03 Sep 2026

feedPlanet Mozilla

Firefox Tooling Announcements: PerfCompare just deployed (Sept 3, 2026)

The latest version of PerfCompare is now live! This includes the launch of PerfCompare's Simple View. Read the following blog post, Introducing PerfCompare's Simple View, to learn more.

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

Highlights:

[kala-moz]

Other changes:

[kala-moz]

[gopar]

[bug-2021092] Update light/dark toggle wording to be less ambiguous (#1073)

Thank you for the contributions!

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

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03 Sep 2026 6:34pm GMT

Mozilla Performance Blog: Introducing PerfCompare’s Simple View

We heard you. You don't want to open your PerfCompare performance comparison and feel like you need a background in statistics to interpret the Mann-Whitney-U results. Cliff's Delta, CLES, and normality tests, oh my! It should not feel like you've blindly walked into a college stats exam, unless of course, you love statistics. For the rest, all you want is an easy answer to the question: Did my changes make things faster or slower?

So we fixed that. PerfCompare now shows the bare essentials by default and tucks the heavy statistics one click away.

An approachable default for beginners and occasional users

The results table now leads with the platform, the Base and New values, how far the median moved and its direction, the status (which includes the option to filter out noise) and the size of the change. To create a more advanced view, a single "Advanced options" dropdown allows you to add the expert columns (Cliff's Delta, CLES, Significance) and the info-rich expanded row details below the full-width graph. A small help section at the top of the results provides a quick explanation of the columns. The purpose is to serve a diverse audience while preserving your selections in the URL so a shared link reproduces your view.

PerfCompare Results Table

The "Advanced options" dropdown lets you add the precise statistics columns:

Each one toggles on independently, so you can surface only Significance or the whole trio.

PerfCompare Results Table

Expand a result and you get a clean panel: a full-width graph of how your Base and New runs are spread out, a quick summary (platform, app, which direction is "better"), and a little "how to read this graph" tool-tip for anyone who hasn't stared at a distribution curve before. You can still poke at the graph's mode controls if you want to dig in.

PerfCompare Expanded Row

You can access the extra expanded row details such as effect size and confidence intervals or the full stats table in the "Advanced options" dropdown. Turn on what you need, and the expanded extras lay out in neat two-column rows. If you turn on mode analysis and there aren't multiple modes to show, we now say "No mode analysis available" instead of a blank space.

Lastly, the advanced columns and expanded-row details you've switched on, along with your filters and sorting, are all encoded in the URL. Paste it to a coworker and they'll land on the same result you were looking at; for filters and sorting, the shared link takes precedence over whatever they had saved locally. (One exception: the "How to read the results" panel is a personal per-browser preference, so it stays with you rather than tagging along on the link.)

What's next

Several follow-ups on the way:

Share your feedback

We want PerfCompare to work for all users, whether you're chasing your first regression, or you're an expert who doesn't want to lose their favorite metrics to a cleaner view. Our answer is separating the simple view from the power user view, and making the toggle obvious and shareable.

The great news is it's live now! Go expand a Mann-Whitney-U result, click around the Advanced options, and tell us what you think. Please share your feedback, suggestions, or comments on the #perfcompare channel on Matrix. If you've encountered a bug or have a feature request, please file them in Bugzilla. We're listening.

03 Sep 2026 6:29pm GMT

Firefox Tooling Announcements: Firefox Profiler Deployment (September 3, 2026)

The latest version of the Firefox Profiler is now live! Check out the full changelog below to see what's changed:

Highlights:

Other Changes:

Big thanks to our amazing localizers for making this release possible:

Find out more about the Firefox Profiler on profiler.firefox.com! If you have any questions, join the discussion on our Matrix channel!

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03 Sep 2026 3:36pm GMT

The Rust Programming Language Blog: Announcing Rust 1.98.1

The Rust team has published a new point release of Rust, 1.98.1. Rust is a programming language that is empowering everyone to build reliable and efficient software.

If you have a previous version of Rust installed via rustup, getting Rust 1.98.1 is as easy as:

rustup update stable

If you don't have it already, you can get rustup from the appropriate page on our website.

What's in 1.98.1

Rust 1.98.1 fixes a miscompilation in vtable generation.

In Rust 1.98.0, in some circumstances, rustc would incorrectly generate a trait object vtable with a null pointer where a function pointer should be. This leads to undefined behavior in the emitted code. In some cases this may 'just' cause segfaults due to the null pointer being loaded, but it is possible for it to be justification for arbitrary effects (as is typical for UB).

If you'd like to help us out by testing future releases, you might consider using the beta (rustup default beta) and nightly (rustup default nightly) channels locally and in your CI. Please report any bugs you might come across!

Contributors to 1.98.1

Many people came together to create Rust 1.98.1. We couldn't have done it without all of you. Thanks!

03 Sep 2026 12:00am GMT

02 Sep 2026

feedPlanet Mozilla

Thunderbird Blog: Thunderbird Desktop New Protocol Support: Microsoft Graph API

As many of you know, Microsoft will be disabling Exchange Web Services (EWS) support on its Microsoft 365 platform later this year. The Thunderbird Desktop team has been hard at work to ensure continuity of functionality through this transition, and today we are pleased to announce the release of native Thunderbird support for the Microsoft Graph API! Curious what this means for you? Then keep reading to find out!

IMAP and EWS and Graph, Oh My!

When connecting to either on-premises or hosted Microsoft accounts, Thunderbird users now have an array of options. Users might not know which option is best for their case, so we'll start with a little bit of guidance to help users decide what will be best for them. First off, if you are currently using IMAP with an on-premises Exchange account or a Microsoft 365 hosted account, and it's currently working for you, then there's nothing you need to do! EWS and Graph support are for users whose organizations do not allow IMAP access.

Now for the more complicated part. Last year, we released EWS email support for both on-premises Exchange accounts and Microsoft 365 hosted accounts. If you are using the EWS protocol with an on-premises Exchange account, then you don't need to do anything.

However, if your organization uses Microsoft 365 for email, and you're currently using EWS to connect to that account, then you need to take action before October of this year. Earlier this year, Microsoft announced that they would begin a phased shutdown of EWS on Microsoft 365/Exchange Online starting in October of 2026 with the goal of a complete shutdown in 2027. If you are currently using Thunderbird's EWS support to connect to a Microsoft 365/Exchange Online email account, then you will need to switch to the Microsoft Graph API support to keep using Thunderbird beyond these dates. See below for details.

So the TL;DR is:

Microsoft only supports the Graph protocol on Microsoft 365, so that is the only provider that offers it. On-premises Exchange servers do not implement the Graph API, so Thunderbird's EWS support will continue to be the primary mechanism for users on these servers to connect to their email accounts.

Microsoft Graph Account Set Up

Thunderbird does not support migrating accounts to different protocols. Instead, you will need to set up a new account. If you have an existing EWS-connected Microsoft 365 account, first you'll need to delete that account in Thunderbird. You can do this from the account settings. Deleting the account locally in Thunderbird will not affect any data stored on Microsoft 365, so your data is safe.

