Oxidation is a project to integrate Rust code in and around Firefox.

Rust support has been required on all platforms since Firefox 54, and the first major Rust components were shipped in Firefox 56 (encoding_rs) and 57 (Stylo). Moving forward, the goal of Oxidation is to make it easier and more productive to use Rust in Firefox, and correspondingly to increase the amount of Rust code in Firefox.

This page is intended to serve as the starting point for all matters relating to Rust code in Firefox: the what, the why, and the how.



The goal of this section is to provide some high-level guidelines about when Rust should be used.

In summary, Rust should be used in the following situations.

  • For new components and completely rewritten components there should be a strong bias towards using Rust, especially for code around Firefox but not within Firefox.
  • For existing components it's more complicated!

The following sections have more detail. Ultimately, choice of language for a code component is an engineering decision, with corresponding trade-offs, and is best decided by individual teams.

Rust Strengths

Rust has the following strengths.

  • Memory-safety and thread-safety. Crashes and security vulnerabilities are much less likely. (Roughly 70% of critical security vulnerabilities are due to memory safety bugs.)
  • High performance. In particular, the safety enables code that is designed more actively for performance, especially parallel performance.
  • Nimbleness. The safety enables significant changes to existing code to be made quickly and with confidence.
  • Expressiveness. It is powerful and pleasant to use, particularly once a moderate level of experience has been reached.
  • Excellent package management and an extensive ecosystem.
  • Excellent compiler error messages.
  • Excellent documentation.
  • A friendly and helpful community.

Rust Weaknesses

One major issue with Rust relates to personnel.

  • There is a wide variety of experience levels within Mozilla, for both coding and reviewing.
  • Rust's learning curve is steep at the start, which can be intimidating. (The new borrow checker released in Rust 1.31 helped greatly with this.)

There are also technical challenges.

See "Blockers and obstacles" below for more details about work being done to remedy these weaknesses.


Therefore, Rust is most suitable in the following situations.

  • For components that are relatively standalone, with small and simple APIs.
    • This minimizes the C++/Rust boundary layer issues.
    • Infrastructure tools that are standalone programs are ideal.
    • Note that it's good software engineering practice to write loosely-coupled components anyway.
  • For components that process untrusted input, e.g. parsers.
  • For components where parallelism can provide big performance wins.
  • For components where Servo has demonstrated success.

In terms of where to keep Rust crates, there are three options.

  • Put the crate in mozilla-central or in Servo's repository.
    • For binding code, the decision to put it into Gecko or Servo can be difficult. The best choice depends on the details of the binding code in question.
  • Put the crate on crates.io and use Cargo to access it at build-time.
    • This is only suitable for highly general-purpose crates, such as smallvec.
  • Put the crate somewhere else (e.g. a separate GitHub repository), and regularly vendor it into mozilla-central.
    • This makes sense for pre-existing third-party crates that we choose to import.
    • Otherwise, this option is not recommended, because vendoring is something of a hassle.

In general, erring on the side of tighter coupling is advisable. For example, the heapsize crate used in memory reporting was moved to crates.io, and then other crates came to depend on it. Later on it needed major API changes, and we ended up replacing it with a new crate called malloc_size_of (stored in Servo's repository) because that was easier than modifying heapsize.

Documentation and assistance

Rust in general

Rust in Firefox


Rust in Firefox docs

Getting extra help

  • The Rust channel on Mozilla's Matrix network contains lots of people who know about both Rust and Gecko.
  • Are you new to Rust and not sure if your Rust code could be improved? The following people can review Rust patches for Firefox from an "is this good Rust code?" point of view.
    • Alexis Beingessner (:Gankra)
    • Josh Bowman-Matthews (:jdm)
    • Emilio Cobos Alvarez (:emilio)
    • Manish Goregaokar (:manishearth)
    • Nika Layzell (:nika)
    • Cameron McCormack (:heycam)

