# [−][src]Trait core::marker::Unsize

    #[lang = "unsize"]pub trait Unsize<T: ?Sized> { }

🔬 This is a nightly-only experimental API. (`unsize` #27732)

Types that can be "unsized" to a dynamically-sized type.

For example, the sized array type `[i8; 2]` implements `Unsize<[i8]>` and `Unsize<dyn fmt::Debug>`.

All implementations of `Unsize` are provided automatically by the compiler.

`Unsize` is implemented for:

- `[T; N]` is `Unsize<[T]>`
- `T` is `Unsize<dyn Trait>` when `T: Trait`
- `Foo<..., T, ...>` is `Unsize<Foo<..., U, ...>>` if:
  - `T: Unsize<U>`
  - Foo is a struct
  - Only the last field of `Foo` has a type involving `T`
  - `T` is not part of the type of any other fields
  - `Bar<T>: Unsize<Bar<U>>`, if the last field of `Foo` has type `Bar<T>`

`Unsize` is used along with `ops::CoerceUnsized` to allow "user-defined" containers such as `rc::Rc` to contain dynamically-sized types. See the DST coercion RFC and the nomicon entry on coercion for more details.

## Implementors

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