Rust's type system ensures the safety of Rust programs; however,
programmers can side-step some of the strict typing rules by using the
unsafe keyword. A common use of unsafe Rust is by libraries.
Bugs in these libraries undermine the safety of the entire
Rust program. Therefore, it is crucial to thoroughly test library APIs
to rule out bugs. Unfortunately, such testing relies on programmers to
manually construct test cases, which is an inefficient and ineffective
process.

The goal of this paper is to develop a methodology for automatically
generating Rust programs to effectively test Rust library APIs. The
main challenge is to synthesize well-typed Rust programs to
account for proper chaining of API calls and Rust's ownership type
system and polymorphic types. We develop a program synthesis
technique for Rust library API testing, which relies on a novel
logical encoding of typing constraints from Rust's ownership type
system. We implement SyRust, a testing framework for Rust
libraries that automatically synthesizes semantically valid test
cases. Our experiments on $30$ popular open-source
Rust libraries found $4$ new bugs.