15 Apr 2026

feedPlanet Mozilla

Mozilla Open Policy & Advocacy Blog: Mozilla Urges the FTC to Tackle Harmful Design Practices

In response to concerns from both consumers and the industry, the US Federal Trade Commission (FTC) invited public comment on whether it should amend the current Rule Concerning the Use of Prenotification Negative Option Plans to address deceptive or unfair negative option practices.

Negative option marketing is a practice in which a seller treats a consumer's silence or failure to take action as consent to be charged for goods or services. This technique is often used in subscription services, where users may be guided toward accepting recurring charges through default selections or obscure disclosures. These design practices, also known as "dark patterns," successfully manipulate and influence user behavior on a systematic level and are often employed in all aspects of digital markets, not just with subscriptions.

As a browser developer, Mozilla is well-acquainted with the negative impacts of manipulative design. The web browser market provides a documented case study illustrating how operating systems deploy deceptive design practices to weaponize friction and status-quo bias to influence consumer behavior. As such, Mozilla was eager to provide feedback and encourage the Commission to examine the breadth of deceptive design practices that undermine choice.

Dark patterns are a byproduct of power asymmetry between companies and consumers. If we don't protect meaningful choice and effective competition now, we risk giving even more control to the biggest players - and losing what makes the web open and innovative in the first place.

The FTC has a critical opportunity, both in this rulemaking and more broadly, to modernize consumer protection for the realities of digital markets. We encourage the FTC to:

We welcome the opportunity to share our relevant experiences in the browser space and look forward to continuing the conversation.

Read our full comments to the FTC for more details on our recommendations.

The post Mozilla Urges the FTC to Tackle Harmful Design Practices appeared first on Open Policy & Advocacy.

15 Apr 2026 4:29pm GMT

Firefox Tooling Announcements: MozPhab 2.13.0 Released

Bugs resolved in Moz-Phab 2.13.0:

Discuss these changes in #engineering-workflow on Slack or #Conduit Matrix.

1 post - 1 participant

Read full topic

15 Apr 2026 3:30pm GMT

14 Apr 2026

feedPlanet Mozilla

Firefox Application Security Team: Firefox Security & Privacy Newsletter 2026 Q1

Welcome to the Q1 2026 edition of the Firefox Security & Privacy Newsletter.

Security and privacy are foundational to Mozilla's manifesto and central to how we build Firefox. In this edition, we highlight key security and privacy work from Q1 2026, organized into the following areas:

Preface

Note: Some of the bugs linked below might not be accessible to the general public and restricted to specific work groups. We de-restrict fixed security bugs after a grace-period, until the majority of our user population have received Firefox updates. If a link does not work for you, please accept this as a precaution for the safety of all Firefox users.

Firefox Product Security & Privacy

Collaboration with Anthropic: A few weeks ago, Anthropic's Frontier Red Team shared the results of a new AI-assisted vulnerability detection approach. Using this method, we have identified more than a dozen confirmed security issues, each supported by reproducible test cases. Learn more in our blog: Hardening Firefox with Anthropic's Red Team. Leveraging our Firefox Security expertise, we ended up finding dozens of additional vulnerabilities that were fixed in the following Firefox updates.

YouTube coverage of Firefox at pwn2own 2025: To demonstrate Firefox's focus on user security and Mozilla's commitment to openness, we invited LiveOverflow to follow us during the prestigious hacking competition pwn2own last year. LiveOverflow's four-party documentary provides behind-the-scenes coverage of our quick response to fixing two Firefox 0-day security bugs. The videos go from preparation (part 1), to exploit analysis (part 2) and disclosure (part 3), all the way to the rapid release of a Firefox update (part 4) for the 2-day event coverage.

Trustworthy JavaScript for the Open Web: Alongside partners from Meta, Proton AG, Cloudflare, and the Freedom of the Press Foundation, we presented our plans to improve the trustworthiness of JavaScript on the Web at Real World Crypto.

SafeBrowsing: Firefox 147 shipped with SafeBrowsing v5 support, allowing to protect users against malicious URLs. And starting with v149, Firefox blocks and revokes websites permissions for sites on the SafeBrowsing lists (Bug 1986300), leveling-up the built-in protection from online threats.

