Migrating C/C++ Projects to Rust: A Practical Guide to Incremental Rewriting

Introduction: A Rational Migration

Recently on Hacker News, a seemingly simple question sparked heated discussion among developers: "How do you rewrite a C/C++ project in Rust?" While the buzz wasn't exactly explosive, it touched on a real and widespread pain point in systems programming—how to safely and incrementally migrate mature C/C++ codebases to embrace Rust's memory safety guarantees.

Rewriting a large C/C++ project into Rust all at once is almost never a wise choice. The real challenge isn't "can we write it" but rather "how do we complete the migration without breaking existing functionality or disrupting the business." This article draws on community consensus and engineering practice to systematically outline migration strategies from C/C++ to Rust.

Why Migrate to Rust

Before diving into methodology, it's worth clarifying the motivation for migration. Rust's rapid rise in the systems programming space comes down to a unique value proposition: guaranteed memory safety without sacrificing performance.

Practical Benefits of Memory Safety

According to publicly available research from Microsoft and Google, approximately 70% of serious security vulnerabilities stem from memory safety issues—null pointer dereferences, buffer overflows, use-after-free, and the like. These are precisely the scenarios where C/C++ is most error-prone. Rust's Ownership and Borrow Checker mechanisms eliminate the vast majority of such risks at compile time.

Specifically, Rust's ownership system dictates that every value has exactly one owner at any given time, and the value is automatically freed when the owner goes out of scope. The borrow checker enforces two core rules at compile time: either there exists one mutable reference (&mut T), or any number of immutable references (&T), but never both simultaneously. This design fundamentally eliminates the possibility of data races and dangling pointers without relying on a garbage collector (GC). By contrast, while C++'s RAII and smart pointers can also manage resource lifetimes, they are library-level conventions rather than language-level guarantees—developers can still bypass these protections through raw pointers or improper reference passing.

A Modern Engineering Experience

Beyond safety, Rust brings a modern package manager (Cargo), excellent error handling mechanisms, a powerful type system, and a vibrant ecosystem. For projects requiring long-term maintenance, these engineering qualities can significantly reduce ongoing maintenance costs.

However, it's important to emphasize: migration is not the goal—solving real problems is. If your existing C/C++ code runs stably, has no security concerns, and your team lacks Rust experience, a blind rewrite is likely to do more harm than good.

Core Strategy: Incremental Migration, Not Starting from Scratch

The community consensus is crystal clear: avoid a "Big Bang" rewrite. Countless failed rewrite projects throughout history have proven that replacing an entire codebase at once is not only extremely risky but also freezes new feature development for extended periods.

One of the most famous rewrite failures in software engineering history is the Netscape browser. In 1998, Netscape decided to rewrite its browser engine from scratch, a decision that led to nearly three years of development stagnation. During that time, Internet Explorer rapidly captured market share, and Netscape ultimately met its demise. Joel Spolsky cited this event in his classic article "Things You Should Never Do" as one of the most serious strategic mistakes a software company can make. The fundamental problem with big-bang rewrites is that old code contains vast amounts of implicit knowledge accumulated through long-term operation (bug fixes, edge case handling, performance optimizations), and this knowledge is easily lost during a rewrite. Incremental migration allows teams to gradually replace components while keeping the system in a working state at all times—this aligns closely with Martin Fowler's "Strangler Fig Pattern."

The Basic Path of Incremental Migration

Incremental migration is currently the most recommended approach. The basic idea is:

• Keep the existing C/C++ project buildable and runnable at all times
• Rewrite modules one by one in Rust, interoperating with the original code via FFI (Foreign Function Interface)
• Run the complete test suite after each migration step to ensure behavioral consistency
• Gradually expand Rust code coverage

This approach lets teams validate each migration step in real production environments, keeping risk under control at all times.

Start with Leaf Modules to Reduce Complexity

A highly practical tip: prioritize migrating "leaf node" modules—those with few dependencies that are called by other modules but don't themselves heavily call external components. These modules have clear boundaries and introduce minimal FFI complexity after migration, making them ideal starting points for gaining experience.

Key Technology: Cross-Language Interoperability

The technical foundation enabling C/C++ and Rust coexistence is cross-language interoperability, which is the key to making incremental migration viable.

FFI Calls Based on the C ABI

Rust natively supports the C Application Binary Interface (ABI). The ABI defines calling conventions at the machine code level, including how parameters are passed (registers or stack), how return values are retrieved, and how stack frames are organized. Due to its simplicity and long history, the C ABI has become the de facto standard for cross-language interoperability—virtually all modern programming languages support function calls compatible with the C ABI.

FFI (Foreign Function Interface) is the mechanism in a programming language for calling functions written in other languages. Rust's FFI design is particularly elegant: it marks all cross-language calls as unsafe, explicitly informing developers that the compiler cannot verify safety invariants at these boundaries and that safety responsibility lies with the developer. This explicit marking prevents implicit degradation of safety guarantees.

Through extern "C" declarations, Rust functions can be called by C/C++ code and vice versa. This is the most fundamental and stable form of interoperability.

