# 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`.
* The [never type] `!`

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

* [Tuples]
* [Arrays]
* [Slices]
* [Function pointers]
* [References]
* [Pointers]

[machine types]: #machine-types
[machine-dependent integer types]: #machine-dependent-integer-types
[textual types]: #textual-types
[never-type]: #never-type
[Tuples]: #tuple-types
[Arrays]: #array-and-slice-types
[Slices]: #array-and-slice-types
[References]: #pointer-types
[Pointers]: #raw-pointers-const-and-mut
[Function pointers]: #function-pointer-types
[function]: #function-types
[closure]: #closure-types

## Numeric types

### Machine types

The machine types are the following:

* The unsigned word types `u8`, `u16`, `u32`, `u64`, and `u128` with values drawn from
  the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1],
  [0, 2^64 - 1], and [0, 2^128 - 1] respectively.

* The signed two's complement word types `i8`, `i16`, `i32`, `i64`, and `i128`, 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], and
  [-(2^127), 2^127 - 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](
http://www.unicode.org/glossary/#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`.

## Never type

The never type `!` is a type with no values, representing the result of
computations that never complete. Expressions of type `!` can be coerced into
any other type.

## 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:

```rust
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:

```rust
// 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.[^structtype]

New instances of a `struct` can be constructed with a [struct
expression](expressions/struct-expr.html).

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](visibility-and-privacy.html), 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.

[^structtype]: `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](items/enumerations.html). [^enumtype]

An [`enum` item](items/enumerations.html) 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](expressions/enum-variant-expr.html).

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](items/enumerations.html).

[^enumtype]: 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](items/unions.html).

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.

[`Copy`]: special-types-and-traits.html#copy

## Recursive types

Nominal types &mdash; [structs](#struct-types),
[enumerations](#enumerated-types) and [unions](#union-types) &mdash; 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](../grammar.html#type-definitions), or other structural types
  such as [arrays](#array-and-slice-types) or [tuples](#tuple-types)). 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](#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:

```rust
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](interior-mutability.html) 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](expressions.html#temporary-lifetimes) 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](unsafe-functions.html), 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](dynamically-sized-types.html) 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:

```rust,compile_fail,E0308
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](#function-pointer-types) 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`:

[coercion]: type-coercions.html

```rust
# 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>
};
```

All function items implement [`Fn`], [`FnMut`], [`FnOnce`], [`Copy`],
[`Clone]`, [`Send`], and [`Sync`].

## 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](#function-item-types) and
non-capturing [closures](#closure-types).

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:

```rust
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. A closure type is approximately equivalent to a
struct which contains the captured variables. For instance, the following
closure:

```rust
fn f<F : FnOnce() -> String> (g: F) {
    println!("{}", g());
}

let mut s = String::from("foo");
let t = String::from("bar");

f(|| {
    s += &*t;
    s
});
// Prints "foobar".
```

generates a closure type roughly like the following:

```rust,ignore
struct Closure<'a> {
    s : String,
    t : &'a String,
}

impl<'a> (FnOnce() -> String) for Closure<'a> {
    fn call_once(self) -> String {
        self.s += &*self.t;
        self.s
    }
}
```

so that the call to `f` works as if it were:

```rust,ignore
f(Closure{s: s, t: &t});
```

The compiler prefers to capture a closed-over variable by immutable borrow,
followed by unique immutable borrow (see below), by mutable borrow, and finally
by move. It will pick the first choice of these that allows the closure to
compile. The choice is made only with regards to the contents of the closure
expression; the compiler does not take into account surrounding code, such as
the lifetimes of involved variables.

If the `move` keyword is used, then all captures are by move or, for `Copy`
types, by copy, regardless of whether a borrow would work. The `move` keyword is
usually used to allow the closure to outlive the captured values, such as if the
closure is being returned or used to spawn a new thread.

Composite types such as structs, tuples, and enums are always captured entirely,
not by individual fields. It may be necessary to borrow into a local variable in
order to capture a single field:

