Trait Unsize

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. 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.
• Trait objects dyn TraitA + AutoA... + 'a implement Unsize<dyn TraitB + AutoB... + 'b> if all of these conditions are met:
  • TraitB is a supertrait of TraitA.
  • AutoB... is a subset of AutoA....
  • 'a outlives 'b.
• 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 coercion for more details.