Rust Components

Within Firefox



  • MP4 metadata parser: bug 1161350 (shipped for desktop in Firefox 48)
    • Why Rust? Parses untrusted input, replaces libstagefright, a 3rd-party library with a history of security vulnerabilities.
  • Replace uconv with encoding-rs: bug 1261841 (shipped in Firefox 56)
  • CSS style calculation (from Servo): bug stylo (shipped for desktop in Firefox 57)
    • Why Rust? Code taken from Servo, uses parallel algorithms.
  • U2F HID backend: bug 1388843 (shipped in Firefox 57)
  • libcubeb Audio backend for Linux (PulseAudio): bug 1346665 (shipped in Firefox 59)
  • libcubeb Audio backend for macOS (CoreAudio): bug 1530713 (shipped in Firefox 74)
    • Why Rust? Safer implementation avoids data racing by leveraging lifetime, variable mutability check and better mutex pattern.
  • XPIDL binding generator (bug 1293362) (shipped in Firefox 60)
  • New prefs parser: bug 1423840 (shipped in Firefox 60)
    • Why Rust? Old parser needed replacing. Well-separated component, simple interface, parses untrusted input.
  • Audio remoting for Linux: bug 1434156 (shipped in Firefox 60)
  • WebRender: bug webrender (shipped in Firefox 67, enabled for users with appropriate hardware)
    • Why Rust? Code taken from Servo, has high performance; Rust's memory and thread safety assist productivity, and allow more aggressive optimizations.
  • kvstore (key-value storage backed by LMDB): bug 1490496 (shipped in Firefox 67)
    • Why Rust? The rkv crate provides a safe, ergonomic wrapper around LMDB, our choice for simple key-value storage in Firefox. kvstore wraps rkv in an asynchronous XPCOM API for JS and C++ callers.
  • Profiler symbolication: bug 1509549 (shipped in Firefox 67)
    • Why Rust? Makes use of existing crates that handle object file parsing and symbol iteration. Easy to compile to WebAssembly.
  • XUL store, backed by rkv: bug 1460811 (landed in Firefox 68, used in Nightly only)
  • TLS certificate store, backed by rkv: bug 1429796 (shipped in Firefox 68)
  • Synced bookmark merger: bug 1482608 (shipped in Firefox 68, on by default in Nightly and early Beta)
  • Windows BITS interface: bug 1520321 (shipped in Firefox 68)
  • Japanese encoding detector: bug 1543077 (shipped in Firefox 69)
    • Why Rust? Builds upon encoding_rs, has tiny FFI surface, subject matter prone to accesses past the bounds of a buffer.
  • audio_thread_priority bug 1429847 (shipped in Firefox 69), allow promoting threads to a real-time scheduling class, on Windows/Linux/macOS.
    • Why Rust? This crate is being used by C++ code and by Rust code (audioipc), Rust is nicer to write than C++ (especially for what is essentially just a series of system calls, the error checking style is nice), and cbindgen made it trivial to expose a C ABI.
  • Dogear a bookmark merger for Sync. Shipped pref'd-off in bug 1482608 (Firefox 68), enabled by default in bug 1588005 (shipped in Firefox 72)
    • Why Rust? A single performant and safe implementation shared between desktop and the bookmarks engine in application-services
  • Unicode Language Identifier: bug 1571915 (shipped in Firefox 72)
    • Why Rust? Much faster, parser-heavy, easier to handle low-memory footprint thanks to `tinystr`.
  • Language Negotiation: bug 1581960 (shipped in Firefox 72)
    • Why Rust? Ties into `unic-langid`, easier to handle list filtering and ordering.
  • Replace libhyphen with mapped_hyph, bug 1590167 (shipped in Firefox 72).
  • Encoding detector: bug 1551276 (shipped in Firefox 73)
    • Why Rust? Builds upon encoding_rs, has tiny FFI surface, subject matter prone to accesses past the bounds of a buffer, potentially parallelizable with Rayon.
  • Integrate fluent-rs, a localization system: bug 1560038 (shipped in Firefox 76)
    • Why Rust? Performance and memory wins are substantial over previous JS implementation. It brings zero-copy parsing, and memory savvy resolving of localization strings. It also paves the way for migrating the rest of the Fluent APIs away from JS which is required for Fission.
  • qcms [1] ported from C to Rust
    • Why Rust? Memory safety