Once your previous EWS account has been deleted, you can use Account Hub to set up a new account. In the new account set up dialog, enter your Microsoft 365 hosted email account. Thunderbird should automatically detect that your email is hosted on Microsoft 365 and will give you a few options for getting connected. Choose the Microsoft Graph option, and you should be good to go! If for some reason automatic configuration doesn't work, you can still configure your account manually using these instructions.

What is currently supported?

As with EWS, Thunderbird's current Graph support extends only to email. Thunderbird does not yet support Calendar and Address Book functionality with either EWS or Graph.

What's in ESR?

Thunderbird's current Extended Support Release, version 153, supports the EWS protocol, but does not support the Microsoft Graph API, which was first enabled in the Thunderbird Release channel in version 154. Users on the ESR channel who are using Microsoft 365 will need to migrate to the Release channel in order to use the Microsoft Graph API support to continue accessing Microsoft 365 hosted mailboxes.

What's Next?

Calendar and Address Book! The Thunderbird Desktop team has already started working on support for connecting to Microsoft 365-hosted calendars using the Graph protocol. We will be busily working on this throughout the rest of the year. As with our previous work on Thunderbird support for EWS, this is the first new calendar protocol to be added to Thunderbird in many years. We are currently doing some up-front design work to ensure a sustainable product going forward and hopefully make the next calendar protocol easier to add.

How can users give us feedback?

As with all new functionality, there may be features we are missing or unexpected issues. As always, Bugzilla is always open! You can send us feature requests and issue reports using this Bugzilla link. Thunderbird's greatest strength is its community of users, developers, and supporters who help us deliver the best tools we can to connect people with one another. We welcome community involvement to help us make everything better!

Nitty-Gritty development details

Those of you who have followed along with development throughout EWS and into Graph support might have noticed that, compared to EWS, we were able to implement support for the Microsoft Graph API relatively quickly. This is largely due to the up-front thought and work that went into the design and development of Thunderbird's EWS support. The Graph implementation was the team's first test of the extensibility of the new architecture, and we were very pleased with the results. It took 11 months to enable basic synchronization with EWS. With the new architecture, we were able to cut that time down to 4 months with Graph.

Graph's operations are similar to the operations that EWS provides, but different enough that it served as a great first test case for how the new email client architecture can support future protocols, such as JMAP. Given the results of the Graph implementation, we are optimistic that the same architecture will extend to JMAP once the team decides to move forward to that new protocol. We are hopeful that what we have built here can provide a sustainable base on which to build future protocol support in Thunderbird.

This would not have been possible without many contributions from Thunderbird developers, past and present, and the community of users who have helped us improve Thunderbird by submitting issue reports and feature requests. We are grateful to everyone for their support!

The post Thunderbird Desktop New Protocol Support: Microsoft Graph API appeared first on The Thunderbird Blog.

02 Sep 2026 7:58pm GMT

The Mozilla Blog: AI on your terms: Firefox meets you where you are

People feel differently about AI, and here at Firefox, we think that is completely reasonable. Something else that is completely reasonable: deciding for yourself how and when you engage with AI features.

For some of you, AI is part of your everyday life, whether you are using it to summarize a large amount of information, brainstorm ideas for work, or research trips and personal purchases. Some of you interact with AI as little as possible, and prefer to keep it that way. And a lot of you are somewhere in between: curious, but exploring on your own terms and at your own pace.

Your personal use of AI is just that, personal. Firefox offers various levels of AI integration into your browser, and the choice between them is entirely yours.

Block new and current AI features in a single switch

Opting out of upcoming and current AI features on your browser should not require endless navigation through multiple Settings pages. That's why Firefox offers an AI controls section within its General Settings panel. A single, easily located place where you can block current and future AI features and related pop-ups with the swipe of a toggle.

Not only do we want people to have the choice to remove certain AI features from their Firefox browsing experience, we make sure doing so is as easy and accessible as possible.

Choose the features that work for you

For the many people who sit in the middle of the AI usage spectrum, we made sure you can opt in and out of specific features in line with your preferences. Capabilities like AI translations, image alt text in Firefox PDF viewer, tab group suggestions, and key points in link previews can all be individually switched on and off, ensuring you can enjoy such offerings on a case by case basis as it suits your needs.

In addition, if there is an AI chatbot provider you already trust or pay for, you can use it directly inside Firefox's built-in desktop sidebar. This means you can use your chatbot of choice and browse side by side without ping-ponging between tabs.

While some browsers integrate a singular AI model or chatbot into their system, this can restrict flexibility and concentrate data sharing. Firefox prioritizes transparency and user control by letting you select from multiple providers, allowing you to review each option's policies before you engage with them.

You can turn on AI chatbot providers in the AI controls section of browser settings and select one of the following: Anthropic Claude, ChatGPT, Google Gemini, and Le Chat Mistral. Once this feature is turned on, you can switch between chatbots within the sidebar itself based on your preferences and task at hand.

Smart Window, for when you want help finishing what you started

Smart Window is Firefox's most integrated AI experience, but that doesn't mean it compromises our commitment to choice, privacy, and transparency. Our newest window type, which we've been polishing and testing in beta, uses only the context you share with it to help you move work forward and across the finish line.

When permitted by you, its built-in, AI-powered assistant can work directly with your open tabs and browsing history to connect the dots. This means comparing information, generating recommendations, summarizing pages, and planning projects without having to feed every crumb of context from your previous and current browsing activity each time you enter a new prompt.

As we've built out Smart Window, we've added a few additional features to align its design with how people actually browse. These include the ability to group related tabs together - so you can make sense of what's open without spending time organizing everything yourself - and visual history previews that help you spot a page you visited earlier when you can't remember the exact URL or phrase searched. In addition, a new partnership with Exa enables Smart Window to not only locate current web information to include in responses, but also display the sources behind these responses so you can verify what you're seeing, without breaking your flow.

We understand people use different Firefox windows for different reasons, and often switch between options based on the task at hand. Smart Window is the window you reach for when your browsing turns into a project. When your quick search becomes inspo for the girls' trip you've been meaning to plan, or finding the perfect gift for your partner becomes a dozen tabs comparing prices and reviews, turn the window you're already using into a Smart Window. Your tabs and progress stay in place while Smart Window helps organize the task and move it forward. Use it for one complex project, use it all the time, or switch it back when you're done.

Smart Window remains in beta as we continue to learn and improve from our community's feedback. At present, it's available in English to people in the U.S. and Canada.

To try Smart Window, visit: https://www.firefox.com/smart-window

Always your call

Your opinions and preferences on AI usage are individual to you.

We don't think there is one right answer for how and when to engage with AI, and we're not going to pretend there is one. What we can do is make sure that whatever level of engagement is right for you, Firefox supports it well. Because the only person telling you how much AI you need should be you.

