nomicon/exotic-sizes.md

% Exotically Sized Types

Most of the time, we think in terms of types with a fixed, positive size. This
is not always the case, however.





# Dynamically Sized Types (DSTs)

Rust also supports types without a statically known size. On the surface, this
is a bit nonsensical: Rust *must* know the size of something in order to work
with it! DSTs are generally produced as views, or through type-erasure of types
that *do* have a known size. Due to their lack of a statically known size, these
types can only exist *behind* some kind of pointer. They consequently produce a
*fat* pointer consisting of the pointer and the information that *completes*
them.

For instance, the slice type, `[T]`, is some statically unknown number of
elements stored contiguously. `&[T]` consequently consists of a `(&T, usize)`
pair that specifies where the slice starts, and how many elements it contains.
Similarly, Trait Objects support interface-oriented type erasure through a
`(data_ptr, vtable_ptr)` pair.

Structs can actually store a single DST directly as their last field, but this
makes them a DST as well:

```rust
// Can't be stored on the stack directly
struct Foo {
    info: u32,
    data: [u8],
}
```

**NOTE: As of Rust 1.0 struct DSTs are broken if the last field has
a variable position based on its alignment.**





# Zero Sized Types (ZSTs)

Rust actually allows types to be specified that occupy *no* space:

```rust
struct Foo; // No fields = no size

// All fields have no size = no size
struct Baz {
    foo: Foo,
    qux: (),      // empty tuple has no size
    baz: [u8; 0], // empty array has no size
}
```

On their own, ZSTs are, for obvious reasons, pretty useless. However as with
many curious layout choices in Rust, their potential is realized in a generic
context.

Rust largely understands that any operation that produces or stores a ZST can be
reduced to a no-op. For instance, a `HashSet<T>` can be effeciently implemented
as a thin wrapper around `HashMap<T, ()>` because all the operations `HashMap`
normally does to store and retrieve values will be completely stripped in
monomorphization.

Similarly `Result<(), ()>` and `Option<()>` are effectively just fancy `bool`s.

Safe code need not worry about ZSTs, but *unsafe* code must be careful about the
consequence of types with no size. In particular, pointer offsets are no-ops,
and standard allocators (including jemalloc, the one used by Rust) generally
consider passing in `0` as Undefined Behaviour.





# Empty Types

Rust also enables types to be declared that *cannot even be instantiated*. These
types can only be talked about at the type level, and never at the value level.
Empty types can be declared by specifying an enum with no variants:

```rust
enum Void {} // No variants = EMPTY
```

Empty types are even more marginal than ZSTs. The primary motivating example for
Void types is type-level unreachability. For instance, suppose an API needs to
return a Result in general, but a specific case actually is infallible. It's
actually possible to communicate this at the type level by returning a
`Result<T, Void>`. Consumers of the API can confidently unwrap such a Result
knowing that it's *statically impossible* for this value to be an `Err`, as
this would require providing a value of type Void.

In principle, Rust can do some interesting analyses and optimizations based
on this fact. For instance, `Result<T, Void>` could be represented as just `T`,
because the Err case doesn't actually exist. The following *could* also compile:

```rust,ignore
enum Void {}

let res: Result<u32, Void> = Ok(0);

// Err doesn't exist anymore, so Ok is actually irrefutable.
let Ok(num) = res;
```

But neither of these tricks work today, so all Void types get you today is
the ability to be confident that certain situations are statically impossible.

One final subtle detail about empty types is that raw pointers to them are
actually valid to construct, but dereferencing them is Undefined Behaviour
because that doesn't actually make sense. That is, you could model C's `void *`
type with `*const Void`, but this doesn't necessarily gain anything over using
e.g. `*const ()`, which *is* safe to randomly dereference.