Types - The Rust Reference

# Types

Every variable, item and value in a Rust program has a type. The type of a value defines the interpretation of the memory holding it.

Built-in types and type-constructors are tightly integrated into the language, in nontrivial ways that are not possible to emulate in user-defined types. User-defined types have limited capabilities.

## Primitive types

The primitive types are the following:

- The boolean type `bool` with values `true` and `false`.
- The machine types (integer and floating-point).
- The machine-dependent integer types.
- Arrays
- Tuples
- Slices
- Function pointers

### Machine types

The machine types are the following:

- The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1] and [0, 2^64 - 1] respectively.

- The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with values drawn from the integer intervals [-(2^(7)), 2^7 - 1], [-(2^(15)), 2^15 - 1], [-(2^(31)), 2^31 - 1], [-(2^(63)), 2^63 - 1] respectively.

- The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and `f64`, respectively.

### Machine-dependent integer types

The `usize` type is an unsigned integer type with the same number of bits as the platform's pointer type. It can represent every memory address in the process.

The `isize` type is a signed integer type with the same number of bits as the platform's pointer type. The theoretical upper bound on object and array size is the maximum `isize` value. This ensures that `isize` can be used to calculate differences between pointers into an object or array and can address every byte within an object along with one byte past the end.

## Textual types

The types `char` and `str` hold textual data.

A value of type `char` is a Unicode scalar value (i.e. a code point that is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to 0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` array is effectively an UCS-4 / UTF-32 string.

A value of type `str` is a Unicode string, represented as an array of 8-bit unsigned bytes holding a sequence of UTF-8 code points. Since `str` is of unknown size, it is not a first-class type, but can only be instantiated through a pointer type, such as `&str`.

## Tuple types

A tuple type is a heterogeneous product of other types, called the elements of the tuple. It has no nominal name and is instead structurally typed.

Tuple types and values are denoted by listing the types or values of their elements, respectively, in a parenthesized, comma-separated list.

Because tuple elements don't have a name, they can only be accessed by pattern-matching or by using `N` directly as a field to access the `N`th element.

An example of a tuple type and its use:

    # #![allow(unused_variables)]
    #fn main() {
    type Pair<'a> = (i32, &'a str);
    let p: Pair<'static> = (10, "ten");
    let (a, b) = p;

    assert_eq!(a, 10);
    assert_eq!(b, "ten");
    assert_eq!(p.0, 10);
    assert_eq!(p.1, "ten");

    #}

For historical reasons and convenience, the tuple type with no elements (`()`) is often called ‘unit’ or ‘the unit type’.

## Array, and Slice types

Rust has two different types for a list of items:

- `[T; N]`, an 'array'
- `&[T]`, a 'slice'

An array has a fixed size, and can be allocated on either the stack or the heap.

A slice is a 'view' into an array. It doesn't own the data it points to, it borrows it.

Examples:

    # #![allow(unused_variables)]
    #fn main() {
    // A stack-allocated array
    let array: [i32; 3] = [1, 2, 3];

    // A heap-allocated array
    let vector: Vec<i32> = vec![1, 2, 3];

    // A slice into an array
    let slice: &[i32] = &vector[..];

    #}

As you can see, the `vec!` macro allows you to create a `Vec<T>` easily. The `vec!` macro is also part of the standard library, rather than the language.

All in-bounds elements of arrays and slices are always initialized, and access to an array or slice is always bounds-checked.

## Struct types

A `struct` type is a heterogeneous product of other types, called the fields of the type.1

1

`struct` types are analogous to `struct` types in C, the record types of the ML family, or the struct types of the Lisp family.

New instances of a `struct` can be constructed with a struct expression.

The memory layout of a `struct` is undefined by default to allow for compiler optimizations like field reordering, but it can be fixed with the `#[repr(...)]` attribute. In either case, fields may be given in any order in a corresponding struct expression; the resulting `struct` value will always have the same memory layout.

The fields of a `struct` may be qualified by visibility modifiers, to allow access to data in a struct outside a module.

A tuple struct type is just like a struct type, except that the fields are anonymous.

A unit-like struct type is like a struct type, except that it has no fields. The one value constructed by the associated struct expression is the only value that inhabits such a type.

## Enumerated types

An enumerated type is a nominal, heterogeneous disjoint union type, denoted by the name of an `enum` item. 2

2

The `enum` type is analogous to a `data` constructor declaration in ML, or a pick ADT in Limbo.

