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 parametersSelf: ?Sized by default in trait definitions.
• Structs may contain a DST as the last field, this makes the struct itself a DST.

Notably: variables, function parameters, const and static items must be Sized.