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 and the operations that may be performed on the value.

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.

The list of types is:

- Primitive types:
  - Boolean — `true` or `false`
  - Numeric — integer and float
  - Textual — `char` and `str`
  - Never — `!` — a type with no values
- Sequence types:
  - Tuple
  - Array
  - Slice
- User-defined types:
  - Struct
  - Enum
  - Union
- Function types:
  - Functions
  - Closures
- Pointer types:
  - References
  - Raw pointers
  - Function pointers
- Trait types:
  - Trait objects
  - Impl trait

## Type expressions

> Syntax
> Type :
> TypeNoBounds
> | ImplTraitType
> | TraitObjectType

> TypeNoBounds :
> ParenthesizedType
> | ImplTraitTypeOneBound
> | TraitObjectTypeOneBound
> | TypePath
> | TupleType
> | NeverType
> | RawPointerType
> | ReferenceType
> | ArrayType
> | SliceType
> | InferredType
> | QualifiedPathInType
> | BareFunctionType
> | MacroInvocation

A type expression as defined in the Type grammar rule above is the syntax for referring to a type. It may refer to:

- Sequence types (tuple, array, slice).
- Type paths which can reference:
  - Primitive types (boolean, numeric, textual).
  - Paths to an item (struct, enum, union, type alias, trait).
  - `Self` path where `Self` is the implementing type.
  - Generic type parameters.
- Pointer types (reference, raw pointer, function pointer).
- The inferred type which asks the compiler to determine the type.
- Parentheses which are used for disambiguation.
- Trait types: Trait objects and impl trait.
- The never type.
- Macros which expand to a type expression.

### Parenthesized types

> ParenthesizedType :
> `(` Type `)`

In some situations the combination of types may be ambiguous. Use parentheses around a type to avoid ambiguity. For example, the `+` operator for type boundaries within a reference type is unclear where the boundary applies, so the use of parentheses is required. Grammar rules that require this disambiguation use the TypeNoBounds rule instead of Type.

    # #![allow(unused_variables)]
    #fn main() {
    # use std::any::Any;
    type T<'a> = &'a (dyn Any + Send);
    #}

## 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 aliases, 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))));
    #}