The post AI on your terms: Firefox meets you where you are appeared first on The Mozilla Blog.

02 Sep 2026 1:00pm GMT

This Week In Rust: This Week in Rust 667

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
Research
Miscellaneous

Crate of the Week

This week's crate is buf_read_splitter, a crate to split (separate) a stream into sub-subtreams.

Thanks to flo 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

522 pull requests were merged in the last week

Compiler
Library
Cargo
Rustdoc
Rustfmt
Clippy
Rust-Analyzer
Rust Compiler Performance Triage

This week continues a steady stream of compile time improvements. Most of the impact this week comes from type system micro-optimization in #160473 and dead_code lint propagation fix in #161571. We've also hit unexpected regression in a standard library refactor, but we expect that to be addressed soon.

Triage done by @panstromek. Revision range: 9a4ad59a..5321a4f4

Summary:

(instructions:u) mean range count
Regressions ❌
(primary)		 0.6% [0.2%, 1.8%] 27
Regressions ❌
(secondary)		 0.6% [0.2%, 1.8%] 27
Improvements ✅
(primary)		 -0.7% [-2.4%, -0.1%] 135
Improvements ✅
(secondary)		 -0.7% [-2.2%, -0.1%] 120
All ❌✅ (primary) -0.5% [-2.4%, 1.8%] 162

5 Regressions, 4 Improvements, 4 Mixed; 9 of them in rollups 39 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

Rust RFCs

Leadership Council

No Items entered Final Comment Period this week for Cargo, Compiler Team, Language Team, Language Reference 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 2026-09-02 - 2026-09-30 🦀

Virtual
Africa
Europe
North 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

I think you are inventing provenance for integers. Please, let's not.

- Ralf Jung in an RFC discussion

Thanks to kleines Filmröllchen 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

02 Sep 2026 4:00am GMT

01 Sep 2026

feedPlanet Mozilla

Firefox Developer Experience: Firefox WebDriver Newsletter 155

WebDriver is a remote control interface that enables introspection and control of user agents. As such, it can help developers to verify that their websites are working and performing well with all major browsers. The protocol is standardized by the W3C and consists of two separate specifications: WebDriver classic (HTTP) and the new WebDriver BiDi (Bi-Directional).This newsletter gives an overview of the work we've done as part of the Firefox 155 release cycle.

Contributions

Firefox is an open source project, and we are always happy to receive external code contributions to our WebDriver implementation. We want to give special thanks to everyone who filed issues, bugs, and submitted patches. In Firefox 155, several WebDriver bugs were fixed by contributors:

WebDriver code is written in JavaScript, Python, and Rust, so any web developer can contribute! Read how to set up the work environment and check the list of mentored issues for Marionette or the list of mentored JavaScript bugs for WebDriver BiDi. Join our chatroom if you need any help to get started!

All Changes

A complete list of developer-facing changes included in this Firefox release is available in the MDN Firefox 155 Release Notes.

01 Sep 2026 4:00pm GMT

Mozilla Privacy Blog: Browsers compete on privacy. When the operating system allows.

For more than two decades, Mozilla has advocated for an internet where people, not powerful platforms, determine how technology works for them. Meaningful consumer choice requires more than the ability to select a browser. People should also be able to choose what their browser can do, from the privacy protections it provides to the ways they can customize their experience.

A recent Firefox product initiative brings that longstanding policy concern into focus.

Firefox is bringing a built-in Ad Blocker to Firefox for iOS, giving people an optional way to reduce intrusive ads and ad-related trackers while browsing. The feature uses an EasyList-based filter list and Apple's WebKit Content Blocker framework. It is off by default and can be enabled or disabled at any time.

The feature responds to years of requests from Firefox users on iPhone. It also illustrates a wider policy problem: dominant mobile platforms can determine which privacy and customization tools competing browsers are able to offer.

On Firefox for desktop and Android, people can install add-ons that change how they experience the web.

Almost 40% of all Firefox users take advantage of this functionality. They include content blockers, accessibility tools, password managers and other extensions that allow people to shape their browser around their own priorities.

Firefox cannot offer that same add-on ecosystem on iOS. Apple's App Review Guidelines generally require apps to remain self-contained and restrict them from downloading or executing code that introduces or changes an app's functionality.

Apple also controls the conditions under which browsers may use alternative browser engines, with entitlement pathways available only in certain jurisdictions and subject to detailed requirements that no browser developer has been able to meet. In practice, the combined effect of Apple's app distribution and browser policies prevents Mozilla from bringing the full Firefox add-on experience available on other platforms to iPhone users.

This is not simply a technical inconvenience but a competition and consumer-choice problem.

A mobile platform owner controls the operating system, the primary app-distribution channel and its own browser.

When that company can also determine which technologies, add-on models and operating-system capabilities competing browsers may use, it sets the boundaries within which browser competition takes place.

Operating system restrictions can prevent independent browsers from differentiating themselves.

Even when users want browsers to differentiate on privacy, operating system restrictions can prevent independent browsers from doing so. They can also force browser developers to recreate individual tools as platform-specific, built-in features rather than allowing people to choose from a broader add-on ecosystem. That increases development costs, slows or restricts innovation and produces uneven experiences across devices.

The new Firefox Ad Blocker is a practical response to those constraints. It gives iPhone users more control today using the technical options currently available to us. But building one feature directly into a browser is not a replacement for a healthy, independent add-on ecosystem. It addresses a specific capability gap for Firefox users on iOS at this time.

A competitive browser ecosystem should focus on the entire user experience.

The EU's Digital Markets Act has shown that policy interventions can improve user choice, with Mozilla and other browsers reporting significant growth following the introduction of browser choice screens. However, meaningful browser competition must extend to the full user experience. This includes whether people can easily choose and change their default browser. It also includes whether people can benefit from the full set of innovations developed by rival browsers by ensuring adequate interoperability with the operating system. With this deeper focus on the full user experience, people can benefit from differentiated privacy and security features.

People's privacy choices should not depend on whether a dominant platform permits their browser to offer them.

The post Browsers compete on privacy. When the operating system allows. appeared first on Open Policy & Advocacy.

01 Sep 2026 12:59pm GMT

The Mozilla Blog: Introducing Ad Blocker for Firefox on iOS: More Control, Fewer Distractions

There's only so much room on your screen. Pop-ups, overlays, and ads can take over fast, getting between you and what you came to do.

That's where Ad Blocker for Firefox on iOS comes in: a built-in option that blocks many third-party ads and ad-related trackers before they load, helping reduce clutter and distractions while you browse.


How it works

Ad Blocker uses Apple's WebKit Content Blocker technology and the EasyList filter list to determine what gets blocked. There's no separate extension to install, and you can turn it on in Settings > Browsing > Ad Blocker. It's off by default, so you decide whether to use it.

Ad Blocker won't block every ad. Ads served directly by the site you're visiting and ads shown in search results will still appear. Sponsored shortcuts and other sponsored content shown by Firefox when you open a new tab are separate from ads on the web pages you visit, so Ad Blocker doesn't affect them.

