Types with a constant size known at compile time.

All type parameters have an implicit bound of Sized. The special syntax ?Sized can be used to remove this bound if it’s not appropriate.

struct Foo<T>(T);
struct Bar<T: ?Sized>(T);

// struct FooUse(Foo<[i32]>); // error: Sized is not implemented for [i32]
struct BarUse(Bar<[i32]>); // OK

The one exception is the implicit Self type of a trait. A trait does not have an implicit Sized bound as this is incompatible with trait objects where, by definition, the trait needs to work with all possible implementors, and thus could be any size.

Although Rust will let you bind Sized to a trait, you won’t be able to use it to form a trait object later:

trait Foo { }
trait Bar: Sized { }

struct Impl;
impl Foo for Impl { }
impl Bar for Impl { }

let x: &dyn Foo = &Impl;    // OK
// let y: &dyn Bar = &Impl; // error: the trait `Bar` cannot be made into an object