Stronger XSS Protection through the Sanitizer API: Starting with v148, Firefox was the first browser to add support for the Sanitizer API, helping prevent XSS attacks on the web. Learn more in our blog post, Goodbye innerHTML, Hello setHTML: Stronger XSS Protection in Firefox 148, or tune in to the ShopTalk Show podcast, where Freddy Braun discusses the details of the Sanitizer API.

2048-bit Minimum for RSA Certificates: Firefox now enforces a minimum 2048-bit RSA key size for certificates issued by Mozilla's built-in root CAs. As publicly trusted CAs already meet this requirement, no significant impact to the broader web is expected.

Community Engagement

Bug Bounty Program Updates: As the threat landscape evolves, addressing the increasing volume of AI-assisted security bug reports, we're evolving our security program alongside it. With continued advances in browser security architecture, our bug bounty program is refining its incentives to prioritize the highest-impact research and the most critical classes of vulnerabilities while focusing on novelty. Learn more in our blogpost: Bug Bounty Program Updates 2026. We have also just updated our Bug Bounty hall of fame, to list all people who helped us find and fix security vulnerabilities in Q1 of 2026.

Web Security & Standards

Storage-Access Headers: Firefox 147 is shipping an extension of the Storage Access API to improve both web compatibility and parity with Chrome. These Storage Access headers allow web pages to opt out of storage isolation upfront and without the need to first load a document.

Going Forward

As a Firefox user, you automatically benefit from the security and privacy improvements described above through Firefox's regular automatic updates. If you're not using Firefox yet, you can download it to enjoy a fast, secure browsing experience-while supporting Mozilla's mission of a healthy, safe, and accessible web for everyone.

We'd like to thank everyone who helps make Firefox and the open web more secure and privacy-respecting.

See you next time with the Q2 2026 report.

- The Firefox Security and Privacy Teams

14 Apr 2026 11:00pm GMT

13 Apr 2026

feedPlanet Mozilla

Mozilla Data YouTube Channel: Responsible Data Collection is Good, Actually (Ubisoft Data Summit 2021)

Firefox Telemetry Engineer and Data Steward Chris H-C (:chutten) gives a talk at Ubisoft's Data Summit 2021 about how Responsible Data Collection as practised at Mozilla makes cataloguing easy, stops instrumentation mistakes before they ship, and allows you to build self-serve analysis tooling that gets everyone invested in data quality. Oh, and it's cheaper, too.

13 Apr 2026 5:07pm GMT

Spidermonkey Development Blog: Benchmark Mode in SpiderMonkey

You ever get to the end of running benchmarks, maybe a long running one, and realize… "Oh no. I forgot to set that important option, and these results are useless"

Yeah. I have. Too many times.

So I've added --benchmark-mode and --strict-benchmark-mode to SpiderMonkey.

These options configure the shell for benchmarking, taking the wisdom of the team and boiling multiple shell options down to a single --benchmark-mode flag, and in --strict-benchmark-mode will abort the run if the shell is configured in a way where effective benchmarking is unlikely to be possible (e.g. benchmarking a debug build!)

The nice thing about nailing this down is that this is something we can point anyone to and know that their shell is following the rules any of us would follow.

The general design philosophy of benchmark mode is to disable things you wouldn't see enabled in Firefox in normal configuration, as well as debugging code that maybe makes sense for test suites but doesn't make sense for a benchmark.

Hopefully this is the end of me realizing that I forgot to pass --no-async-stacks yet again.

13 Apr 2026 5:00pm GMT

Mozilla Open Policy & Advocacy Blog: Anti-hacking laws should not be used to lock up the open internet

Mozilla has joined EFF, the Alliance for Responsible Data Collection, Digital Medusa, and EleutherAI in filing an amicus brief in Amazon v. Perplexity, urging the Ninth Circuit not to stretch the Computer Fraud and Abuse Act (CFAA) far beyond its intended purpose.

We have said this before, and it remains true: laws designed to protect the security of the internet should not be used to undermine how people want to use it.