Ad Blocker works alongside the privacy protections already built into Firefox, including Enhanced Tracking Protection, which blocks many trackers and limits tracking across the web.

More control over how you experience the web

On Desktop and Android, Firefox already supports a strong ecosystem of ad-blocking and privacy extensions, giving people the flexibility to choose the tools that work best for them. We value that ecosystem and will keep supporting it.

iOS works differently. Extensions aren't available in the same way, and we know people want more options. Bringing ad blocking to Firefox on iOS meant building it directly into the browser.

Giving people choice in how they experience the web is important to us. Advertising helps fund much of the open web, supporting the publishers, creators and websites people rely on. We also know that ads can sometimes crowd the screen or interrupt what you're trying to do.

That's why Ad Blocker is optional: you decide whether it's part of how you browse. It's part of a broader approach across Firefox to give you more control over your experience, from the extensions you use to how AI shows up in your browser.

Try it

To turn on Ad Blocker, go to Settings > Browsing > Ad Blocker.

If you find an ad you expected to be blocked, a site that behaves strangely or something we should improve, let us know on Mozilla Connect.

Visit our Support page for more details on Ad Blocker for Firefox on iOS. For more on Firefox's built-in privacy protections, check out How Firefox Protects Your Data.

The post Introducing Ad Blocker for Firefox on iOS: More Control, Fewer Distractions appeared first on The Mozilla Blog.

01 Sep 2026 12:45pm GMT

The Rust Programming Language Blog: Announcing rustup 1.29.1

The rustup team is happy to announce the release of rustup version 1.29.1.

Rustup is the recommended tool to install Rust, a programming language that empowers everyone to build reliable and efficient software.

What's new in rustup 1.29.1

The headlines of this release are:

In addition, rustup now officially supports aarch64-pc-windows-gnullvm as a host platform. pr#4523

Further details are available in the changelog!

How to update

If you have a previous version of rustup installed, getting the new one is as easy as stopping any programs which may be using rustup (e.g. closing your IDE) and running:

$ rustup self update

Rustup will also automatically update itself at the end of a normal toolchain update:

$ rustup update

If you don't have it already, you can get rustup from the appropriate page on our website.

Rustup's documentation is also available in the rustup book.

Caveats

Rustup releases can come with problems not caused by rustup itself but just due to having a new release.

In particular, anti-malware scanners might block rustup or stop it from creating or copying files, especially when installing rust-docs which contains many small files.

Issues like this should be automatically resolved in a few weeks when the anti-malware scanners are updated to be aware of the new rustup release.

Thanks

Thanks again to all the contributors who made this rustup release possible!

01 Sep 2026 12:00am GMT

31 Aug 2026

feedPlanet Mozilla

Mozilla Privacy Blog: How Challengers Thrive and Competition Survives In Today’s Ecosystem

The web should be a place where people can choose how they connect, which tools they use, and who they trust. At Mozilla, we live these values, prioritizing products that ensure users can control their experience and that the web remains a global public resource.

Challengers across the tech ecosystem provide these competitive opportunities, but they contend with structural barriers imposed by tech giants leveraging market dominance. A healthy internet depends on ensuring that independent companies have a fair opportunity to innovate and compete on merit. That's why Mozilla and many others support key legislation like AICOA.

So how do we promote proposals like AICOA and ensure that challengers can thrive in markets controlled by powerful gatekeepers? We explored these questions and more at the recent Mozilla Meetup, "How Challengers Thrive: Why Competition Matters for AI, Security, and the Open Internet." Across panel events, a lightning discussion, and a networking happy hour, we discussed promoting an innovative, secure internet.

Partnering with Internet Works, Mozilla invited leading minds in tech policy to explore the importance of Middle Tech, AI's impact on security and competition, and new research on deceptive design practices. Below are some highlights.

On today's competition landscape, and Middle Tech as a user-first alternative:

Peter Chandler, Executive Director, Internet Works:

"Less competition equals less innovation equals less choices for consumers. It's that simple."

Charlotte Slaiman, Principal, Charlotte Slaiman Impact Advising:

"There are all sorts of reasons that competition is not just a click away. There are lock-in effects. There is gatekeeper power. Those things are protecting incumbents and making it much harder for consumers to switch to an alternative."

On the impact of deceptive design tactics:

Gemma Petrie, Principal Researcher, Competition & Regulation, Mozilla:

"For independent browsers like Firefox, dominant operating systems are both the gateway to consumers and the direct competitor. When these platforms use harmful design to undermine free choice, this is not simply inconvenient. It can create consumer harm by overriding informed choice, social harm by eroding public trust in technology, and market harm by undermining competition and innovation."

On the role of AI in competition and security:

Helen Toner, Executive Director, Georgetown's Center for Security and Emerging Technology (CSET):

"It is really notable that in open source software, if you have more eyes on software and you can identify vulnerabilities, that makes it easier and quicker to patch those vulnerabilities to solve the problem. With AI, often we actually don't have very good solutions when we identify problems, when we identify security risks, and so I think that can change some of the dynamics."

Chris Lewis, President & CEO, Public Knowledge:

"We have an opportunity. It's not too late in the AI era to not only design for competition through open technology, open source AI, model weights, some of the things we heard in the introduction, but also to set policies that promote competition."

Avery Gardiner, Director of Global Competition Policy, Spotify:

"A big part of what I'm thinking about with AI is how quickly it allows us to innovate as an app developer, right, and how quickly we can speed up our coding, bring more features to consumers, bring more things to market, and be better at innovating for the consumer. And all of our competitors can do the same thing. So there's this remarkable thing where the AI opportunities, the AI services, are getting better every passing day."

There's a reason we convene this community. As the web becomes both more complex and even more integral to our lives, competition offers people the ability to make meaningful choices about privacy, security, and control. Competition fosters innovation and spurs better products and services. Now is a particularly important moment to advance these priorities.

The post How Challengers Thrive and Competition Survives In Today's Ecosystem appeared first on Open Policy & Advocacy.

31 Aug 2026 6:16pm GMT

Mozilla Localization (L10N): Minding My Language: My Internship Story

Hello everyone! It's Jamie. My internship at Mozilla is quite unfortunately coming to an end, after which I will be heading back to study at the University of Toronto. In this post, I want to share a rundown of my story working on the Localization (L10N) team, break down the projects I built, and reflect on what made this experience so special.

The First Day

Before joining Mozilla I was quite a bit nervous, as I had never worked at such a large organization before. The first day shattered this perception, as getting an office tour and having a live Q&A with VPs of Engineering really set the expectations of what working at Mozilla would be like.

The Toronto office became the go-to place for connecting with passionate, mission-oriented people across different teams. After getting my computer set up, I immediately got to work on Pontoon, Mozilla's Translation Management System (TMS). Since Pontoon's codebase relies on React and Django - frameworks I've worked with before - I was able to set up my local environment running smoothly and ship code early on. Over time, I had the opportunity to meet my fellow interns, each on their own teams, and we had chances to bond over our shared experiences and different problems.

