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 valuestrue
andfalse
. - The machine types (integer and floating-point).
- The machine-dependent integer types.
- The textual types
char
andstr
.
There are also some primitive constructs for generic types built in to the language:
Numeric types
Machine types
The machine types are the following:
-
The unsigned word types
u8
,u16
,u32
andu64
, 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
andi64
, 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
andbinary64
floating-point types:f32
andf64
, 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.
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.
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> }; #}
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 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 asFnOnce
can move out of its captured values. -
FnMut
: The closure can be called multiple times as mutable. A closure called asFnMut
can mutate values from its environment.FnMut
inherits fromFnOnce
(i.e. anything implementingFnMut
also implementsFnOnce
). -
Fn
: The closure can be called multiple times through a shared reference. A closure called asFn
can neither move out from nor mutate captured variables, but read-only access to such values is allowed. Usingmove
to capture variables by value is allowed so long as they aren't mutated or moved in the body of the closure.Fn
inherits fromFnMut
, which itself inherits fromFnOnce
.
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 implement the base trait, its auto traits, and any super traits of the base trait.
Trait objects are written as the path to the base trait followed by the list
of auto traits followed optionally by a lifetime bound all separated by +
. For
example, given a trait Trait
, the following are all trait objects: Trait
,
Trait + Send
, Trait + Send + Sync
, Trait + 'static
,
Trait + Send + 'static
.
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, 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 fromSync + 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 thanTrait + 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 implementsSomeTrait
- a virtual method table, often just called a vtable, which contains, for
each method of
SomeTrait
thatT
implements, a pointer toT
'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
.