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 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

Some types are defined by the language, rather than as part of the standard library, these are called primitive types. Some of these are individual types:

- The boolean type `bool` with values `true` and `false`.
- The machine types (integer and floating-point).
- The machine-dependent integer types.
- The textual types `char` and `str`.

There are also some primitive constructs for generic types built in to the language:

- Tuples
- Arrays
- Slices
- Function pointers
- References
- Pointers

## Numeric types

### 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]` is effectively a 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 a dynamically sized type, 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 dynamically sized type representing a 'view' into an array. To use a slice type it generally has to be used behind a pointer for example as

- `&[T]`, a 'shared slice', often just called a 'slice', it doesn't own the data it points to, it borrows it.
- `&mut [T]`, a 'mutable slice', mutably borrows the data it points to.
- `Box<[T]>`, a 'boxed slice'

Examples:

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

    // A heap-allocated array, coerced to a slice
    let boxed_array: Box<[i32]> = Box::new([1, 2, 3]);

    // A (shared) slice into an array
    let slice: &[i32] = &boxed_array[..];
    #}

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

> Note: The `Vec<T>` standard library type provides a heap-allocated resizable array type.

## Struct types

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

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.

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.

## Enumerated types

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

An `enum` item declares both the type and a number of variants, each of which is independently named and has the syntax of a struct, tuple struct or unit-like struct.

New instances of an `enum` can be constructed in an enumeration variant expression.

Any `enum` value consumes as much memory as the largest variant for its corresponding `enum` type, as well as the size needed to store a discriminant.

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

2

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

## Union types

A union type is a nominal, heterogeneous C-like union, denoted by the name of a `union` item.

A union contains the value of any one of its fields. Since the accessing the wrong field can cause unexpected or undefined behaviour, `unsafe` is required to read from a union field or to write to a field that doesn't implement `Copy`.

The memory layout of a `union` is undefined by default, but the `#[repr(...)]` attribute can be used to fix a layout.

## Recursive types

Nominal types — structs, enumerations and unions — may be recursive. That is, each `enum` variant or `struct` or `union` 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). So `type Rec = &'static [Rec]` is not allowed.
- 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 moved or copied, stored into data structs, and returned from functions.

### Shared references (`&`)

These point to memory owned by some other value. When a shared reference to a value is created it prevents direct mutation of the value. Interior mutability provides an exception for this in certain circumstances. As the name suggests, any number of shared references to a value may exit. A shared 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, that is, pointers are `Copy`. Releasing a reference has no effect on the value it points to, but referencing of a temporary value will keep it alive during the scope of the reference itself.

### Mutable references (`&mut`)

These also point to memory owned by some other value. A mutable reference type is written `&mut type` or `&'a mut type`. A mutable reference (that hasn't been borrowed) is the only way to access the value it points to, so is not `Copy`.

### Raw pointers (`*const` and `*mut`)

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 is an `unsafe` operation, this can also be used to convert a raw pointer to a reference by reborrowing it (`&*` or `&mut *`). Raw pointers are generally discouraged in Rust code; they exist to support interoperability with foreign code, and writing performance-critical or low-level functions.

When comparing pointers they are compared by their address, rather than by what they point to. When comparing pointers to dynamically sized types they also have their addition data compared.

### Smart Pointers

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

## Function item types

When referred to, a function item, or the constructor of a tuple-like struct or enum variant, yields a zero-sized value of its function item type. That type explicitly identifies the function - its name, its type arguments, and its early-bound lifetime arguments (but not its late-bound lifetime arguments, which are only assigned when the function is called) - so the value does not need to contain an actual function pointer, and no indirection is needed when the function is called.

There is no syntax that directly refers to a function item type, but the compiler will display the type as something like `fn(u32) -> i32 {fn_name}` in error messages.

