What Is the Sized Trait in Rust?

The Sized trait in Rust marks types with a known size at compile time, which is required for most values to be stored on the stack. By default, generic types are implicitly bounded by Sized, but you can opt out of this requirement using the ?Sized syntax to allow unsized types like slices or trait objects.

fn generic<T: Sized>(t: T) {
    // T has a known size at compile time
}

fn generic_maybe_sized<T: ?Sized>(t: &T) {
    // T can be unsized, so we take a reference
}

The Sized trait tells the compiler that a type has a fixed, known size so it can be stored directly in memory. Most types you use are sized, but some special types like slices don't have a fixed size and need to be handled differently. Think of it like packing a suitcase: Sized items fit in a specific slot, while unsized items need a flexible bag.