Our mission is grounded in the idea that the internet must remain open and accessible to all, and that privacy and security online are fundamental. Mozilla joined this brief because overly broad interpretations of computer crime laws can put those values at risk.

The CFAA is an anti-hacking law. It was meant to address break-ins to computer systems - not to criminalize tools that enable people to access and engage with information that is publicly available on the web. While there are no-doubt many challenging legal and policy questions around the growth and use of agentic AI tools, we believe expanding the reach of CFAA to address these issues would threaten innovation, chill the development of useful tools and services for researchers and journalists, and undermine competition online.

The post Anti-hacking laws should not be used to lock up the open internet appeared first on Open Policy & Advocacy.

13 Apr 2026 4:51pm GMT

The Servo Blog: Servo is now available on crates.io

Today the Servo team has released v0.1.0 of the servo crate. This is our first crates.io release of the servo crate that allows Servo to be used as a library.

We currently do not have any plans of publishing our demo browser servoshell to crates.io. In the 5 releases since our initial GitHub release in October 2025, our release process has matured, with the main "bottleneck" now being the human-written monthly blog post. Since we're quite excited about this release, we decided to not wait for the monthly blog post to be finished, but promise to deliver the monthly update in the coming weeks.

As you can see from the version number, this release is not a 1.0 release. In fact, we still haven't finished discussing what 1.0 means for Servo. Nevertheless, the increased version number reflects our growing confidence in Servo's embedding API and its ability to meet some users' needs.

In the meantime we also decided to offer a long-term support (LTS) version of Servo, since breaking changes in the regular monthly releases are expected and some embedders might prefer doing major upgrades on a scheduled half-yearly basis while still receiving security updates and (hopefully!) some migration guides. For more details on the LTS release, see the respective section in the Servo book.

13 Apr 2026 12:00am GMT

10 Apr 2026

feedPlanet Mozilla

Andreas Farre: How to make Firefox builds1 17% faster2

In the previous post, I mentioned that buildcache has some unique properties compared to ccache and sccache. One of them is its Lua plugin system, which lets you write custom wrappers for programs that aren't compilers in the traditional sense. With Bug 2027655 now merged, we can use this to cache Firefox's WebIDL binding code generation.

What's the WebIDL step?

When you build Firefox, one of the earlier steps runs python3 -m mozbuild.action.webidl to generate C++ binding code from hundreds of .webidl files. It produces thousands of output files: headers, cpp files, forward declarations, event implementations, and so on. The step isn't terribly slow on its own, but it runs on every clobber build, and the output is entirely deterministic given the same inputs. That makes it a perfect candidate for caching.

The problem was that the compiler cache was never passed to this step. Buildcache was only wrapping actual compiler invocations, not the Python codegen.

The change

The fix in Bug 2027655 is small. In dom/bindings/Makefile.in, we now conditionally pass $(CCACHE) as a command wrapper to the py_action call:

WEBIDL_CCACHE=
ifdef MOZ_USING_BUILDCACHE
WEBIDL_CCACHE=$(CCACHE)
endif

webidl.stub: $(codegen_dependencies)
        $(call py_action,webidl $(relativesrcdir),$(srcdir),,$(WEBIDL_CCACHE))
        @$(TOUCH) $@

The py_action macro in config/makefiles/functions.mk is what runs Python build actions. The ability to pass a command wrapper as a fourth argument was also introduced in this bug. When buildcache is configured as the compiler cache, this means the webidl action is invoked as buildcache python3 -m mozbuild.action.webidl ... instead of just python3 -m mozbuild.action.webidl .... That's all buildcache needs to intercept it.

Note the ifdef MOZ_USING_BUILDCACHE guard. This is specific to buildcache because ccache and sccache don't have a mechanism for caching arbitrary commands. Buildcache does, through its Lua wrappers.

The Lua wrapper

Buildcache's Lua plugin system lets you write a script that tells it how to handle a program it doesn't natively understand. The wrapper for WebIDL codegen, webidl.lua, needs to answer a few questions for buildcache:

With that information, buildcache can hash the inputs, check the cache, and either replay the cached outputs or run the real command and store the results.