Spending my break time in the lounge with my fellow interns.

Spending my break time in the lounge with my fellow interns.

Since Mozilla is a remote-first company, the L10N team is distributed across Europe and North America. Working with them was an absolute highlight. To put it simply, they are a brilliant group of people of engineers, project managers and open-source contributors who keep Mozilla, and by extension, the wider web open and accessible.

Group photo with the L10N team! Taken August 2026

Group photo with the L10N team! Taken August 2026

So, what was working at Mozilla like?

Working at Mozilla, I had the opportunity to contribute to a variety of features involving Pontoon, associated localization workflows and other cool projects. During the duration of my internship, a great deal of localization related work was centralized and continues to be that way, which significantly influenced the projects that I worked on, and more generally, how the L10N team will continue to operate when I am gone.

This work unfolded against the backdrop of an ever changing technological landscape, particularly involving industry adoption of AI tools for code generation. In 2025 and 2026, agentic coding assistants like Claude Code and Cursor began to achieve mass adoption in software development. At Mozilla, I was introduced to Claude Code as a tool later in my internship, which was excellent, as it allowed me to gain a broad understanding of how the codebase worked without the use of AI. After the introduction of Claude Code to my workflow, I was able to push code and conduct code reviews on a much greater scale. I appreciate that Mozilla does not shy away from the practical uses of AI, especially in the realm of improving engineer and localizer workflows, and importantly, making it a choice.

The Work

Here is a shortlist of the varying projects I had the opportunity to work on at my time with the L10N team.

GraphQL Deprecation and REST API Implementation

The first task I was assigned to primarily revolved around the deprecation of Pontoon's public-facing GraphQL API endpoints in favor of a Django REST Framework implementation. The motivations for this project included a preference for simplicity, desire for faster API response times and consistency with existing vanilla Django REST endpoints. The new public facing API lives here.

Personal Access Tokens

The next project I was given was to devise a new personal access token (PAT) implementation in Pontoon in order to gate permissions to the newly created REST API endpoints. I was inspired by the way that GitHub did their Classic personal access tokens so I tried to model my design around their implementation as much as possible.

Pontoon Translation Search

The next project I took up was to reduce the feature gap between Pontoon and Transvision. For context, Transvision was and is used for searching available translations of strings across certain Mozilla products. It uses GitHub as its primary source, which is why it is limited only to the big projects. Thus the motivation was to create a similar service directly on Pontoon that has the most up to date strings with all Mozilla products, with data manipulation and access being a non-issue due to existing on Pontoon directly. Translation Search lives here.

Pontoon's Translation Search feature, which searches for strings across Mozilla's localized projects.

Pontoon's Translation Search, which searches for strings across Mozilla's localized projects.

Taking responsibility for Pontoon Add-on

The Pontoon Add-on was a community maintained browser extension that enabled localizers to keep track of their locale's progress and receive any notifications from Pontoon directly in the browser. As part of the Pontoon API rework mentioned earlier, we needed to refactor the add-on code in order to maintain functionality. The main contributor had limited availability to continue maintaining the add-on anymore, so Mozilla assumed ownership of the extension, and we managed to clean up some of the backlog and redeploy.

Firefox for Android localization workflow changes

For years, Firefox Desktop and Firefox for Android used to operate under different localization workflows. Firefox for Android's workflow was written such that Android developers needed to maintain deprecated strings to prevent previous versions of localizations of Firefox for Android breaking. Firefox Desktop does not have this issue due to different logic, so I rewrote the whole Firefox for Android workflow to be closer in substance to Firefox Desktop's localization process. This subsequently deprecated the need for Android engineers to maintain the string deprecation process needed, which was great.

Insights Project Manager Dashboard and Alert system

The last main feature set I worked on was the Insights dashboard and accompanying alert system. For context, the Project Managers on the L10N team needed to calculate a score assigned to locales each month for community health analysis purposes using a mish mash of spreadsheets and Python scripts. This project automated the capture and calculation of these statistics to the first of each month and displayed them on a dashboard along with other useful information. The alert system was also implemented which notifies admins of significant changes of scores for specific locales, so certain locales can get increased focus.

Insights dashboard, which enables our Project Managers to track locale health over time.

Final Thoughts

Working at Mozilla has undoubtedly been an enriching experience. I have met a great number of incredible people who have guided me, encouraged me and shaped me into a better engineer and a better person. It has certainly widened my perspective on localization as a global effort, where peoples across different languages, cultures and creeds can work together to build a better web. I leave with sharper technical skills, amazing memories, and zero regrets, only immense gratitude for the opportunity to contribute to software that empowers millions. 🙂

Acknowledgements

To Matjaž (engineer and mentor): Your presence has been invaluable, both as a mentor and a friend. Thank you for your patience with my 10 PM deployment requests, endless questions, and edge-case bugs.

To Flod (manager): You are as always the backbone of the team, and have certainly taught me many lessons regarding leadership, professionalism and dependability. Thank you for reminding me about my deadlines and being the go-to for questions and feedback.

To Eemeli (engineer): Your Zoom call escapades have always been inspiring to me, from burning detritus, cooking random stuff, remote data lag spikes and such much more. I truly wish to become like you with such a vast and bottomless knowledge for so many facets of software engineering, before AI completely rots my brain.

To Ayush (fellow intern): I hope my mentorship was as helpful to you as your camaraderie was to me. You are going to accomplish great things, my friend.

To Eric (engineer): I very much enjoyed your unique German humor, your kindness and passion for building cool technologies.

To Camila (PM): Your personality is incredibly energetic, and I'm sure you will continue to bring so much positive energy to the team. I hope that you continue to be ambitious in everything that you do.

To Eda (PM): I really hope you enjoy working as part of the L10N team. I hope that you continue to make an excellent contribution to Mozilla!

Thank you for reading about my time at Mozilla! If you would like to learn more or wish to connect, feel free to contact me on LinkedIn.

Fin.

31 Aug 2026 6:07pm GMT

The Servo Blog: July in Servo: more platforms, faster canvas, web fonts in SVG, and more!

Servo 0.5.0 contains all of the changes we landed in July, which came out to 488 commits, and we now publish binaries for Linux aarch64 (@mukilan, #46760)!

DOM text selections are now visible (@mrobinson, @SimonSapin, #46698, #46864, #46742, #46889, #46126). Interactive selection is coming soon!

For security fixes, see § Security.

servoshell 0.5.0 showing several new features: ‘text-decoration-thickness’, ‘box-decoration-break’, web fonts in inline `<svg>`, and DOM selection rendering

We've shipped several new web platform features:

Plus a bunch of new DOM APIs:

This is another big update, so here's an outline:

You can help!