An `enum` item declares both the type and a number of variant constructors, each of which is independently named and takes an optional tuple of arguments.

New instances of an `enum` can be constructed by calling one of the variant constructors, in a call expression.

Any `enum` value consumes as much memory as the largest variant constructor for its corresponding `enum` type.

Enum types cannot be denoted structurally as types, but must be denoted by named reference to an `enum` item.

## Recursive types

Nominal types — enumerations and structs — may be recursive. That is, each `enum` constructor or `struct` field may refer, directly or indirectly, to the enclosing `enum` or `struct` type itself. Such recursion has restrictions:

- Recursive types must include a nominal type in the recursion (not mere type definitions, or other structural types such as arrays or tuples).
- A recursive `enum` item must have at least one non-recursive constructor (in order to give the recursion a basis case).
- The size of a recursive type must be finite; in other words the recursive fields of the type must be pointer types.
- Recursive type definitions can cross module boundaries, but not module visibility boundaries, or crate boundaries (in order to simplify the module system and type checker).

An example of a recursive type and its use:

    # #![allow(unused_variables)]
    #fn main() {
    enum List<T> {
        Nil,
        Cons(T, Box<List<T>>)
    }

    let a: List<i32> = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil))));

    #}

## Pointer types

All pointers in Rust are explicit first-class values. They can be copied, stored into data structs, and returned from functions. There are two varieties of pointer in Rust:

- References (`&`) : These point to memory owned by some other value. A reference type is written `&type`, or `&'a type` when you need to specify an explicit lifetime. Copying a reference is a "shallow" operation: it involves only copying the pointer itself. Releasing a reference has no effect on the value it points to, but a reference of a temporary value will keep it alive during the scope of the reference itself.

- Raw pointers (`*`) : Raw pointers are pointers without safety or liveness guarantees. Raw pointers are written as `*const T` or `*mut T`, for example `*const i32` means a raw pointer to a 32-bit integer. Copying or dropping a raw pointer has no effect on the lifecycle of any other value. Dereferencing a raw pointer or converting it to any other pointer type is an `unsafe` operation. Raw pointers are generally discouraged in Rust code; they exist to support interoperability with foreign code, and writing performance-critical or low-level functions.

The standard library contains additional 'smart pointer' types beyond references and raw pointers.

## Function types

The function type constructor `fn` forms new function types. A function type consists of a possibly-empty set of function-type modifiers (such as `unsafe` or `extern`), a sequence of input types and an output type.

An example of a `fn` type:

    # #![allow(unused_variables)]
    #fn main() {
    fn add(x: i32, y: i32) -> i32 {
        x + y
    }

    let mut x = add(5,7);

    type Binop = fn(i32, i32) -> i32;
    let bo: Binop = add;
    x = bo(5,7);

    #}

### Function types for specific items

Internal to the compiler, there are also function types that are specific to a particular function item. In the following snippet, for example, the internal types of the functions `foo` and `bar` are different, despite the fact that they have the same signature:

    # #![allow(unused_variables)]
    #fn main() {
    fn foo() { }
    fn bar() { }

    #}

The types of `foo` and `bar` can both be implicitly coerced to the fn pointer type `fn()`. There is currently no syntax for unique fn types, though the compiler will emit a type like `fn() {foo}` in error messages to indicate "the unique fn type for the function `foo`".

## Closure types

A closure expression produces a closure value with a unique, anonymous type that cannot be written out.

Depending on the requirements of the closure, its type implements one or more of the closure traits:

- `FnOnce` : The closure can be called once. A closure called as `FnOnce` can move out values from its environment.

- `FnMut` : The closure can be called multiple times as mutable. A closure called as `FnMut` can mutate values from its environment. `FnMut` inherits from `FnOnce` (i.e. anything implementing `FnMut` also implements `FnOnce`).

- `Fn` : The closure can be called multiple times through a shared reference. A closure called as `Fn` can neither move out from nor mutate values from its environment, but read-only access to such values is allowed. `Fn` inherits from `FnMut`, which itself inherits from `FnOnce`.

