Unsize in core::marker - Rust (original) (raw)

pub trait Unsize<T: ?Sized> { }

🔬This is a nightly-only experimental API. (unsize #18598)

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

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

All implementations of Unsize are provided automatically by the compiler. Those implementations are:

• Arrays [T; N] implement Unsize<[T]>.
• A type implements Unsize<dyn Trait + 'a> if all of these conditions are met:
  • The type implements Trait.
  • Trait is dyn-compatible1.
  • The type is sized.
  • The type outlives 'a.
• Structs Foo<..., T1, ..., Tn, ...> implement Unsize<Foo<..., U1, ..., Un, ...>>where any number of (type and const) parameters may be changed if all of these conditions are met:
  • Only the last field of Foo has a type involving the parameters T1, …, Tn.
  • All other parameters of the struct are equal.
  • Field<T1, ..., Tn>: Unsize<Field<U1, ..., Un>>, where Field<...> stands for the actual type of the struct’s last field.

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