If you're working on a pull request that you think might be interesting for the next monthly update, even if you're not 100% sure, tell us about it by following the steps below:

  1. You add the monthly update label to your pull request, or comment @servo-highfive monthly update

  2. Highfive posts a comment asking you some questions

  3. You answer those questions in a comment containing @servo-highfive monthly update answer

Security

Servo was potentially affected by vulnerabilities in quick-xml and crossbeam-epoch that have been fixed in Servo 0.5.0 (@atouchet, @Loirooriol, #46737, #46324). For more details, see RUSTSEC-2026-0194, RUSTSEC-2026-0195, and RUSTSEC-2026-0204.

We've updated ANGLE from a version based on Firefox 115.x ESR (02755361e26d8) to a version based on Firefox 140.12.0 ESR (f8025617e815f), which likely includes many security fixes (@jschwe, @sagudev, #46455, mozangle#100).

Real world compat

The duck on the DuckDuckGo (duckduckgo.com) landing page now renders in v0.5.0, after we fixed a preload bug that affected SVG images (@jdm, #46668).

v0.4.0
v0.5.0
duckduckgo.com

Most of Gumroad (gumroad.com), except for the landing page, did not render at all in v0.4.0, but as of v0.5.0, pages like the Discover page or this product page render almost perfectly.

v0.4.0
v0.5.0
gumroad.com
v0.4.0
v0.5.0
gumroad.com

We're interested to hear how well your favourite websites run in Servo! Report successes in this Zulip thread, and failures in our GitHub issues.

Work in progress

The upgrade to Stylo 2026-07-01 brings several changes to built-in CSS functions (@Loirooriol, #46129):

Web­GPU content can now enjoy better conformance and use GPU­External­Texture and import­External­Texture() on GPU­Device, under --pref dom­_webgpu­_enabled (@sagudev, #45873, #46178, #46286).

servoshell 0.5.0 showing four tilted rectangles (in WebGPU), textured with a picture of a dog (which is a frame from an external video)

Indexed­DB content can now use the name property on IDB­Index, under --pref dom­_indexeddb­_enabled (@skyz1, #45512).

document.fonts now includes a Font­Face for each valid '@font-face', under --pref dom­_fontface­_enabled (@simonwuelker, #46509, #46537).

All of the features above are enabled in servoshell's experimental mode.

We've started implementing WebVTT for native subtitles and captions, enabled by default (no --pref). While they don't render just yet, we can now fetch each <track src>, parse the WebVTT, and expose cues via the track property on HTML­Track­Element (@TimvdLippe, #46289, #46383).

July was a big month for accessibility in Servo, under --pref accessibility­_enabled. The focus for this month has been on performance, with the accessibility tree now supporting incremental updates (@alice, @delan, #45578, #45971, #46589, #46691, #46385), requiring fewer HashMap lookups and tree walks (@alice, @delan, #45798, #46740, #46348), and allowing for faster DOM mutations (@alice, #46348, #46530).

We've also started working on the File and Directory Entries API, to allow users to select and upload entire directories via <input type=file> and drag-and-drop. To that end, we now have webkit­Get­As­Entry() on Data­Transfer­Item, plus minimal support for File­System­Entry, File­System­Directory­Entry, and File­System­File­Entry, under --pref dom­_entries­_api­_enabled (@yezhizhen, #46456, #46879, #46832).

Embedding API

We've improved the docs for the servo crate, and for Web­View­Delegate (@mukilan, #46193).

Breaking change: Servo­Builder::webxr­_registry() has been removed. Instead use the new Servo::register­_webxr­_registry, which is a lazy design that has allowed servoshell to halve its startup time (@Narfinger, #46494).

For users and developers

servoshell for Android now runs on Android 10+ (91% market share), not just Android 13+ (68% market share), improving adaptability and reducing waste (@jschwe, #46142, #46308). We've also fixed a problem with building for Android on macOS (@jschwe, #46128).

servoshell for Windows is now better behaved when run in a console window, making the command prompt wait until servoshell exits (@yezhizhen, #43010).

When using the Firefox DevTools, the Console tab now supports some basic autocomplete (@freyacodes, #46382).

We've finished modernising servoshell for Android to use Compose UI (@veyndan, #46085, #46164, #46253, #46257, #46317, #46353, #46565, #46612, #46626, #46666, #46663, #46700), and we're now migrating Servo as a library to use Kotlin (@veyndan, #46817, #46895, #46772).

More on the web platform

Inline SVG can now use web fonts defined in the containing page (@yodalee, #45979). We're also implementing the SVG DOM, starting with stub interfaces for SVG­Element, SVG­Circle­Element, SVG­Defs­Element, SVG­Ellipse­Element, SVG­Line­Element, SVG­Linear­Gradient­Element, SVG­Path­Element, SVG­Polygon­Element, SVG­Polyline­Element, SVG­Radial­Gradient­Element, SVG­Stop­Element, SVG­Rect­Element, SVG­Symbol­Element, and SVG­Use­Element (@mu-mostafa98, #46558).

<button> now vertically centers its contents (@Loirooriol, @mrobinson, #46590), and behaves better with 'display: block' and 'display: inline' (@Loirooriol, #46536).

We've improved the conformance of <form> without <form action> (@kevlu93, #46860), <color> values (@Loirooriol, #46129), Gamepad­Event (@log101, #46788), document.execCommand("delete") (@Psychpsyo, #46539), the selector­Text property on CSS­Style­Rule (@simonwuelker, #46687), and Set Window Rect in WebDriver (@janeoa, #46475, #46477).

We've fixed bugs related to <iframe> (@jschwe, @jdm, #46587), <img> (@yodalee, #46892), <textarea> (@SimonSapin, @mrobinson, #46309), custom properties (@Loirooriol, #46129), '::before' and '::after' (@Loirooriol, #46640), 'flex-direction: column' (@simonwuelker, #46697), 'float' (@Loirooriol, @mrobinson, #46407, #46500, #46505), '@font-face' (@simonwuelker, #46568, #46271, #46436), 'position: absolute' (@simonwuelker, #46358, #46637), Blob (@jdm, #46881), IDB­Database and IDB­Object­Store and IDB­Index (@mrobinson, #46615), the adopted­Style­Sheets property on Shadow­Root (@simonwuelker, #46738), delete() on Font­Face­Set (@simonwuelker, #46634), move­Before() on Element (@mrobinson, #46599), resize­To() on Window (@janeoa, #46477), the selected property on HTML­Option­Element (@rhit-kapilaar, #46386), and the value property on HTML­Select­Element (@simonwuelker, #46230).

Performance and stability

2D canvas rendering is now multithreaded, improving frame rates by up to 55% and power consumption per frame by up to 42% (@yezhizhen, #46410), and should use a lot less memory too (@jschwe, @sagudev, #46786).

Text rendering is up to 10x faster for cases with the same text and different 'font-size' (@Loirooriol, #46129).

Flex layout benchmarks are up to 3% faster, and an improvement to get­Elements­By­Class­Name() has made some websites up to 1% faster (@Narfinger, @jdm, #46563, #46595, #46594).