The wrapper uses buildcache's direct_mode capability, meaning it hashes input files directly rather than relying on preprocessed output. This is the right approach here since we're not dealing with a C preprocessor but with a Python script that reads .webidl files.

Numbers

Here are build times for ./mach build on Linux, comparing compiler cachers. Each row shows a clobber build with an empty cache (cold), followed by a clobber build with a filled cache (warm):

tool cold warm with plugin
none 5m35s n/a n/a
ccache 5m42s 3m21s n/a
sccache 9m38s 2m49s n/a
buildcache 5m43s 1m27s 1m12s

The "with plugin" column is buildcache with the webidl.lua wrapper active. It shaves another 15 seconds1, bringing the total down to 1m12s2. Not a revolutionary improvement on its own, but it demonstrates the mechanism. The WebIDL step is just the first Python action to get this treatment; there are other codegen steps in the build that could benefit from the same approach.

More broadly, these numbers show buildcache pulling well ahead on warm builds. Going from a 5m35s clean build to a 1m12s cached rebuild is a nice improvement to the edit-compile-test cycle.

These are single runs on one machine, not rigorous benchmarks, but the direction is clear enough.

Setting it up

If you're already using buildcache with mach, the Makefile change is available when updating to today's central. To enable the Lua wrapper, clone the buildcache-wrappers repo and point buildcache at it via lua_paths in ~/.buildcache/config.json:

{
"lua_paths": ["/path/to/buildcache-wrappers/mozilla"],
"max_cache_size": 10737418240,
"max_local_entry_size": 2684354560
}

Alternatively, you can set the BUILDCACHE_LUA_PATH environment variable. A convenient place to do that is in your mozconfig:

mk_add_options "export BUILDCACHE_LUA_PATH=/path/to/buildcache-wrappers/mozilla/"

The large max_local_entry_size (2.5 GB) is needed because some Rust crates produce very large cache entries.

What's next

The Lua plugin system is the interesting part here. The WebIDL wrapper is a proof of concept, but the same technique applies to any deterministic build step that takes known inputs and produces known outputs. There are other codegen actions in the Firefox build that could get the same treatment, and I plan to explore those next.

Notes

  1. For a clobber build with a warm cache ↩

  2. On my machine ↩

10 Apr 2026 12:00am GMT

09 Apr 2026

feedPlanet Mozilla

The Mozilla Blog: Old habits die hard: Microsoft tries to limit our options, this time with AI

Black-and-white close-up of a hand using a device beside oversized cursor icons.

Microsoft recently announced it's pulling back Copilot from several of its core Windows apps - Photos, Notepad, the Snipping Tool, and Widgets. Rolling back these forced AI integrations is the right move, but this is just the most recent example of Microsoft going too far without user consent.

Copilot was pushed onto users

Over the past year, Copilot wasn't offered to Windows users - it was installed on them. The M365 Copilot app began auto-installing on any Windows device running Microsoft 365 desktop apps, with no prompt and no consent. A new physical keyboard key was added to laptops that launched Copilot by default, with no simple way to remap it. By default, Copilot was pinned to the taskbar starting with Windows 11 PCs. And, going a step further, Microsoft planned to embed it into three of the most fundamental surfaces for the operating system: the Windows notification center, the Settings app, and File Explorer.

Then came the user backlash.

When Microsoft says it now wants to be "intentional" about Copilot, they're really admitting that they made repeated choices to serve their business over their customers.

This isn't the first time - Microsoft has a pattern of deceptive design patterns

The pattern of behavior here isn't new. Independent research commissioned by Mozilla has documented how Microsoft uses design and distribution tactics to override user choice - from deliberately complicated processes for changing your default browser, to UI that routes users back to Microsoft's Edge browser even after they've explicitly chosen something else.

Since Mozilla published that research, Microsoft has continued to escalate its use of dark patterns to force behaviors that help the bottom line, not people's lives. Here are a few examples from the rollout of Windows 11 that have continued to strip users of their choice:

The Copilot rollout followed the same playbook we've come to expect from Microsoft: use automatic installs, physical hardware, and default settings to force behaviors. In the most recent instance, they allowed their AI to learn and gather data as quickly as possible before people had a choice.

