Why Meta’s Billion-User Apps are Switching from C to Rust

When Meta’s popular apps, including Instagram and Facebook Messenger, utilised a decade-old C codebase for their messaging library, it led to problems with memory management and a subpar developer experience. 

The company eventually switched to Rust, one of the most popular languages today. 

In an episode of Meta Tech Podcast, three of the company’s engineers – Eliane W, Buping W, and Pascal Hartig shared their experience with using C, discussing the challenges faced, the migration process, and their success with Rust. 

The Problems With C

The company chose C for its speed, compact binaries, and seamless integration on iOS via Core Foundation and Android via JNI. Moreover, it helped avoid the extra overhead and complexity of C++ runtimes.

But as the codebase grew and more engineers contributed, C’s limitations became increasingly painful bottlenecks. It brought several memory management issues, resulting in a poor user experience that led to crashes. 

The most severe issues weren’t just inconveniences; they were classified as Site Events, Meta’s term for production incidents, demanding weeks of intensive debugging from multiple engineers. Manual memory management led to crashes that were “extremely hard to debug”, often stemming from use-after-free bugs or double-free errors that only surfaced under specific conditions.

“It took maybe half of the people working on this whole big project to really spend weeks to debug that,” Buping, who has been at Meta for 11 years, pointed out. 

C’s absence of modern abstractions led to a development process that hindered productivity. Engineers had to follow error-prone patterns, declaring and initialising potentially hundreds of variables at the start of functions, then manually tracking and freeing each one, often over a thousand lines later.

This pattern forced engineers to scroll through massive functions just to match allocations with deallocations, making code reviews a nightmare and bugs inevitable. 

“We have this terrible practice where you allocate all the variables at the beginning of a do-while loop, and then, 1,000 lines later, when you’re done using them, you release all of them,” Elaine explained. 

Besides, Meta’s open engineering culture, where anyone can modify any codebase, amplified the weakness of C. 

Without modern language features like interfaces or strong type systems, engineers couldn’t create clean abstractions that prevented code pollution.

New contributors would add logging, error handling, or feature flags directly into existing functions, creating increasingly unmaintainable code.

Meta’s transition to Rust systematically tackled these issues while upholding the performance and portability needs of their mobile infrastructure. 

How Rust Eliminated These Challenges

Rust’s ownership system prevents entire types of memory bugs at compile time. The borrow checker avoids use-after-free, double-free, and data races before code reaches production, eliminating bugs that caused Meta’s most serious incidents.

“I think one of the biggest things about Rust is the compile-time memory safety enforcement. C++ doesn’t have that. Since a lot of the issues that we face in the day-to-day are related to memory management, it doesn’t make sense that we wouldn’t choose a language that combats that most effectively,” Elaine added. 

Moreover, Rust’s tooling ecosystem provides the necessary scaffolding for large-team development through an integrated suite of developer tools that work seamlessly together. 

For instance, Rust analyser provides real-time type hints, error feedback, and autocompletion features. “Autocompletion has been great, especially with the AI-based version. You just don’t have to write a lot of code manually nowadays,” Buping said. 

In addition, rustfmt automatically formats code on save, eliminating style debates that often plague large engineering teams. 

Besides, Rust also helped the company not only solve memory safety problems but also establish cleaner, more maintainable coding standards across its team. 

“C++ has a lot of bloat that you can potentially add to, which is very easy to abuse. Everybody has different experiences with C++ and has preferred ways of doing things. It becomes harder to make things standardised,” Elaine said, indicating why Meta chose Rust over other languages like C++.

“So I think Rust being relatively newer, we’re all kind of on an even playing field, and we can all kind of work together to decide what the standard should be, which makes it a lot easier to manage that kind of bloat,” she added. 

Moreover, when Meta initially started that project, there were multiple variations of the errors. Teams had to address a Windows-specific issue, an Android emulator problem, an Apple Silicon issue, or even a Linux problem. However, with Rust, the company could apply all the new code to all the same platforms where their existing C code runs.

“I think we’ve come to a point where we’ve pretty much overcome all those kinks, and we can get to a point where you can just land the Rust code everywhere our code is linked, which is pretty much also everywhere,” Buping explained. 

The debugging experience was also remarkably robust, working seamlessly across C and Rust boundaries, where breakpoints, stack traces, and variable inspection all functioned reliably.

“Basically, can you put a debugger and can you put a breakpoint and have the debugger pause when the breakpoint gets hit? Can you see the stack trace? Can you see the values of the variables? All of those supports are there, which is very good for us,” Buping added. 

For teams managing legacy C/C++ codebases, Meta’s migration demonstrated that modern systems languages can deliver both improved developer experience and production reliability. While Meta’s incremental migration is still ongoing, the early results have been promising for the company.

The post Why Meta’s Billion-User Apps are Switching from C to Rust  appeared first on Analytics India Magazine.

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