The Rust programming language restricts aliasing and mutability to provide static safety guarantees, which developers rely on to write secure and performant applications. However, Rust is frequently used to interoperate with other languages that have far weaker restrictions. These languages support cyclic and self-referential design patterns that conflict with current models of Rust's operational semantics, representing a potentially significant source of undefined behavior that no current tools can detect. We created MiriLLI, a tool which uses existing Rust and LLVM interpreters to jointly execute multi-language Rust applications. We used our tool in a large-scale study of Rust libraries that call foreign functions, and we found 45 instances of undefined or undesirable behavior. These include four bugs from libraries that had over 10,000 daily downloads on average, one from a component of the GNU Compiler Collection (GCC), and one from a library maintained by the Rust Project. Most of these errors were caused by incompatible aliasing and initialization patterns, incorrect foreign function bindings, and invalid type conversion. The majority of aliasing violations were caused by unsound operations in Rust, but they occurred in foreign code. The Rust community must invest in new tools for validating multi-language programs to ensure that developers can easily detect and fix these errors.

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