A Grounded Conceptual Model for Ownership Types in Rust

TL;DR

This paper develops a conceptual model and pedagogical approach to teaching Rust's ownership types, improving learner understanding and bridging the gap between static and dynamic semantics.

Contribution

It introduces a new ownership conceptual model, a visualization tool, and an improved teaching chapter that enhances understanding of Rust's ownership semantics.

Findings

Learners' scores on the Ownership Inventory increased by 9%.

The pedagogy helped connect static and dynamic semantics of Rust.

The Ownership Inventory effectively measures ownership understanding.

Abstract

Programmers learning Rust struggle to understand ownership types, Rust's core mechanism for ensuring memory safety without garbage collection. This paper describes our attempt to systematically design a pedagogy for ownership types. First, we studied Rust developers' misconceptions of ownership to create the Ownership Inventory, a new instrument for measuring a person's knowledge of ownership. We found that Rust learners could not connect Rust's static and dynamic semantics, such as determining why an ill-typed program would (or would not) exhibit undefined behavior. Second, we created a conceptual model of Rust's semantics that explains borrow checking in terms of flow-sensitive permissions on paths into memory. Third, we implemented a Rust compiler plugin that visualizes programs under the model. Fourth, we integrated the permissions model and visualizations into a broader pedagogy of ownership by writing a new ownership chapter for The Rust Programming Language, a popular Rust textbook. Fifth, we evaluated an initial deployment of our pedagogy against the original version, using reader responses to the Ownership Inventory as a point of comparison. Thus far, the new pedagogy has improved learner scores on the Ownership Inventory by an average of 9% ($N = 342, d = 0.56$).