What Is the Sized Trait and When Does It Matter?

The Sized trait guarantees that a type has a known size at compile time, which is required for most generic code to work. By default, Rust assumes generic type parameters implement Sized, but you can opt out of this requirement using the ?Sized bound to accept unsized types like slices or trait objects.

// Default: T must have a known size
fn generic_sized<T>(t: T) {
    // ...
}

// Opt-out: T can be unsized (must be used behind a pointer)
fn generic_maybe_sized<T: ?Sized>(t: &T) {
    // ...
}

The Sized trait is a rule that tells the compiler how much memory a piece of data needs. Most data has a fixed size, but some data (like a list of unknown length) does not. You usually don't need to worry about this, but if you write code that handles both fixed and variable-sized data, you use ?Sized to tell Rust, "I know this might not have a fixed size, and that's okay."