How Meta’s Engineers Shifted a Billion-User Codebase from C to Rust

When Meta began exploring the use of Rust within critical systems such as messaging infrastructure and backend components, they turned to Rust to replace parts of a sprawling C and C++ codebase. The transition, as discussed across Meta engineering talks and in various open-source and internal migrations, provides a detailed look at how large-scale system engineering evolves when safety, maintainability and developer velocity become as important as raw performance.

In this article, we walk through the motivations, architecture, migration strategy and practical lessons from the engineering team’s point of view.

Why the Migration: From C to Rust

The legacy C/C++ codebase had served Meta’s mobile stack well for years—compact binaries, tight integration via JNI (Android), Objective‑C/Swift and Core Foundation (iOS), minimal runtime overhead. But the team identified mounting problems:

• Manual memory management creeping into dozens of modules, making refactoring risky. Engineers described allocations at the top of a file and frees hundreds of lines later.
• Developer productivity suffering: error-prone code, lengthy on-call debugging, opaque ownership semantics. As one engineer put it: “Spaghetti begets spaghetti.”
• Maintainability and onboarding costs rising as the codebase grew and new features (e.g., AR/VR) demanded new integrations.
• Rust offered compelling advantages: memory-safety via ownership and borrowing, strong compile-time checks, modern toolchain (rustfmt, rust‐analyzer), improved developer ergonomics.

In short, the migration was driven by safety, velocity, and long-term maintainability and not simply rewriting for the sake of novelty.

Architecture & Strategy for Migration

Migrating a library used by “billions of users every day” requires careful architectural planning. Meta’s engineers adopted a gradual, incrementally safe strategy rather than a big-bang rewrite.

Key elements of their strategy:

• Library-level replacement: The target was a central messaging library (shared across Messenger, Instagram, Facebook, and more) rather than the full application stack. This scoped the migration to one module with well-defined boundaries.
• Interoperability with C: Since apps still depended on existing C APIs, the new Rust components had to interface seamlessly via FFI / C bindings. That allowed incremental rollout.
• Gradual replacement and dual build support: Both C and Rust variants were supported during the transition phase, with careful runtime feature flags to switch to Rust when verified.
• Component-by-component rewriting: Engineers prioritized hot-paths and modules with the highest risk (e.g., memory-unsafe routines) first. Lower-risk modules stayed in C until engineered out.
• Developer experience uplift: Beyond just safety, the team placed emphasis on using Rust tooling (cargo, rustfmt, clippy) to make developer workflows smoother and more standardized.

Technical Challenges & Solutions

Transitioning to Rust at this scale presented multiple technical hurdles; below are the key ones and how the engineers addressed them.

1. Memory Safety vs. Legacy Code Interactions

Legacy C code contained manual allocations, pointer arithmetic, and implicit ownership. The Rust shift meant:

• Defining clear ownership/borrowing boundaries when wrapping C APIs.
• Isolating unsafe Rust blocks and encapsulating them behind safe abstractions.
• Writing FFI shims so that Rust exposes C-compatible APIs for the rest of the system.

2. Performance Sensitivity

Mobile applications often operate under tight constraints: binary size, startup latency, memory footprint. To ensure the migration maintained performance, the team:

• Benchmarked critical functions in C and in Rust, iterating until Rust matched or exceeded performance.
• Applied Rust’s #[inline], zero-cost abstractions, and careful allocation avoidance (e.g., using no_std where possible, stack-allocations, avoiding Box in hot loops).
• Maintained compatibility with platform-specific runtime constraints (iOS vs Android) and ensured that binding layers did not add undue overhead.

3. Tooling and Developer Onboarding

Many engineers had little prior Rust experience. To smooth adoption, they:

• Used internal training and pair-programming sessions to build Rust fluency.
• Developed linters, formatters, and code review enforce rules (e.g., cargo fmt for formatting, rust-analyzer for real-time feedback).
• Cultivated a “Rust first” mindset for new modules, while still permitting legacy C in stabilized code until rewritten.

4. Testing, Verification, and Roll-out

Safe migration demands rigorous testing. Meta’s team emphasized:

• Extensive unit and integration tests in Rust modules, with cross-language integration tests for C/C++ interop.
• Canary rollout strategies: gradually shifting user traffic to the Rust version while monitoring crash rates, latency, and regressions.
• Feature-flagging so the Rust module could be disabled or switched back if issues arose in production.

