Rust's dyn compatibility (formerly 'object safety') determines whether a trait can be used as a trait object with `dyn Trait`. A trait is not dyn compatible when it has methods that return `Self`, lack a receiver, or have generic type parameters — because the vtable-based dynamic dispatch system requires stable, concrete function signatures at compile time. Four fixes are covered with tradeoffs: (1) use generics instead of trait objects for compile-time dispatch, (2) add `where Self: Sized` to opt problematic methods out of dispatch, (3) return `Box<dyn Trait>` instead of `Self`, or (4) split the trait into two focused traits. A modern gotcha is that `async fn` in traits also breaks dyn compatibility, with workarounds including `async-trait` and `dynosaur` crates. The post also includes a full reference table of dyn compatibility rules and a historical timeline of how the concept evolved in Rust.
