Meta has begun rewriting its mobile messaging infrastructure in Rust, gradually replacing a legacy C codebase that engineers say had become increasingly difficult to maintain and frustrating to work with. In episode 76 of The Metatech Podcast, members of Meta’s Messaging Infrastructure team outlined their motivations for the transition, citing memory safety, developer happiness and long-term maintainability as the main drivers.
The library at the center of the effort, ships in every Facebook, Messenger, Instagram, Instagram Lite, VR-headset and wearable build, touching billions of users each day.
Developers describe the old C runtime in terms of functions that stretched hundreds of lines and manual memory bookkeeping: variables were allocated at the top of a file and freed a thousand lines later, and even small refactors felt perilous.
Spaghetti begets spaghetti
…Meta software engineer Elaine quipped, capturing a broken-windows effect in which messy code encouraged more mess. Memory-management mistakes occasionally slipped into production and escalated into hard-to-debug on-call incidents.
Rust’s compile-time ownership checks remove entire classes of those errors, but the team emphasizes day-to-day happiness as much as safety. Cleaner semantics, deterministic formatting with rustfmt, and real-time feedback from Rust-Analyzer allow easier iteration and faster feedback. Performance still matters, yet the driving metric has shifted to developer velocity and confidence.
The learning curve for Rust can often be seen as daunting. Most of the engineers tackling the rewrite arrived with little or no Rust background—Elaine jokes she only knew “the crab logo” and later dreamt about the move keyword. To support this transition, the team leaned on One-on-one walkthroughs and patient code reviews to speed up the onboarding.
Meta’s open-code culture also helped: posting questions to specific Rust working-groups brought expert answers, turning a steep learning curve into a shared adventure rather than a solitary hurdle.
Tooling improvements have carried over to operations. Today an engineer can set a breakpoint in a mixed C/Rust stack and watch the debugger hop seamlessly into Rust frames, with fully symbolicated mobile crash logs—support that didn’t exist just months ago.
As happier workflows, faster feedback, and safer refactors take hold, Engineers described feeling more confident making changes, with engineer Buping remarking that Rust’s compile-time checks made it easier to identify and remedy broken code.
The Rust working group has attracted engineers across the organization who are motivated to productionize Rust on mobile. While the long-term roadmap isn’t spelled out, early signs of internal interest suggest a growing appetite for adoption.
Meta’s team felt it was too early to quantify time savings from the migration, but they can take encouragement from others further along the path. Cloudflare reports faster, more reliable development and code that’s easier for engineers to reason about. Google reached a similar conclusion in its shift from C++, noting that contributors required less effort to write, review, and build code in Rust. Together, these examples highlight how developer experience, not just raw performance, is becoming a decisive factor in language and tooling migrations.
