozjs", rev = "latest-commit-hash" }
Assuming your local servo
and mozjs
directories are siblings, you can build servo
against mozjs
by adding the following to servo/Cargo.toml
:
[patch."https://github.com/servo/mozjs"]
mozjs = { path = "../mozjs/mozjs" }
In order to upgrade to a new version of SpiderMonkey:
Find the mozilla-esr128 commit for the desired version of SpiderMonkey, at https://treeherder.mozilla.org/#/jobs?repo=mozilla-esr128&filter-searchStr=spidermonkey%20pkg. You are looking for an SM(pkg) tagged with FIREFOX_RELEASE. Take a note of the commit number to the left (a hex number such as ac4fbb7aaca0).
Click on the SM(pkg) link, which will open a panel with details of the commit, including an artefact uploaded link, with a name of the form mozjs-version.tar.xz. Download it and save it locally.
Look at the patches in mozjs-sys/etc/patches/*.patch
, and remove any that no longer apply
(with a bit of luck this will be all of them).
Run python3 ./mozjs-sys/etc/update.py path/to/tarball
.
Update mozjs-sys/etc/COMMIT
with the commit number and mozjs-sys version with SpiderMonkey version.
Run ./mozjs/src/generate_wrappers.sh
to regenerate wrappers.
Build and test the bindings as above, then submit a PR!
Send companion PR to servo, as SpiderMonkey bump PR will not be merged until it's tested against servo.
To get a dev environment with shell.nix:
nix-shell
To configure rust-analyzer in Visual Studio Code:
{
"rust-analyzer.check.overrideCommand": ["nix-shell", "--run", "cargo check --message-format=json"],
"rust-analyzer.cargo.buildScripts.overrideCommand": ["nix-shell", "--run", "cargo check --message-format=json"],
"rust-analyzer.rustfmt.overrideCommand": ["nix-shell", "--run", "cargo fmt"],
}
If you are working on the Rust code only, rust-analyzer should work perfectly out of the box, though NixOS users will need to configure rust-analyzer to wrap cargo invocations (see above).
But if you are working on the C++ code, editor support is only really possible in upstream SpiderMonkey (aka “mozilla-central”), but once you’ve set up your editor in your upstream checkout, you can work on your changes there, then import them here as needed for local testing.
This guide assumes that your code is checked out at:
NixOS users: some steps have a note in [brackets] saying they need to be wrapped in nix-shell. Those commands should be wrapped as follows:
nix-shell ~/code/nixpkgs-mozilla/release.nix -A gecko.x86_64-linux.clang --run '...'
Start by checking out mozilla-unified (Building Firefox on Linux §§ 1 and 2).
NixOS users: it’s ok if the bootstrap command fails with a NotImplementedError due to NixOS not being a supported distro.
Now create your MOZCONFIG file (Building and testing SpiderMonkey). I recommend (and this guide assumes) that the file is named debug.mozconfig
, because simple names like debug
can cause MozconfigFindException problems. The file should look like this:
# Build only the JS shell
ac_add_options --enable-project=js
# Enable the debugging tools: Assertions, debug only code etc.
ac_add_options --enable-debug
# Enable optimizations as well so that the test suite runs much faster. If
# you are having trouble using a debugger, you should disable optimization.
ac_add_options --enable-optimize
# Use a dedicated objdir for SpiderMonkey debug builds to avoid
# conflicting with Firefox build with default configuration.
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-debug-@CONFIG_GUESS@
If you are a NixOS user, clone mozilla/nixpkgs-mozilla next to your mozilla-unified
checkout, and add the following line to the start of your debug.mozconfig
:
. ./.mozconfig.nix-shell
You will need to generate your Visual Studio Code config and compilation database against central at least once, before you can do so against the commit we forked from (mozjs/etc/COMMIT
).
~/code/mozilla-unified $ MOZCONFIG=debug.mozconfig ./mach ide vscode
[NixOS users: wrap the command above in nix-shell]
Otherwise you might get an error with lots of exclamation marks:
~/code/mozilla-unified $ MOZCONFIG=debug.mozconfig ./mach ide vscode
[snip]
0:05.80 Unable to locate clangd in /home/delan/code/mozilla-unified/.mozbuild/clang-tools/clang-tidy/bin.
[snip]
0:07.78 ERROR!!!!!! Could not find artifacts for a toolchain build named `linux64-clang-tidy`. Local commits, dirty/stale files, and other changes in your checkout may cause this error. Make sure you are on a fresh, current checkout of mozilla-central. Beware that commands like `mach bootstrap` and `mach artifact` are unlikely to work on any versions of the code besides recent revisions of mozilla-central.
[NixOS users: wrap the command above in nix-shell]
Now switch to the commit we forked from, and generate them again.
~/code/mozilla-unified $ hg update -r $(cat ../mozjs/mozjs/etc/COMMIT)
~/code/mozilla-unified $ MOZCONFIG=debug.mozconfig ./mach ide vscode
[NixOS users: wrap the mach command above in nix-shell]
At this point, you should be able to open Visual Studio Code, install the clangd extension, and open a file like js/src/vm/ArrayBufferObject.cpp
without seeing any problems in the margin.
If there are no problems in the margin, and you can Go To Definition, you’re done!
If you are a NixOS user and see this in the clangd output panel:
[Error - 7:38:13 pm] Clang Language Server client: couldn't create connection to server.
