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)]
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)]
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))));
}