Over the last several years, the Rust programming language has gathered a following among software developers for its robust memory safety features. Nevertheless, it remains susceptible to potentially harmful side effects in untrusted code and is therefore vulnerable to supply chain attacks. We wish to investigate whether preventing them by retroactively enforcing side effect safety is possible. In this extended abstract, we introduce Coenobita, a Rust library that prevents undesirable side effects using capabilities without additional performance overhead. Our goal was to implement statically enforced, zero-cost, and unobtrusive capabilities. To evaluate Coenobita’s practicality and effectiveness, we conducted two case studies porting popular Rust crates walkdir and remove_dir_all to Coenobita. Porting walkdir required modifying or adding around 242 lines across three files originally containing 1800 lines total. Benchmarks were run on 46 tests provided in walkdir and their equivalents in coenobita-walkdir, demonstrating little change in runtime for most tests.