In progress

  • Port Localization API to Rust: bug 1613705
  • neqo A QUIC implentation.
  • cranelift, a low-level retargetable code generator: bug 1469027
    • Why Rust? It's a new, well-separated component with a clear interface. Also, Rust is a great language for writing compilers, due to algebraic data types and pattern matching.
  • SmooshMonkey A Rust front-end for SpiderMonkey (featuring jsparagus).
    • Why Rust? Parses untrusted input. Also, Rust is a great language for writing compilers, due to algebraic data types and pattern matching.
  • Audio remoting for Windows: bug 1432303
  • Audio remoting for Mac OS: bug 1425788
  • SDP parsing in WebRTC: bug 1365792
    • Why Rust? SDP is a complex text protocol and the existing parser in C has a history of security issues. This also allows us to tailor the SDP parser specifically for the subset used in WebRTC, further reducing its surface area. It is currently run in parallel with the C parser in Nightly.
  • Linebreaking with xi-unicode: bug 1290022 (last update late 2016)
  • Background Update Agent for Windows: bug 1343669
  • wgpu, a WebGPU API implementation: bug webgpu-mvp (in Nightly since 72)
    • Why Rust? Complex tracking logic, wide attack area. Also, leverages Rust ecosystem for building libraries on top of our native implementation and the API that will target the Web.
  • Integrate the Glean SDK, a data collection library.


  • WebMIDI: bug 1201593, bug 1201596, bug 1201598
  • Gamepad code: bug 1286699
  • Replace the telemetry module(?)
    • Why Rust? The existing C++ code has a history of threading problems.
  • Replace DOM serializers (XML, HTML for Save As.., plain text)
    • Why Rust? Need a rewrite anyway. Minor history of security vulnerabilities.
  • Expose Rust API to JS Debugger: bug 1263317
  • Generate Rust bindings for IPDL actors (bug 1379739)
  • WebM demuxer: bug 1267492
  • Parallel JS parsing: fast preparse to find function boundaries, parse non-overlapping functions in parallel with a unification step to handle free names and such (no bug on file yet)
    • Why Rust? Parses untrusted input. Requires safe threading. And generally, Rust is a better language than C++ for parsers, due to strong typing, algebraic data types, and pattern matching.
  • Crash reporter
    • Why Rust? Code needs rewriting, useful Rust crates exist that could be used.
  • Sync/FxA components
    • Why Rust? Single safe and performant implementation which is shared across all our products.
  • libcubeb Audio backend for Windows (WASAPI)
  • Replace the XML parser, possibly via c2rust: bug 1611289
    • Why Rust? Parses untrusted input, replaces expat, a 3rd-party library with a history of frequent security vulnerabilities.
  • Rewrite the ICE stack used by WebRTC; bug 1616966
    • Why Rust? Works with network data.

Outside Firefox


  • Testing
    • GeckoDriver, a WebDriver implementation for Firefox integrated via marionette protocol: bug 1340637 (standalone releases)
    • grcov, a tool to collect and aggregate code coverage data for multiple source files, used in Firefox CI.
  • Build system, etc.
    • sccache, compiler cache with s3 storage. Caching C++ and Rust compilation, used in Firefox CI.
    • Parts of mozsearch, the backend for the Searchfox code indexing tool.
    • makecab, a reimplementation of Microsoft's makecab tool. Used to compress PDB files before uploading to symbol server in Firefox CI.
    • dump_syms, a reimplementation of Google Breakpad's dump_syms tool for Windows. Used to parse PDB files and print out Breakpad-compatible symbol files.
      • Why Rust? Doesn't rely on Microsoft closed-source libraries anymore, can be cross-compiled and run on a non-Windows host, is an order of magnitude faster, takes less than a third of the memory, produces better symbols and relies on an active and friendly upstream symbolic
  • Application Services, server-side
    • autopush-rs Rust async based websocket server that implements Mozilla's push/webpush/broadcast protocols.
      • Why Rust? Concise code with the memory efficiency of C.
    • Megaphone, a real-time update broadcast server for Firefox.
    • fxa_email_service, a service for sending email to Firefox Accounts.
    • pairsona, a tool to associate instances of firefox.
  • Application Services, client-side
  • fix-stacks, a stack frame symbolizer: bug 1596292
    • Why Rust? High performance needed, a single implementation can replace multiple platform-specific scripts, and we can use the symbolic crate to do all the hard parts.

In Progress

  • IPDL Parser: bug 1316754
    • Why Rust? Rust is a much better language than Python for writing compilers, due to strong typing, algebraic data types, and pattern matching.

Blockers and obstacles

This section lists areas where Rust integration could be improved.