What 'genuinely useful' AI integration actually looks like

We, like Microsoft and basically every tech company, have been asking ourselves the same question: What does it mean for AI to be genuinely useful? For us, the answer is simple. AI should work on your terms, not ours. Firefox's goal is to create AI enhancements that are made for people, not just because they can increase profit.

We've rolled out AI-enhanced features that make browsing smarter, faster, and more personalized, such as translations that stay local on your device to help you browse the web in your preferred language, alt text in PDFs to add accessibility descriptions to images in PDF pages and tab grouping which suggests related tabs and group names.

But we also know users deserve a choice. We built our answer into Firefox 148, introducing a centralized AI Controls panel in your browser settings including a single "Block AI Enhancements" switch that turns off every AI feature at once. Each option is also individually controllable.

The premise is simple: You should decide whether AI is part of your browsing experience at all. Not Big Tech. Not Mozilla. You.

And critically, your preferences also persist across browser updates, which means AI tools won't silently re-enable themselves after a major upgrade. No reinstalling. No opting out again after the fact. It's designed for people who care about what's happening on their computer but shouldn't have to become a systems administrator to stay in control of it.

The stakes are bigger than one rollback

When a company with Microsoft's reach continues to control users - and only walks it back when the noise gets loud enough - it shapes what people expect from technology. It tells people that their only real move is to complain until, hopefully, the company relents. It also makes it harder for alternatives to compete when a company uses its reach and control to steer people back into its own products.

We don't think that's the internet we have to accept. People have been clear about what they want when it comes to this era of the internet. They want to feel like they're in control of their own devices and their own data. That's the internet we're trying to build.

The post Old habits die hard: Microsoft tries to limit our options, this time with AI appeared first on The Mozilla Blog.

09 Apr 2026 5:03pm GMT

The Mozilla Blog: 0DIN is open-sourcing AI security and the hard-earned knowledge behind it

Retro-futuristic scientist using an open-source AI scanner to analyze floating vintage technology and digital data streams in space.
Image generated by Nano Banana 2 in response to a request for a "Retro-futuristic collage of a scientist using an open-source AI scanner to analyze floating vintage tech and digital data streams."

We're launching across the developer and security community this week on Product Hunt and Hacker News. If you've been following AI security, we'd love your support and your feedback.

At Mozilla, open source has never been just a licensing choice. It's a conviction: the internet gets healthier when tools and knowledge circulate freely, when anyone can audit what's running, extend what exists, and build on what came before. That's why we built Firefox in the open. It's why we've kept building that way ever since.

0DIN, Mozilla's AI security team, is working from the same premise. This week we're releasing the 0DIN AI Security Scanner as open source software under the Apache 2.0 license, along with 179 community probes covering 35 vulnerability families, plus six specialty probes drawn exclusively from our bug bounty library.

The scanner, and the intelligence behind it

The 0DIN Scanner isn't another benchmark suite built from textbook examples. We're seeding it with probes drawn directly from our bug bounty program, where security researchers compete to find novel techniques to manipulate, extract data from, and subvert AI systems. As new vulnerabilities are discovered and disclosed through that program, we'll continue adding probes to the open-source library over time.

That loop, from researcher discovery to packaged reusable test, is what separates 0DIN Scanner from generic tooling. It's high impact intelligence on jailbreaks, updated frequently as our researchers find new techniques.

Built on NVIDIA's GARAK open-source framework, the 0DIN Scanner adds a graphical interface, automated scan scheduling, cross-model comparative analysis, and enterprise-grade reporting. It runs against frontier models, open source LLMs, chatbots and anything with a prompt interface. Security teams can see attack success rates, a vulnerability breakdown, and a comparison against the frontier models that attackers are also probing every day.

Six of those bug bounty probes are named here for the first time: Placeholder Injection, Incremental Table Completion, Technical Field Guide, Chemical Compiler Debug, Correction, and Hex Recipe Book. Each represents a real technique that worked against production AI systems before we closed the loop.

These probes are scored using JEF (Jailbreak Evaluation Framework), our open-source library for measuring prohibited content output, which is also seeing major updates this week.

