Zero-sized type

A Zero-sized type (ZST) is a type that occupies no memory and is optimized away by the compiler. For example, a Vec<()> never allocates memory.

Zero-sized types are:

• The empty tuple ()
• The never type !
• Structs, unions and tuples, if all their fields are zero-sized, e.g.

  struct Struct;
  struct Struct2(Struct);
  struct Struct3(PhantomData<bool>);
  

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• Enums with at most one variant that isn't uninhabited, if all its fields are zero-sized, e.g.

  #![feature(never_type)]
  
  enum Void {}
  enum Unit { Unit }
  enum Unit2 {
      Unit,
      Uninhabited(!),
      Uninhabited2(Void),
  }
  enum Phantom {
      Phantom(PhantomData<bool>),
  }
  

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Note that PhantomData is also a ZST. It is a special type, because it is generic, even though it has no fields.

Note that struct Foo(i32, !) and enum Foo { Foo(i32, !) } are not zero-sized. Rust doesn't treat them as uninhabited types, even though they have no possible values.

N-ZSTs

One important aspect of memory layout of a type is its alignment. To distinguish ZSTs with different alignments, Rust developers agreed to call them n-ZSTs in natural language and there is a proposal to use ZST1, ZST2, ZST4, ZST8, ZST16, ZST32, ZST64, and ZST128 as identifiers in programs.