Sized trait and ?Sized for dynamically-sized types

Most types in Rust are Sized (known size at compile time), which is an auto trait. Some types like str or [T] are !Sized (dynamically-sized types, DSTs). To work with DSTs, use &T or Box<T>. In generic code, T: Sized is the default bound. To accept DSTs, use T: ?Sized ("maybe sized"). I use ?Sized when writing generic functions for trait objects or slices. For example, a function taking &T where T: ?Sized can accept both &str and &String. Understanding Sized helps when you get confusing compiler errors about trait object sizes or return types. It's a subtle but important part of Rust's type system.