The code is at github.com/0din-ai/ai-scanner. Fork it, extend it, build on it.

Knowing your risk before attackers do

Not every organization has a red team or the bandwidth to run adversarial testing. Many companies are deploying AI in production right now without a clear picture of where they're exposed. To help close that gap, we're offering free security assessments for enterprise AI deployments.

The assessment delivers an attack success rate against your systems, a breakdown across prompt injection, jailbreaks, and data extraction categories, and a benchmark comparison against major frontier models. The process takes a few minutes to setup with scan duration varying based on the number of probes chosen. If you're actively deploying AI and haven't tested it under adversarial conditions, this is a good place to start.

For teams that don't want to manage the open source scanner on their own, we also offer a managed Enterprise edition with access to nearly 500 pre-disclosure probes from the bug bounty program, giving organizations advance notice of emerging techniques before they're publicly known.

Why open source, and why now

AI is moving fast enough that no single team will solve this alone. There are too many threats, too many models, too much attack surface. Keeping our tools locked away would make 0DIN marginally stronger while leaving the broader internet weaker.

The researchers who submitted findings through our bug bounty program earned bounties for their work. We're releasing a meaningful portion of that intelligence as open source and we'll keep doing so as new vulnerabilities are discovered and disclosed. That's the deal Mozilla has always offered: we build in the open, the community helps make it better, and the web gets a little healthier for it.

Get involved

The post 0DIN is open-sourcing AI security and the hard-earned knowledge behind it appeared first on The Mozilla Blog.

09 Apr 2026 4:35pm GMT

Andreas Farre: BuildCache now works with mach

I'm happy to announce that buildcache is now a first-class compiler cache in mach. This has been a long time coming, and I'm excited to finally see it land.

For those unfamiliar, buildcache is a compiler cache that can drastically cut down your rebuild times by caching compilation results. It's similar to ccache, but even more so sccache, in that it supports C/C++ out of the box, as well as Rust. It has some nice unique properties of its own though, which we'll look at more closely in following posts.

Getting started

Setting it up is straightforward. Just add the following to your mozconfig:

ac_add_options --with-ccache=buildcache

Then build as usual:

./mach build

That's it.

Give it a try

If you run into any issues, please file a bug and tag me. I'd love to hear how it works out for people, and any rough edges you might hit.

09 Apr 2026 12:00am GMT

08 Apr 2026

feedPlanet Mozilla

Firefox Tooling Announcements: MozPhab 2.12.0 Released

Bugs resolved in Moz-Phab 2.12.0:

Discuss these changes in #engineering-workflow on Slack or #Conduit Matrix.

1 post - 1 participant

Read full topic

08 Apr 2026 6:04pm GMT

This Week In Rust: This Week in Rust 646

Hello and welcome to another issue of This Week in Rust! Rust is a programming language empowering everyone to build reliable and efficient software. This is a weekly summary of its progress and community. Want something mentioned? Tag us at @thisweekinrust.bsky.social on Bluesky or @ThisWeekinRust on mastodon.social, or send us a pull request. Want to get involved? We love contributions.

This Week in Rust is openly developed on GitHub and archives can be viewed at this-week-in-rust.org. If you find any errors in this week's issue, please submit a PR.

Want TWIR in your inbox? Subscribe here.

Updates from Rust Community

Official
Foundation
Newsletters
Project/Tooling Updates
Observations/Thoughts
Rust Walkthroughs
Miscellaneous

Crate of the Week

This week's crate is aimdb-core, a type-safe and platform-agnostic data pipeline where the Rust type system is the schema and trait implementations define its behavior.

Thanks to sounds.like.lx for the self-suggestion!

Please submit your suggestions and votes for next week!

Calls for Testing

An important step for RFC implementation is for people to experiment with the implementation and give feedback, especially before stabilization.

If you are a feature implementer and would like your RFC to appear in this list, add a call-for-testing label to your RFC along with a comment providing testing instructions and/or guidance on which aspect(s) of the feature need testing.

No calls for testing were issued this week by Rust, Cargo, Rustup or Rust language RFCs.

Let us know if you would like your feature to be tracked as a part of this list.