```rust
# use std::collections::HashSet;
# 
struct SetVec {
    set: HashSet<u32>,
    vec: Vec<u32>
}

impl SetVec {
    fn populate(&mut self) {
        let vec = &mut self.vec;
        self.set.iter().for_each(|&n| {
            vec.push(n);
        })
    }
}
```

If, instead, the closure were to use `self.vec` directly, then it would attempt
to capture `self` by mutable reference. But since `self.set` is already
borrowed to iterate over, the code would not compile.

### Unique immutable borrows in captures

Captures can occur by a special kind of borrow called a _unique immutable
borrow_, which cannot be used anywhere else in the language and cannot be
written out explicitly. It occurs when modifying the referent of a mutable
reference, as in the following example:

```rust
let mut b = false;
let x = &mut b;
{
    let mut c = || { *x = true; };
    // The following line is an error:
    // let y = &x;
    c();
}
let z = &x;
```

In this case, borrowing `x` mutably is not possible, because `x` is not `mut`.
But at the same time, borrowing `x` immutably would make the assignment illegal,
because a `& &mut` reference may not be unique, so it cannot safely be used to
modify a value. So a unique immutable borrow is used: it borrows `x` immutably,
but like a mutable borrow, it must be unique. In the above example, uncommenting
the declaration of `y` will produce an error because it would violate the
uniqueness of the closure's borrow of `x`; the declaration of z is valid because
the closure's lifetime has expired at the end of the block, releasing the borrow.

### Call traits and coercions

Closure types all implement [`FnOnce`], indicating that they can be called once
by consuming ownership of the closure. Additionally, some closures implement
more specific call traits:

* A closure which does not move out of any captured variables implements
  [`FnMut`], indicating that it can be called by mutable reference.

* A closure which does not mutate or move out of any captured variables
  implements [`Fn`], indicating that it can be called by shared reference.

> Note: `move` closures may still implement [`Fn`] or [`FnMut`], even though
> they capture variables by move. This is because the traits implemented by a
> closure type are determined by what the closure does with captured values,
> not how it captures them.

*Non-capturing closures* are closures that don't capture anything from their
environment. They can be coerced to function pointers (`fn`) with the matching
signature.

