"Against the Void": An Interview and Survey Study on How Rust Developers Use Unsafe Code

The Rust programming language is an increasingly popular choice for systems programming, since it can statically guarantee memory safety without automatic garbage collection. Rust provides its safety guarantees by restricting aliasing and mutability, but many key design patterns, such as cyclic aliasing and multi-language interoperation, must bypass these restrictions. Rust's $\texttt{unsafe}$ keyword enables features that developers can use to implement these patterns, and the Rust ecosystem includes useful tools for validating whether $\texttt{unsafe}$ code is used correctly. However, it is unclear if these tools are adequate for all use cases. To understand developers' needs, we conducted a mixed-methods study consisting of semi-structured interviews followed by a survey. We interviewed 19 Rust developers and surveyed 160 developers$\unicode{x2013}$all of whom engaged with $\texttt{unsafe}$ code. We found that 77% of survey respondents and a majority of interview participants were motivated to use $\texttt{unsafe}$ code because they were unaware of a safe alternative. Developers typically followed best-practices such as minimizing and localizing their use of $\texttt{unsafe}$ code, but only 23% were always certain that their encapsulations were sound. Limited tooling support for inline assembly and foreign function calls prevented developers from validating $\texttt{unsafe}$ code, and differences between Rust and other languages made foreign functions difficult to encapsulate. Verification tools were underused, and developers rarely audited their dependencies. Our results indicate a pressing need for production-ready tools that can validate the most frequently used $\texttt{unsafe}$ features.