What is a function for structs like Java's instanceof?

I'm making an OOP chat client in Rust. The module messages.rs creates and handles messages to other modules as structs: SimpleMessage and ComplexMessage structs: //! # Messages use time::SteadyTime; /// Represents a simple text message pub struct SimpleMessage<'a> { pub user: ... pub time: &'a SteadyTime<'a>, pub content: &'a str, } /// Represents attachments, like text or multimedia files. pub st