The Usability of Advanced Type Systems: Rust as a Case Study

Advanced type systems that enforce various correctness and safety guarantees--such as linear and ownership types--have a long history in the Programming Languages research community. Despite this history, a human-centered evaluation of these type systems and their usability was all but absent, with empirical evaluations limited to testing their expressiveness in programs written by experts, i.e. the creators of the type system. In the past few years, this has begun to change with the adoption of a version of affine types and ownership in the popular Rust programming language. With the increase in Rust's popularity, various studies have begun empirically evaluating the usability of Rust's Ownership and Lifetime rules, providing a breadth of qualitative and quantitative information on the usability of such type systems. They found that despite Rust's general success in achieving its promise of safety and performance, these rules come with a steep learning curve and have been repeatedly cited as a barrier to adopting Rust. In this report, I provide a brief history of linear types and region-based memory management, which directly inspired Rust's type system. I then introduce Rust's Ownership and Lifetime rules, and present the state-of-the-art in academic research into their usability. I discuss both theoretical arguments and empirical evidence for why these rules are difficult to learn and apply, and survey existing work on addressing some of these difficulties. I also draw from broader works in the HCI and CS Education communities to recommend future work in this area.