Bringing Rust-Style Memory Safety to C++: A Practical Static Analyzer Built with AI Assistance

## Introduction Memory safety errors—segmentation faults, dangling pointers, use‑after‑free—continue to plague even the most mature C++ codebases. For a developer with 15 years of systems research experience, the sheer volume of such bugs can feel like a relentless “15‑year itch”. While rewriting the entire codebase in Rust would theoretically eliminate these issues, the cost of migration is prohibitive and often unrealistic. Instead of a wholesale rewrite, the goal becomes clear: transplant Rust’s borrow‑checking model directly into C++ so that the same safety guarantees can be enforced without touching the compiler. The result is the open‑source project **Rusty‑Cpp** (see ), a static analyzer that adds Rust‑style memory safety checks to existing C++ projects. ## The Interoperability Gap Rust is growing steadily, but cross‑language interop remains a challenge. Modern C++ codebases rarely coexist neatly with Rust modules, and existing solutions such as D or Swift expose only limited support. Attempts by Google and others to encode borrow‑checking semantics with macros proved ineffective due to fundamental C++ constraints, making a language‑level solution infeasible. Instead, the focus shifted to a **source‑level static analyzer**. An analyzer can examine the abstract syntax tree (AST) produced by an existing compiler, perform the necessary borrow‑check logic, and report violations—all while preserving compatibility with standard build tools. ## Design Principles 1. **Non‑intrusive annotations** – To remain compiler‑agnostic, Rusty‑Cpp uses *comment‑based markers* rather than new syntax: ```cpp // @safe void foo() { /* … */ } ``` Unannotated code defaults to `@unsafe`, ensuring that existing libraries (STL, Boost, etc.) are treated conservatively. 2. **Clear audit boundaries** – `@safe` code may only call other `@safe` functions directly. Calling external or legacy code requires wrapping the call in an `@unsafe` block, which the analyzer permits but does not check. 3. **Rust‑style mutable/immutable distinction** – C++’s `const` matches Rust’s immutable variables. By flipping the default usage and treating non‑`const` as mutable, the analyzer can apply identical borrow‑checking rules. 4. **Borrow‑checking semantics** – The analyzer enforces these core rules: * Multiple immutable (read‑only) references are permitted. * Multiple mutable references to the same variable are disallowed. * Mixing immutable and mutable references to the same variable is prohibited. * Moving a value nullifies its binding, preventing use‑after‑move. 5. **External annotations** – For library functions that cannot be modified, Rusty‑Cpp allows declarative annotations describing lifetimes and side‑effects (e.g., `strlen`, `sqlite3_column_text`). This informs the analyzer of proper pointer ownership and lifecycle. 6. **Custom Rust‑like types** – To match Rust’s ownership concepts, the tool ships C++ equivalents for `Box`, `Arc`, `Option`, `Vec`, and `Result`. These wrappers offer the same API as their Rust counterparts but enforce non‑nullable guarantees and explicit cloning semantics. 7. **Send/Sync semantics** – Thread‑safety is modeled with C++ concepts marking types as `Send` or `Sync`. While currently manual, this provides compile‑time guarantees that types can safely cross thread boundaries. ## Implementation Details Rusty‑Cpp leverages **libclang** to walk the AST and perform analysis. The analyzer’s core is a data‑flow engine that tracks borrow states across variables and function calls. Although libclang’s name resolution can be imprecise in complex namespaces, iterative refinement via AI‑assistance (Claude) resolved most ambiguities. AI Coding Assistants played a pivotal role: after an initial prototype, the author asked Claude to generate test suites, identify missing edge cases, and iteratively improve the engine. The result was a usable prototype in less than a year, a process that would have taken a full-time team months or years. ## Usage Running the analyzer is straightforward: ```bash $ rusty-cpp-checker myfile.cpp Rusty C++ Checker Analyzing: myfile.cpp ✗ Found 3 violation(s) in myfile.cpp: - Cannot create mutable reference to 'value': already mutably borrowed - Cannot create mutable reference to 'value': already immutably borrowed - Use after move: variable 'ptr' has been moved ``` For larger projects, supply a `compile_commands.json` file generated by CMake or other build systems: ```bash cmake -DCMAKE_EXPORT_COMPILE_COMMANDS=ON .. rusty-cpp-checker src/main.cpp --compile-commands build/compile_commands.json ``` A CMake integration can be added to enforce checks as part of the build pipeline. ## Practical Impact The developer behind Rusty‑Cpp applied the tool to the `rpc` component of the Mako project. The analyzer surfaced additional bugs that went unnoticed by developers, leading to rapid fixes and a more robust codebase. Beyond immediate bug detection, the project demonstrates a broader trend: AI‑driven tools accelerating traditionally labor‑intensive compiler research. The author observes that an AI assistant can write, test, and refine sophisticated static‑analysis logic faster than an average PhD candidate, reshaping the skill set required for future systems engineers. ## Conclusion Rusty‑Cpp illustrates that bringing Rust‑style safety guarantees to existing C++ projects is not a distant dream but an attainable reality. By combining comment‑based annotations, a libclang‑based analysis engine, and AI‑assisted development, the project offers a practical, non‑intrusive solution for memory‑safety‑centric teams. The tool is available on GitHub; try it on your own codebase and experience the peace of mind that comes from knowing your legacy C++ code now benefits from borrow‑checking semantics without rewriting.