#[no_mangle]
pub extern "C" fn process_data(input: *const u8, len: usize) -> i32 {
    // Rust implementation logic
}

Automated Binding Generation Tools

Manually writing FFI bindings is both tedious and error-prone. The community provides a mature toolchain for this:

• bindgen: Automatically generates Rust binding code from C/C++ header files, suitable for calling existing C libraries from Rust
• cbindgen: The reverse operation—generates C/C++ header files from Rust code, allowing C/C++ to call Rust functions
• cxx: A safe interoperability solution designed specifically for Rust and C++, capable of handling more complex C++ types (such as std::string, std::vector, etc.)

cxx differs fundamentally from traditional FFI binding tools. bindgen/cbindgen essentially translate type declarations from one language into equivalent declarations in another, and developers still need to handle large amounts of unsafe code and manual memory management. cxx instead uses a shared type definition approach: developers declare both Rust-side and C++-side interfaces within a #[cxx::bridge] macro, and cxx automatically generates glue code for both ends while verifying type compatibility at compile time. It has built-in mapping support for C++ standard library types (e.g., std::string → rust::String, std::vector<T> → rust::Vec<T>), avoiding the need to manually handle ABI layout issues for complex C++ types. This makes migrating C++ projects more feasible than pure C projects, since C++'s name mangling, template instantiation, and exception mechanisms are otherwise major obstacles to cross-language interoperability.

For pure C projects, the bindgen + cbindgen combination is usually sufficient. For scenarios involving C++ templates, class inheritance, and the like, cxx can dramatically reduce the amount of hand-written unsafe code.

Common Pitfalls During Migration

Even with an incremental strategy, there are several common traps to watch out for during migration.

Managing and Encapsulating unsafe Boundaries

All FFI calls are marked as unsafe in Rust. Early in migration, the code will be filled with unsafe blocks. The key principle is: encapsulate unsafe within the thinnest possible boundary layer and expose safe Rust APIs externally. Never let unsafe spread into business logic—otherwise the migration loses its purpose.

Cross-Language Memory Ownership Alignment

C/C++ and Rust have fundamentally different philosophies about memory management. At cross-language boundaries, "whoever allocates, deallocates" must be clearly agreed upon. The common practice is to have the allocating side responsible for deallocation, using explicit API contracts to avoid double-free or memory leaks.

Build System Integration Challenges

Integrating Cargo into existing CMake, Make, or Bazel build systems is often a seriously underestimated effort. Modern C/C++ projects typically rely on complex build system ecosystems: CMake is the de facto standard for cross-platform builds, Bazel is widely used in large monorepos, and many legacy projects still depend on GNU Make or custom scripts. While Rust's Cargo build system offers an excellent experience for pure Rust projects, it has its own independent dependency resolution, compilation caching, and linking logic.

Seamlessly integrating two build systems faces numerous challenges: coordinating compilation order (C++ libraries may depend on Rust-generated libraries and vice versa, creating circular dependencies), unified configuration of cross-compilation toolchains, correct invalidation of incremental compilation caches, and CI/CD pipeline adaptation. It's recommended to use corrosion (formerly cmake-cargo, a dedicated CMake-Cargo integration tool) to reduce integration friction—it registers Cargo targets as external projects within CMake and automatically handles library search paths and linker flags. For Bazel users, rules_rust provides similar integration capabilities.

Tool-Assisted Migration: The Reality of Automatic Transpilation

Many developers hope to use tools to automatically complete code translation. The c2rust automatic transpilation tool does exist, capable of mechanically converting C code into semantically equivalent Rust code.

c2rust's workflow consists of two phases: first, it uses the Clang frontend to parse C code into an Abstract Syntax Tree (AST), then mechanically maps each AST node to its corresponding Rust syntax construct. For example, a C raw pointer int *p gets converted to *mut i32, and malloc calls map to corresponding unsafe memory allocations. This statement-by-statement translation guarantees semantic equivalence, but the generated code is completely non-idiomatic Rust—there's no use of enums for exhaustive matching, no Result/Option for error handling, and no ownership transfer for resource management.

However, it's essential to recognize clearly: code generated by c2rust is almost entirely wrapped in unsafe blocks. It essentially re-expresses C's pointer operations in Rust syntax without actually gaining Rust's safety advantages.

The real value of c2rust lies in providing a compilable starting point. From there, developers still need to invest significant effort in manual refactoring to transform the code into safe, idiomatic Rust. Refactoring c2rust output into idiomatic Rust is itself an active research area—the Immunant team has developed the c2rust-refactor tool to automate some refactoring steps, but full automation remains a distant goal. Therefore, automatic transpilation tools can only serve as aids and cannot replace deep understanding of code logic.

Practical Recommendations Summary

Combining community discussion and engineering experience, a viable migration roadmap can be summarized in six steps:

1. Assess migration necessity: Clarify what specific problems migration will solve—avoid migrating for migration's sake
2. Establish a test baseline: Ensure sufficient test coverage before starting, as a guarantee of behavioral consistency
3. Choose an appropriate entry point: Start with leaf modules that have few dependencies and clear boundaries to accumulate experience
4. Build the interop layer: Use tools like bindgen, cbindgen, or cxx to establish stable FFI bridges
5. Replace incrementally and verify continuously: Run a full test pass after migrating each module to confirm no regressions
6. Continuously reduce unsafe code: Gradually encapsulate unsafe into smaller scopes, ultimately achieving complete safety at the business logic layer

Conclusion

Migrating a C/C++ project to Rust is fundamentally an exercise in engineering discipline, not merely a language technology challenge. It tests a team's patience with incremental refactoring, rigorous management of interoperability boundaries, and rational judgment about when to migrate and when not to.

As the community discussion reflects in its consensus: there are no silver bullets, but there are proven paths to follow. For critical systems truly plagued by memory safety issues, steadily migrating to Rust is undoubtedly an engineering decision worth long-term investment.