Because the function item type explicitly identifies the function, the item types of different functions - different items, or the same item with different generics - are distinct, and mixing them will create a type error:

    # #![allow(unused_variables)]
    #fn main() {
    fn foo<T>() { }
    let x = &mut foo::<i32>;
    *x = foo::<u32>; //~ ERROR mismatched types
    #}

However, there is a coercion from function items to function pointers with the same signature, which is triggered not only when a function item is used when a function pointer is directly expected, but also when different function item types with the same signature meet in different arms of the same `if` or `match`:

    # #![allow(unused_variables)]
    #fn main() {
    # let want_i32 = false;
    # fn foo<T>() { }

    // `foo_ptr_1` has function pointer type `fn()` here
    let foo_ptr_1: fn() = foo::<i32>;

    // ... and so does `foo_ptr_2` - this type-checks.
    let foo_ptr_2 = if want_i32 {
        foo::<i32>
    } else {
        foo::<u32>
    };
    #}

## Function pointer types

Function pointer types, written using the `fn` keyword, refer to a function whose identity is not necessarily known at compile-time. They can be created via a coercion from both function items and non-capturing closures.

A function pointer 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 where `Binop` is defined as a function pointer 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);
    #}

## 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 of its captured values.

- `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 captured variables, but read-only access to such values is allowed. Using `move` to capture variables by value is allowed so long as they aren't mutated or moved in the body of the closure. `Fn` inherits from `FnMut`, which itself inherits from `FnOnce`.

Closures that don't use anything from their environment, called 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

A trait object is an opaque value of another type that implements a set of traits. The set of traits is made up of an object safe base trait plus any number of auto traits.

Trait objects are written as the path to the base trait followed by the list of auto traits all separated by `+`. For example, given a trait `Trait`, the following are all trait objects: `Trait`, `Trait + Send`, `Trait + Send + Sync`.

Two trait object types alias each other if the base traits alias each other and if the sets of auto traits are the same. For example, `Trait + Send + UnwindSafe` is the same as `Trait + Unwindsafe + Send`.

> Warning: With two trait object types, even when the complete set of traits is the same, if the base traits differ, the type is different. For example, `Send + Sync` is a different type from `Sync + Send`. See issue 33140.

> Warning: Including the same auto trait multiple times is allowed, and each instance is considered a unique type. As such, `Trait + Send` is a distinct type than `Trait + Send + Send`. See issue 47010.

Due to the opaqueness of which concrete type the value is of, trait objects are dynamically sized types. Like all DSTs, trait objects are used behind some type of pointer; for example `&SomeTrait` or `Box<SomeTrait>`. Each instance of a pointer to 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.

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`.

### Trait Object Lifetime Bounds

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:

- If there is a unique bound from the containing type then that is the default.
- If there is more than one bound from the containing type then an explicit bound must be specified.
- Otherwise the default bound is `'static`.

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

    // These two are the same as Box<T> has no lifetime bound on T
    Box<Foo>
    Box<Foo + 'static>

    // ...and so are these:
    impl Foo {}
    impl Foo + 'static {}

    // ...so are these, because &'a T requires T: 'a
    &'a Foo
    &'a (Foo + 'a)

    // std::cell::Ref<'a, T> also requires T: 'a, so these are the same
    std::cell::Ref<'a, Foo>
    std::cell::Ref<'a, Foo + 'a>

    // This is an error:
    struct TwoBounds<'a, 'b, T: ?Sized + 'a + 'b>
    TwoBounds<'a, 'b, Foo> // Error: the lifetime bound for this object type cannot
                           // be deduced from context

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 a single lifetime bound of their own then, instead of infering 'static as the default bound, the bound on the trait is used instead

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

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

    // ...and so are these:
    impl<'a> Foo<'a> {}
    impl<'a> Foo<'a> + 'a {}

    // This is still an error:
    struct TwoBounds<'a, 'b, T: ?Sized + 'a + 'b>
    TwoBounds<'a, 'b, Foo<'c>>

## 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: it refers to 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`.