Call for Participation; projects and speakers

CFP - Projects

Always wanted to contribute to open-source projects but did not know where to start? Every week we highlight some tasks from the Rust community for you to pick and get started!

Some of these tasks may also have mentors available, visit the task page for more information.

If you are a Rust project owner and are looking for contributors, please submit tasks here or through a PR to TWiR or by reaching out on Bluesky or Mastodon!

CFP - Events

Are you a new or experienced speaker looking for a place to share something cool? This section highlights events that are being planned and are accepting submissions to join their event as a speaker.

If you are an event organizer hoping to expand the reach of your event, please submit a link to the website through a PR to TWiR or by reaching out on Bluesky or Mastodon!

Updates from the Rust Project

479 pull requests were merged in the last week

Compiler
Library
Cargo
Clippy
Rust-Analyzer
Rust Compiler Performance Triage

A shorter week than normal (probably due to later perf triage last week). Overall fairly small changes scattered across various PRs, though the net effect was slightly positive (-0.5% avg change). All changed ended up either mixed or improvements this week.

Triage done by @simulacrum. Revision range: cf7da0b7..e73c56ab

0 Regressions, 3 Improvements, 8 Mixed; 5 of them in rollups 26 artifact comparisons made in total

Full report here

Approved RFCs

Changes to Rust follow the Rust RFC (request for comments) process. These are the RFCs that were approved for implementation this week:

Final Comment Period

Every week, the team announces the 'final comment period' for RFCs and key PRs which are reaching a decision. Express your opinions now.

Tracking Issues & PRs

Rust

Cargo

Compiler Team (MCPs only)

No Items entered Final Comment Period this week for Rust RFCs, Language Reference, Language Team, Leadership Council or Unsafe Code Guidelines.

Let us know if you would like your PRs, Tracking Issues or RFCs to be tracked as a part of this list.

New and Updated RFCs

Upcoming Events

Rusty Events between 2026-04-08 - 2026-05-06 🦀

Virtual
Asia
Europe
North America
Oceania
South America

If you are running a Rust event please add it to the calendar to get it mentioned here. Please remember to add a link to the event too. Email the Rust Community Team for access.

Jobs

Please see the latest Who's Hiring thread on r/rust

Quote of the Week

Rust tried to have polymorphic generics in the early pre-1.0 days, and they quite reasonably gave up because it was too much work. For real Swift, great fucking working for getting all of this to work!

- Aria Desires on her blog

llogiq thanks himself for the suggestion!

Please submit quotes and vote for next week!

This Week in Rust is edited by:

Email list hosting is sponsored by The Rust Foundation

Discuss on r/rust

08 Apr 2026 4:00am GMT

07 Apr 2026

feedPlanet Mozilla

Firefox Tooling Announcements: New Deploy of PerCompare April 7th

The latest version of PerfCompare is now live!

Check out the change-log below to see the updates:

[kala]

[moijes12]

[padenot]

[mgaudet]

Thank you for the contributions!

Bugs or feature request can be filed on Bugzilla. The team can also be found on the #perfcompare channel on Element. Come and chat!

1 post - 1 participant

Read full topic

07 Apr 2026 10:05pm GMT

Firefox Tooling Announcements: Engineering Effectiveness Newsletter (Q1 2026 Edition)

Welcome to the Q1 edition of the Engineering Effectiveness Newsletter! The Engineering Effectiveness org makes it easy to develop, test and release Mozilla software at scale. See below for some highlights, then read on for more detailed info!

Highlights

Detailed Project Updates

AI for Development

Bugzilla

Build System and Mach Environment

Firefox-CI, Taskcluster and Treeherder

Lint, Static Analysis and Code Coverage

PDF.js

Firefox Translations

Phabricator, moz-phab, and Lando

Release Engineering and Release Management

Release Operations

Other

Thanks for reading and see you next quarter!

1 post - 1 participant

Read full topic

07 Apr 2026 3:37pm GMT

04 Apr 2026

feedPlanet Mozilla

The Rust Programming Language Blog: Changes to WebAssembly targets and handling undefined symbols

Rust's WebAssembly targets are soon going to experience a change which has a risk of breaking existing projects, and this post is intended to notify users of this upcoming change, explain what it is, and how to handle it. Specifically, all WebAssembly targets in Rust have been linked using the --allow-undefined flag to wasm-ld, and this flag is being removed.