We've also reduced memory usage, allocations, GC rooting steps, and other operations in many parts of Servo (@mrobinson, @jdm, @yezhizhen, @Narfinger, @Gae24, @SimonSapin, @Taym95, @cychronex-labs, @arayaryoma, #46499, #46411, #46659, #45974, #46377, #45758, #46440, #46762, #46301, #46349, #46419, #46418, #46420, #46460, #46633, #46638, #46690, #46745, #46726, #46564, #46144, #46664, #46462, #46139, #46430, #46446, #46498, #46548, #46598, #46632, #46656, #46678, #46718, #46722, #46238, #46072, #46408, #46438, #46437, #46528, #46124, #46330, #46412, #46807).

We've fixed a crash regression with memory corruption (@mrobinson, #46316), several dynamic-borrow-related crashes (@Narfinger, @SharanRP, @Taym95, @agrawalx, @amittenak47, @sungmen, #46381, #46384, #46405, #46684, #46452, #46770, #46830, #46763), plus crashes related to:

We've continued our long-running effort to use the Rust type system to make Servo's integration with SpiderMonkey safer and more reliable (@Gae24, @Narfinger, @TimvdLippe, @jdm, @kunalmohan, @lumiscosity, @simonwuelker, #46191, #46777, #46890, #46243, #46248, #46246, #46310, #46312, #46333, #46147, #46150, #46151, #46229, #46262, #46375, #46374, #46529, #46584, #46585, #46593, #46693, #46166, #46156, #46254, #46267, #46268, #46269, #46270, #46284, #46285, #46318, #46435, #46461).

New contributors

A special thanks to the following people for landing their first patch in Servo:

Interested in helping build a web browser? Take a look at our curated list of issues that are good for new contributors!

Donations

Thanks again for your generous support! We are now receiving 7824 USD/month (+1.8% from June) in recurring donations. This helps us cover the cost of our speedy CI and benchmarking servers, one of our latest Outreachy interns, and funding maintainer work that helps more people contribute to Servo.

Servo is also on thanks.dev, and already 35 GitHub users (same as June) that depend on Servo are sponsoring us there. If you use Servo libraries like url, html5ever, selectors, or cssparser, signing up for thanks.dev could be a good way for you (or your employer) to give back to the community.

We now have sponsorship tiers that allow you or your organisation to donate to the Servo project with public acknowlegement of your support. If you're interested in this kind of sponsorship, please contact us at join@servo.org.

7824 USD/month
10000

Use of donations is decided transparently via the Technical Steering Committee's public funding request process, and active proposals are tracked in servo/project#187. For more details, head to our Sponsorship page.

31 Aug 2026 12:00am GMT

27 Aug 2026

feedPlanet Mozilla

Firefox Tooling Announcements: MozPhab 2.18.0 Released

Bugs resolved in Moz-Phab 2.18.0:

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

1 post - 1 participant

Read full topic

27 Aug 2026 6:43pm GMT

26 Aug 2026

feedPlanet Mozilla

Serge Guelton: Pros and Cons of Unified Build

Unified builds (also know as Jumbo Builds) is a build techniques that aims at improving build time through the concatenation of several sources as a single unified source before compilation.

The goal is obtained through implicit caching of header instantiation, although it implies a trade-off with parallelism.

Let's illustrate this behavior through a simple example, two codes that implement variation of the same approach:

/* algo0.cpp */
#include <iostream>
#include <string>
#include <vector>
void translate(std::vector<std::string>& w, void (&t)(std::string&));
void translate(std::vector<std::string>& w_out, std::vector<std::string> const & w_in, void (&t)(std::string&)) {
    std::cout << "[log] through transform\n";
    w_out = w_in;
    translate(w_out, t);
}

/* algo1.cpp */
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
void translate(std::vector<std::string>& w, void (&t)(std::string&)) {
    std::cout << "[log] through for_each\n";
    std::for_each(w.begin(), w.end(), [&t](std::string& s) { t(s); });
}

Compiling individual files take the following times:

% hyperfine --warmup 5 "/usr/bin/clang++ -O2 algo0.cpp -c"
Benchmark 1: /usr/bin/clang++ -O2 algo0.cpp -c
  Time (mean ± σ):     267.7 ms ±   7.4 ms    [User: 234.6 ms, System: 30.4 ms]
  Range (min … max):   259.7 ms … 277.3 ms    11 runs

% hyperfine --warmup 5 "/usr/bin/clang++ -O2 algo1.cpp -c"
Benchmark 1: /usr/bin/clang++ -O2 algo1.cpp -c
  Time (mean ± σ):     173.6 ms ±  46.3 ms    [User: 149.6 ms, System: 22.1 ms]
  Range (min … max):   130.4 ms … 231.6 ms    13 runs

Creation of the unified file is just a matter of invoking cat, let's benchmark the compilation of the unified source:

% cat algo{0,1}.cpp > unified_algo.cpp
% hyperfine --warmup 5 "/usr/bin/clang++ -O2 unified_algo.cpp -c"
Benchmark 1: /usr/bin/clang++ -O2 unified_algo.cpp -c
  Time (mean ± σ):     223.8 ms ±  64.5 ms    [User: 193.0 ms, System: 28.4 ms]
  Range (min … max):   160.8 ms … 301.8 ms    10 runs

In that simple case, the weight of headers with respect to actual user code is such that compilation of the unified file takes almost the same time as the max compilation time among each individual file. That's roughly a 1.97x speedup on compilation time.

That's the promise given by unified builds. And it's a promise held.

Now let's have a look at the consequences of that deal.

Beforehand, we still need to introduce another parameter tied to unified builds: the unification parameter, say P. That parameter bounds the number of files that are unified together. Let's imagine we have a hundred of individual source files compiled with exactly the same compilation flags. Setting P to 5 leads to the generation of 20 unified sources compiled independently.

Remember the parameter P.

Quality of the Generated Code

Let's create a shared object from algo{0,1}.o (this implies a recompilation with -fPIC of the sources):

% /usr/bin/clang++ -O2 algo0.cpp -c -fPIC
% /usr/bin/clang++ -O2 algo1.cpp -c -fPIC
% /usr/bin/clang++ -shared algo{0,1}.o -fPIC -o algo.so

And do the same from unified_algo.o:

% /usr/bin/clang++ -O2 unified_algo.cpp -c -fPIC
% /usr/bin/clang++ -shared unified_algo.o -fPIC -o unified_algo.so

After stripping, comparing the size of the binaries yield a difference of a few bytes. After disassembling, it turns out the compiler decides to inline the call to void translate(std::vector<std::string>& w, void (&t)(std::string&)) from algo0.cpp when compiling the unified source, something the compiler cannot do when doing split compilation, as it does not know anything about the implementation of that function.

Interestingly, compiling with -flto=thin still lead the compiler instantiation through different optimization path.

Falling back to -flto=full finally yields to the same shared object, which makes sense because Full LTO is very close to performing source unification at the bytecode level and our sources are very simple. It's not a given though because the actual optimisation pipeline is still different in the two scenario.