Closures that don't use anything from their environment ("non capturing closures") can be coerced to function pointers (`fn`) with the matching signature. To adopt the example from the section above:

    # #![allow(unused_variables)]
    #fn main() {
    let add = |x, y| x + y;

    let mut x = add(5,7);

    type Binop = fn(i32, i32) -> i32;
    let bo: Binop = add;
    x = bo(5,7);

    #}

## Trait objects

In Rust, a type like `&SomeTrait` or `Box<SomeTrait>` is called a trait object. Each instance of a trait object includes:

- a pointer to an instance of a type `T` that implements `SomeTrait`
- a virtual method table, often just called a vtable, which contains, for each method of `SomeTrait` that `T` implements, a pointer to `T`'s implementation (i.e. a function pointer).

The purpose of trait objects is to permit "late binding" of methods. Calling a method on a trait object results in virtual dispatch at runtime: that is, a function pointer is loaded from the trait object vtable and invoked indirectly. The actual implementation for each vtable entry can vary on an object-by-object basis.

Note that for a trait object to be instantiated, the trait must be object-safe. Object safety rules are defined in RFC 255.

Given a pointer-typed expression `E` of type `&T` or `Box<T>`, where `T` implements trait `R`, casting `E` to the corresponding pointer type `&R` or `Box<R>` results in a value of the trait object `R`. This result is represented as a pair of pointers: the vtable pointer for the `T` implementation of `R`, and the pointer value of `E`.

An example of a trait object:

    trait Printable {
        fn stringify(&self) -> String;
    }

    impl Printable for i32 {
        fn stringify(&self) -> String { self.to_string() }
    }

    fn print(a: Box<Printable>) {
        println!("{}", a.stringify());
    }

    fn main() {
        print(Box::new(10) as Box<Printable>);
    }

In this example, the trait `Printable` occurs as a trait object in both the type signature of `print`, and the cast expression in `main`.

Since a trait object can contain references, the lifetimes of those references need to be expressed as part of the trait object. The assumed lifetime of references held by a trait object is called its default object lifetime bound. These were defined in RFC 599 and amended in RFC 1156.

For traits that themselves have no lifetime parameters, the default bound is based on what kind of trait object is used:

    // For the following trait...
    trait Foo { }

    // ...these two are the same:
    Box<Foo>
    Box<Foo + 'static>

    // ...and so are these:
    &'a Foo
    &'a (Foo + 'a)

The `+ 'static` and `+ 'a` refer to the default bounds of those kinds of trait objects, and also to how you can directly override them. Note that the innermost object sets the bound, so `&'a Box<Foo>` is still `&'a Box<Foo + 'static>`.

For traits that have lifetime parameters of their own, the default bound is based on that lifetime parameter:

    // For the following trait...
    trait Bar<'a>: 'a { }

    // ...these two are the same:
    Box<Bar<'a>>
    Box<Bar<'a> + 'a>

The default for user-defined trait objects is based on the object type itself. If a type parameter has a lifetime bound, then that lifetime bound becomes the default bound for trait objects of that type. For example, `std::cell::Ref<'a, T>` contains a `T: 'a` bound, therefore trait objects of type `Ref<'a, SomeTrait>` are the same as `Ref<'a, (SomeTrait + 'a)>`.

### Type parameters

Within the body of an item that has type parameter declarations, the names of its type parameters are types:

    # #![allow(unused_variables)]
    #fn main() {
    fn to_vec<A: Clone>(xs: &[A]) -> Vec<A> {
        if xs.is_empty() {
            return vec![];
        }
        let first: A = xs[0].clone();
        let mut rest: Vec<A> = to_vec(&xs[1..]);
        rest.insert(0, first);
        rest
    }

    #}

Here, `first` has type `A`, referring to `to_vec`'s `A` type parameter; and `rest` has type `Vec<A>`, a vector with element type `A`.

## Self types

The special type `Self` has a meaning within traits and impls. In a trait definition, it refers to an implicit type parameter representing the "implementing" type. In an impl, it is an alias for the implementing type. For example, in:

    # #![allow(unused_variables)]
    #fn main() {
    pub trait From<T> {
        fn from(T) -> Self;
    }

    impl From<i32> for String {
        fn from(x: i32) -> Self {
            x.to_string()
        }
    }

    #}

The notation `Self` in the impl refers to the implementing type: `String`. In another example:

    # #![allow(unused_variables)]
    #fn main() {
    trait Printable {
        fn make_string(&self) -> String;
    }

    impl Printable for String {
        fn make_string(&self) -> String {
            (*self).clone()
        }
    }

    #}

The notation `&self` is a shorthand for `self: &Self`. In this case, in the impl, `Self` refers to the value of type `String` that is the receiver for a call to the method `make_string`.