std::marker::Sized - Rust
Description: API documentation for the Rust `Sized` trait in crate `std`.
Keywords: rust, rustlang, rust-lang, Sized

# Trait std::marker::Sized 1.0.0 [−] [src]

    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]>); // OKRun

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 objectRun

## Implementors

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