Why does it matter? Depending on the value of P, the compiler will see different sets of files per unified file, which will result in different binary code. It's actually even worse: depending on the way we fill those unification sets, event with the same parameter P, we end up with different binaries. Let's call that the reunifying problem.

Even if we have an algorithm that seems to guarantee reproducibility, for instance working on a sorted list of files with a fixed P, variation can arise: the introduction of a new source file can lead to changes in every unified file (e.g. if the split is done by chunks and the new file ends up at the beginning of the file list).

So unified builds tend to improve performance, but they do not interact in a gentle way with performance reproducibility.

Recompilation Times

Let's denote S as the number of sources and C as the number of CPUs.

Intuitively, setting P=1 yields to the faster recompilation time when a single file is touched---a usual scenario when developing a new feature.

On the opposite, setting P=S yields to the slower recompilation time (if S >> C!) under the same scenario as all sources are recompiled under that scenario.

The form of the curve between those two extreme varies depending on the nature of the files, and the amount of header sharing between individual sources.

Caching tools like sccache is impacted by the same mechanism: as P gets greater, more cache misses are hit and more recompilation are done.

Marginally, introducing a new source also pollutes the cache or triggers recompilation for the unified source it gets added to, and eventually for all the unified sources derived from the associated file list. The reunifying problem strikes again.

So unified build make compilation faster, but recompilation slower. Setting P to an acceptable value is important depending on the usage scenario.

Correctness

Unified build changing the compilation unit frontier, which in turns modifies the semantic of the program. This change can be straight-forward or complex to debug, and even remain silent. I've listed a few instances of the two first categories below, and a crafted one for the latter category.

Macro / Symbol Redefinition

This one is trivial to spot (a preprocessor-warning is issued for the macro, and a compiler error is issued for the symbol redefinition):

/* pi0.cpp */
#define PI 3.141593
constexpr double pi() { return 3.141593; }

/* pi1.cpp */
#define PI 3.14159265
constexpr double pi() { return 3.14159265; }

The solution usually lies in moving the definition in a shared header, moving the declaration in a shared header and the definition in a single file, or renaming identifiers to avoid the name conflict. Note that depending on the solution we may change the visibility of the symbols, or impact code readability (assuming the identifier name was perfectly chosen in the first place).

Overload Conflicts

This one is also trivial to spot and may hint toward debatable design. But it exists and may be more complex to understand than the above:

/* overload0.cpp */
static float doit(float f) { return f;}
const float f = doit(1);

/* overload1.cpp */
static double doit(double d) { return d;}
const double d = doit(1);

The fix is generally to provide a perfect match for the overload, change the call site to avoid the ambiguity, or rename the functions/change their namespace to make the call site explicit.

Using Namespace Confusion

This one tends to creep a lot in codebase where using namespace is used. It generates ambiguity among potential symbols.

A caricatured situation is exhibited with the following situation:

/* using.h */
#pragma once
namespace a {
    namespace a {}
}

/* using0.cpp */
#include "using.h"
using namespace a;

/* using1.cpp */
#include "using.h"
using namespace a;

Once using{0,1}.cpp unified, the second using namespace a; directive is ambiguous.

A more realistic (but similar in spirit) situation arises when the same symbol is defined in different namespaces:

/* namespace0.cpp */
namespace a0 {
    int var;
}
using namespace a0;
int foo = var;

/* namespace1.cpp */
namespace a1 {
    int var;
}
using namespace a1;
int bar = var;

The problem with that category is that the fix is quite unsatisfying: there is no way to limit the scope of a using directive, removing using directive can lead to very verbose codebase, renaming symbols to avoid conflicts goes against the very purpose of namespaces...

Delicatessen

I spent a lot of time nailing that one down, so I wrote a small reproducer to illustrate the problem.

% tail -n +1 *.h *.cpp
==> header0.h <==
#ifndef H0
#define H0
namespace mozilla::dom {

class Lock final {};

}
#endif

==> header1.h <==
#ifndef H1
#define H1

#include "header0.h"

class Lock {};

class AutoUnlock {
    Lock *lock_;
};
#endif

==> src0.cpp <==
#include "header1.h"

==> src1.cpp <==
#include "header0.h"

==> src2.cpp <==
namespace mozilla::dom {};
using namespace mozilla::dom;
using namespace mozilla;


==> src3.cpp <==
#include "header1.h"

Let me comment that layout a bit: We basically have two different classes named Lock: one lives in the mozilla::dom namespace, and one lives at top-level. In header1.h, although we include the definition of mozilla::dom::Lock, we also get the definition of ::Lock, so a straight reference to Lock is not ambiguous.

Concerning source files, src0.cpp, src1.cpp and src3.cpp just include headers while src2.cpp contains the infamous using namespace modilla::dom; statement.

Let's now consider various partition of the file list src0.cpp, src1.cpp, src2.cpp, src3.cpp:

% for perm in 0,1 2,3 0,1,2 1,2,3 0,1,2,3; do printf "unifying $perm... " ; cat `eval echo src{$perm}.cpp` | clang++ -xc++ - -fsyntax-only 2>/dev/null && echo ok || echo ko ; done
unifying 0,1... ok
unifying 2,3... ko
unifying 0,1,2... ok
unifying 1,2,3... ko
unifying 0,1,2,3... ok

Isn't that amazing? Some intermediate unification, namely 0,1;2,3 and 0;1,2,3 fail, but other unifications, namely 0,1,2,3 and 0;1,2,3 fail. Did you notice that both non-unified and full unified build succeeds, while some intermediate unification fail? What a disaster. This basically mean that given a set of sources, and without putting restriction on the language (like banning using statement), the only way to be sure that a unified build always succeeds whatever the chosen partition is to test every partition. Not very satisfying.

As a side effect, we can also deduce that adding a new source file to a set of files to be unified can break compilation in files that used to compile fine. That's another instance of the reunifying problem.

Changing Semantic

It is quite easy to derive from the above an example whose semantic change once unified. Let's slightly change the overload conflict example from above:

/* silent0.cpp */
#include <cstdio>
static int doit(int f) { putchar('0'); return f;}
const int f = doit(1);

/* silent1.cpp */
#include <cstdio>
static double doit(double d) { putchar('1'); return d;}
const double d = doit(1);

When compiled independently, this results in a binary that prints a 0 and a 1 on the screen. But when compiled as a unified source, we only get a pair of 0.

Concluding Words

Remember that discussion between Luke and Yoda?

LUKE Vader. Is the dark side stronger?

YODA No… no… no. Quicker, easier, more seductive.

That's exactly my thoughts on unified builds: they give you quick wins in term of cold build speed and give faster builds. That's very good properties, and you rip the benefit of them very quickly. Then you realize that you're tied to a monster in terms of maintainability and developer experience, but you're already addict to the speed it gave you.

26 Aug 2026 10:00pm GMT