What is --allow-undefined?

WebAssembly binaries in Rust today are all created by linking with wasm-ld. This serves a similar purpose to ld, lld, and mold, for example; it takes separately compiled crates/object files and creates one final binary. Since the first introduction of WebAssembly targets in Rust, the --allow-undefined flag has been passed to wasm-ld. This flag is documented as:

  --allow-undefined       Allow undefined symbols in linked binary. This options
                          is equivalent to --import-undefined and
                          --unresolved-symbols=ignore-all

The term "undefined" here specifically means with respect to symbol resolution in wasm-ld itself. Symbols used by wasm-ld correspond relatively closely to what native platforms use, for example all Rust functions have a symbol associated with them. Symbols can be referred to in Rust through extern "C" blocks, for example:

unsafe extern "C" {
    fn mylibrary_init();
}

fn init() {
    unsafe {
        mylibrary_init();
    }
}

The symbol mylibrary_init is an undefined symbol. This is typically defined by a separate component of a program, such as an externally compiled C library, which will provide a definition for this symbol. By passing --allow-undefined to wasm-ld, however, it means that the above would generate a WebAssembly module like so:

(module
    (import "env" "mylibrary_init" (func $mylibrary_init))

    ;; ...
)

This means that the undefined symbol was ignored and ended up as an imported symbol in the final WebAssembly module that is produced.

The precise history here is somewhat lost to time, but the current understanding is that --allow-undefined was effectively required in the very early days of introducing wasm-ld to the Rust toolchain. This historical workaround stuck around till today and hasn't changed.

What's wrong with --allow-undefined?

By passing --allow-undefined on all WebAssembly targets, rustc is introducing diverging behavior between other platforms and WebAssembly. The main risk of --allow-undefined is that misconfiguration or mistakes in building can result in broken WebAssembly modules being produced, as opposed to compilation errors. This means that the proverbial can is kicked down the road and lengthens the distance from where the problem is discovered to where it was introduced. Some example problematic situations are:

All native platforms consider undefined symbols to be an error by default, and thus by passing --allow-undefined rustc is introducing surprising behavior on WebAssembly targets. The goal of the change is to remove this surprise and behave more like native platforms.

What is going to break, and how to fix?

In theory, not a whole lot is expected to break from this change. If the final WebAssembly binary imports unexpected symbols, then it's likely that the binary won't be runnable in the desired embedding, as the desired embedding probably doesn't provide the symbol as a definition. For example, if you compile an application for wasm32-wasip1 if the final binary imports mylibrary_init then it'll fail to run in most runtimes because it's considered an unresolved import. This means that most of the time this change won't break users, but it'll instead provide better diagnostics.

The reason for this post, however, is that it's possible users could be intentionally relying on this behavior. For example your application might have:

unsafe extern "C" {
    fn js_log(n: u32);
}

// ...

And then perhaps some JS code that looks like:

let instance = await WebAssembly.instantiate(module, {
    env: {
        js_log: n => console.log(n),
    }
});

Effectively it's possible for users to explicitly rely on the behavior of --allow-undefined generating an import in the final WebAssembly binary.

If users encounter this then the code can be fixed through a #[link] attribute which explicitly specifies the wasm_import_module name:

#[link(wasm_import_module = "env")]
unsafe extern "C" {
    fn js_log(n: u32);
}

// ...

This will have the same behavior as before and will no longer be considered an undefined symbol to wasm-ld, and it'll work both before and after this change.

Affected users can also compile with -Clink-arg=--allow-undefined as well to quickly restore the old behavior.

When is this change being made?

Removing --allow-undefined on wasm targets is being done in rust-lang/rust#149868. That change is slated to land in nightly soon, and will then get released with Rust 1.96 on 2026-05-28. If you see any issues as a result of this fallout please don't hesitate to file an issue on rust-lang/rust.

04 Apr 2026 12:00am GMT