```rust
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);
```

### Other traits

All closure types implement [`Sized`]. Additionally, closure types implement the
following traits if allowed to do so by the types of the captures it stores:

* [`Clone`]
* [`Copy`]
* [`Sync`]
* [`Send`]

The rules for [`Send`] and [`Sync`] match those for normal struct types, while
[`Clone`] and [`Copy`] behave as if [derived][derive]. For [`Clone`], the order
of cloning of the captured variables is left unspecified.

Because captures are often by reference, the following general rules arise:

* A closure is [`Sync`] if all captured variables are [`Sync`].
* A closure is [`Send`] if all variables captured by non-unique immutable
  reference are [`Sync`], and all values captured by unique immutable or mutable
  reference, copy, or move are [`Send`].
* A closure is [`Clone`] or [`Copy`] if it does not capture any values by
  unique immutable or mutable reference, and if all values it captures by copy
  or move are [`Clone`] or [`Copy`], respectively.

## Trait objects

> **<sup>Syntax</sup>**
> _TraitObjectType_ :
> &nbsp;&nbsp; `dyn`<sup>?</sup> _TypeParamBounds_

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 implement the base trait, its auto traits, and any [supertraits]
of the base trait.

Trait objects are written as the optional keyword `dyn` followed by a set of
trait bounds, but with the following restrictions on the trait bounds. All
traits except the first trait must be auto traits, there may not be more than
one lifetime, and opt-out bounds (e.g. `?sized`) are not allowed. Furthermore,
paths to traits may be parenthesized.

For example, given a trait `Trait`, the following are all trait objects: 

* `Trait`
* `dyn Trait`
* `dyn Trait + Send`
* `dyn Trait + Send + Sync`
* `dyn Trait + 'static`
* `dyn Trait + Send + 'static`
* `dyn Trait +`
* `dyn 'static + Trait`.
* `dyn (Trait)`

If the first bound of the trait object is a path that starts with `::`, then the
`dyn` will be treated as a part of the path. The first path can be put in
parenthesis to get around this. As such, if you want a trait object with the
trait `::your_module::Trait`, you should write it as
`dyn (::your_module::Trait)`.

> Note: For clarity, it is recommended to always use the `dyn` keyword on your
> trait objects unless your codebase supports compiling with Rust 1.26 or lower.

Two trait object types alias each other if the base traits alias each other and
if the sets of auto traits are the same and the lifetime bounds are the same.
For example, `dyn Trait + Send + UnwindSafe` is the same as
`dyn 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,
> `dyn Send + Sync` is a different type from `dyn Sync + Send`. See
> [issue 33140].

Due to the opaqueness of which concrete type the value is of, trait objects are
[dynamically sized types]. Like all
<abbr title="dynamically sized types">DSTs</abbr>, trait objects are used
behind some type of pointer; for example `&dyn SomeTrait` or
`Box<dyn 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` and its [supertraits] 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:

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

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

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

fn main() {
    print(Box::new(10) as Box<dyn 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. This lifetime is written as
`Trait + 'a`. There are [defaults] that allow this lifetime to usually be
inferred with a sensible choice.

[defaults]: lifetime-elision.html#default-trait-object-lifetimes

## Type parameters

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

```rust
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`.

## Anonymous type parameters

> Note: This section is a placeholder for more comprehensive reference
> material.

> Note: This is often called "impl Trait in argument position".

Functions can declare an argument to be an anonymous type parameter where the
callee must provide a type that has the bounds declared by the anonymous type
parameter and the function can only use the methods available by the trait
bounds of the anonymous type parameter.

They are written as `impl` followed by a set of trait bounds.

## Abstract return types

> Note: This section is a placeholder for more comprehensive reference
> material.

> Note: This is often called "impl Trait in return position".

Functions, except for associated trait functions, can return an abstract
return type. These  types stand in for another concrete type where the
use-site may only use the trait methods declared by the trait bounds of the
type.

They are written as `impl` followed by a set of trait bounds.

## Self types

The special type `Self` has a meaning within traits and implementations: it
refers to the implementing type. For example, in:

```rust
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:

```rust
trait Printable {
    fn make_string(&self) -> String;
}

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

> Note: The notation `&self` is a shorthand for `self: &Self`.

[`Fn`]: ../std/ops/trait.Fn.html
[`FnMut`]: ../std/ops/trait.FnMut.html
[`FnOnce`]: ../std/ops/trait.FnOnce.html
[`Copy`]: special-types-and-traits.html#copy
[`Clone`]: special-types-and-traits.html#clone
[`Send`]: special-types-and-traits.html#send
[`Sync`]: special-types-and-traits.html#sync
[`Sized`]: special-types-and-traits.html#sized
[derive]: attributes.html#derive
[`Vec<T>`]: ../std/vec/struct.Vec.html
[dynamically sized type]: dynamically-sized-types.html
[dynamically sized types]: dynamically-sized-types.html
[struct expression]: expressions/struct-expr.html
[closure expression]: expressions/closure-expr.html
[auto traits]: special-types-and-traits.html#auto-traits
[object safe]: items/traits.html#object-safety
[issue 47010]: https://github.com/rust-lang/rust/issues/47010
[issue 33140]: https://github.com/rust-lang/rust/issues/33140
[_PATH_]: paths.html
[_LIFETIME_OR_LABEL_]: tokens.html#lifetimes-and-loop-labels
[supertraits]: items/traits.html#supertraits