Launching server using command /home/delan/code/mozilla-unified/.mozbuild/clang-tools/clang-tidy/bin/clangd failed. Error: spawn /home/delan/code/mozilla-unified/.mozbuild/clang-tools/clang-tidy/bin/clangd ENOENT
Then you need to replace the mozbuild toolchain’s clangd with one that has been patchelf’d:
~/code/mozilla-unified $ ln -sf ~/.nix-profile/bin/clangd .mozbuild/clang-tools/clang-tidy/bin/clangd
If you see this in the clangd output panel:
I[19:20:28.001] Indexed /home/delan/code/mozilla-unified/js/src/jit/LIR.cpp (61040 symbols, 244267 refs, 738 files)
I[19:20:28.001] Failed to compile /home/delan/code/mozilla-unified/js/src/jit/LIR.cpp, index may be incomplete
I[19:20:28.087] --> $/progress
Then the commands in your compilation database might be incorrect. You can try running one of the commands in a terminal to see what happens:
~/code/mozilla-unified $ ( f=$PWD/obj-debug-x86_64-pc-linux-gnu/clangd/compile_commands.json; set -x; cd $(< $f jq -r '.[0].directory'); $(< $f jq -r '.[0].command') )
+/run/current-system/sw/bin/zsh:252> jq -r '.[0].directory'
+/run/current-system/sw/bin/zsh:252> cd /home/delan/code/mozilla-unified/obj-debug-x86_64-pc-linux-gnu/js/src
+/run/current-system/sw/bin/zsh:252> jq -r '.[0].command'
+/run/current-system/sw/bin/zsh:252> /nix/store/dkw46jgi8i0bq64cag95v4ywz6g9bnga-gcc-wrapper-11.3.0/bin/cc '-std=gnu99' -o /dev/null -c -I/home/delan/code/mozilla-unified/obj-debug-x86_64-pc-linux-gnu/dist/system_wrappers -include /home/delan/code/mozilla-unified/config/gcc_hidden.h -U_FORTIFY_SOURCE '-D_FORTIFY_SOURCE=2' -fstack-protector-strong '-DDEBUG=1' -DWASM_SUPPORTS_HUGE_MEMORY -DJS_CACHEIR_SPEW -DJS_STRUCTURED_SPEW -DEXPORT_JS_API -DMOZ_HAS_MOZGLUE -I/home/delan/code/mozilla-unified/js/src -I/home/delan/code/mozilla-unified/obj-debug-x86_64-pc-linux-gnu/js/src -I/home/delan/code/mozilla-unified/obj-debug-x86_64-pc-linux-gnu/dist/include -I/nix/store/bsslcbcnrfcv6nl0jfha444nxjky7zxa-zlib-1.2.13-dev/include -include /home/delan/code/mozilla-unified/obj-debug-x86_64-pc-linux-gnu/js/src/js-confdefs.h -DMOZILLA_CLIENT -fPIC -fno-math-errno -pthread -pipe -gdwarf-4 -freorder-blocks -O3 -fno-omit-frame-pointer -funwind-tables -Wall -Wempty-body -Wignored-qualifiers -Wpointer-arith -Wsign-compare -Wtype-limits -Wunreachable-code -Wduplicated-cond -Wlogical-op '-Wno-error=maybe-uninitialized' '-Wno-error=deprecated-declarations' '-Wno-error=array-bounds' '-Wno-error=free-nonheap-object' -Wno-multistatement-macros '-Wno-error=class-memaccess' -Wformat '-Wformat-overflow=2' '-Werror=implicit-function-declaration' -Wno-psabi -fno-strict-aliasing '-ffp-contract=off' '-ferror-limit=0' /home/delan/code/mozilla-unified/js/src/vtune/ittnotify_static.c -Wno-varargs -Wno-sign-compare -Wno-unknown-pragmas -Wno-stringop-overflow -Wno-stringop-truncation
gcc: error: unrecognized command-line option ‘-ferror-limit=0’
In this case, it was because your compiler was gcc (which supports -fmax-errors
but not -ferror-limit
), but it should always be clang (which supports both) when working with clangd. If you are a NixOS user, make sure you use the clang
derivation, not the gcc
derivation, when generating your compilation database:
~/code/mozilla-unified $ nix-shell ~/code/nixpkgs-mozilla/release.nix -A gecko.x86_64-linux.clang --run 'MOZCONFIG=debug.mozconfig ./mach ide vscode'
^^^^^
Start by making a source tarball from your local upstream SpiderMonkey checkout. [TODO(@delan) the default xz compression is very slow here, we should add an option upstream to make it faster]
~/code/mozilla-unified $ AUTOMATION=1 DIST=$PWD/../mozjs/mozjs/etc js/src/make-source-package.py
[NixOS users: wrap the command above in nix-shell]
Now update your vendored copy of SpiderMonkey from that tarball. This creates a commit replacing mozjs/mozjs
with the unpatched contents of the tarball, leaving the changes made by reapplying our patches in your working directory diff (git diff
).
~/code/mozjs $ python3 mozjs/etc/update.py mozjs/etc/mozjs-107.0.0.tar.xz
Then do a (mixed) reset to remove the commit and unstage its changes.
~/code/mozjs $ git reset @~
Your working directory diff (git diff
) should now contain (and only contain) the changes you’ve made to your upstream SpiderMonkey checkout. If you see changes to mozjs/mozjs/js/src/old-configure
[TODO(@delan) why does this happen?], you may need to undo them:
~/code/mozjs $ git restore -W mozjs/mozjs/js/src/old-configure
Otherwise you might get the build failure below:
~/code/mozjs $ cargo build
[snip]
configure: error: can not find sources in /home/delan/code/mozjs/mozjs/mozjs/js/src or ..
--- stderr
WARNING: The value of LD is not used by this build system.
ERROR: old-configure failed
make: *** [/home/delan/code/mozjs/mozjs/makefile.cargo:195: maybe-configure] Error 1
thread 'main' panicked at 'assertion failed: result.success()', mozjs/build.rs:178:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Now you can build the Rust crates against your modified version of SpiderMonkey!
~/code/mozjs $ cargo build