Developer Experience and Team Dynamics

One of the often-under-appreciated benefits the team cited was developer happiness and productivity. Rust’s strong compile-time checks meant fewer runtime surprises, and the enforced structure encouraged code modularity.

• Code review feedback became faster because many memory and concurrency errors were eliminated by the type system.
• New engineers onboarding the messaging library found it easier to reason about ownership and lifetimes thanks to Rust’s explicit semantics.
• The transition instilled discipline: uniform formatting, clearer API boundaries, and improved documentation became standard.

These enhancements helped shift the team’s focus from firefighting bugs to building features—a critical shift for large-scale infrastructure.

Lessons Learned & Best Practices

Engineers and architects can apply several lessons from Meta’s migration journey:

1. Scope your rewrite wisely.

Target a well-defined module or library where the payoff is high (e.g., memory-unsafe routines). Avoid trying to rewrite the entire stack in one go.

2. Support interoperability.

Ensure smooth FFI between old and new components. Maintaining dual support (C + Rust) during transition avoids service disruption.

3. Measure performance from day one.

Rust provides safety but may require tuning to hit performance targets. Benchmark early, optimize aggressively, and treat binary size, latency and memory as first-class metrics.

4. Invest in UX for engineers.

Developer productivity pays long-term dividends. Provide training, tools, and enforce standards (formatting, linting) to get the team on board.

5. Feature-flag and monitor rollout.

Use gradual rollouts with telemetry on crash rate, latency, memory usage and developer build times. Be prepared to revert quickly if anomalies appear.

6. Emphasize documentation and conventions.

Migrated modules should clearly document ownership semantics, FFI boundaries, and migration status (legacy vs Rust). That clarity helps future maintainers.

Why This Migration Matters for System Architects

For system architects managing large-scale mobile or embedded codebases, Meta’s evolving experimentation with Rust adoption signals an inflection point in infrastructure design:

• Safe languages like Rust are now viable not only for greenfield projects but for massive heritage stacks with high user reach across multiple apps.
• The trade-offs (safety vs performance, rewriting cost vs long-term maintainability) are increasingly favourable toward migration when the legacy codebase accumulates risk and complexity.
• Developer experience matters more when scale increases: per-engineer productivity becomes a lever that multiplies across thousands of engineers and millions of builds per day.

Potential Pitfalls and How to Avoid Them

No migration path is free of risk. Teams should watch out for:

• “Rust-fear” backlog: Developers reluctant to switch may stall migration. Strong leadership and training help.
• Hidden C dependencies: Legacy modules may rely on subtle invariants or undocumented behavior making replacement harder. Empirical profiling and instrumentation help uncover them.
• Binary size leakage: careless use of crates or features can inflate mobile binaries—monitor size budgets rigorously.
• Toolchain turbulence: Rust ecosystem evolves quickly; lock crate versions and review dependencies for stability.
• Team fragmentation: Don’t split C and Rust teams too early; maintain shared ownership and coherent review practices to avoid divergence.

Looking Forward

Meta’s work with Rust, alongside other large-scale platform teams highlights how large organizations can adopt modern systems languages without rewinding decades of investment. For other teams working on high-scale systems like mobile apps, IoT firmware, embedded SDKs, the lesson is clear: migrating legacy modules to safer, more maintainable languages is a strategic engineering practice, not a novelty.

As Rust ecosystem matures further and toolchains stabilize, the cost of migration will continue to drop. For architectures that prioritize reliability, maintainability and developer velocity, the window for rewriting is now.

Meta’s migration from C to Rust for its billion-user mobile messaging library offers a blueprint for engineering at scale: one where memory safety, developer experience and long-term maintainability take precedence alongside performance. Their phased strategy, tooling investment, and architectural discipline set a strong example for system engineers and architects.

For ARTIBA’s audience of AI engineers and infrastructure specialists, the core takeaway is this: when legacy code hinders innovation, rewriting modules into safer, modern languages can become a scalable engineering lever but only if approached with pragmatism, measurement and strong developer support.

This article is part of ARTiBA’s series on real-world engineering transformations in large-scale systems.