A Rust developer is responsible for the development and maintenance of software written in the Rust programming language. They may be involved in writing code for new applications, or maintaining existing codebases. In either case, they must have a strong understanding of Rust syntax and semantics. Rust developers typically use the Cargo package manager to manage dependencies and build their projects. They may also contribute to open-source projects written in Rust, or develop their own libraries and frameworks for others to use.