Dynamically Sized Types - The Rust Reference

# Dynamically Sized Types

Most types have a fixed size that is known at compile time and implement the trait `Sized`. A type with a size that is known only at run-time is called a dynamically sized type (DST) or, informally, an unsized type. Slices and trait objects are two examples of DSTs. Such types can only be used in certain cases:

- Pointer types to DSTs are sized but have twice the size of pointers to sized types
  - Pointers to slices also store the number of elements of the slice.
  - Pointers to trait objects also store a pointer to a vtable.
- DSTs can be provided as type arguments when a bound of `?Sized`. By default any type parameter has a `Sized` bound.
- Traits may be implemented for DSTs. Unlike type parameters `Self: ?Sized` by default in trait definitions.
- Structs may contain a DST as the last field, this makes the struct itself a DST.

> Note: variables, function parameters, const items, and static items must be `Sized`.