std::marker::Sized

Trait std::marker::Sized

#[lang = "sized"]
pub trait Sized { }

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, which does not get an implicit Sized bound. This is because a Sized bound prevents the trait from being used to form a trait object:

trait Foo { }
trait Bar: Sized { }

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

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

Implementors