For now, this reference is a best-effort document. We strive for validity and completeness, but are not yet there. In the future, the docs and lang teams will work together to figure out how best to do this. Until then, this is a best-effort attempt. If you find something wrong or missing, file an issue or send in a pull request.

Operator expressions

Syntax
OperatorExpression :
      BorrowExpression
   | DereferenceExpression
   | ErrorPropagationExpression
   | NegationExpression
   | ArithmeticOrLogicalExpression
   | ComparisonExpression
   | LazyBooleanExpression
   | TypeCastExpression
   | AssignmentExpression
   | CompoundAssignmentExpression

Operators are defined for built in types by the Rust language. Many of the following operators can also be overloaded using traits in std::ops or std::cmp.

Overflow

Integer operators will panic when they overflow when compiled in debug mode. The -C debug-assertions and -C overflow-checks compiler flags can be used to control this more directly. The following things are considered to be overflow:

  • When +, * or - create a value greater than the maximum value, or less than the minimum value that can be stored. This includes unary - on the smallest value of any signed integer type.
  • Using / or %, where the left-hand argument is the smallest integer of a signed integer type and the right-hand argument is -1.
  • Using << or >> where the right-hand argument is greater than or equal to the number of bits in the type of the left-hand argument, or is negative.

Borrow operators

Syntax
BorrowExpression :
      (&|&&) Expression
   | (&|&&) mut Expression

The & (shared borrow) and &mut (mutable borrow) operators are unary prefix operators. When applied to a place expression, this expressions produces a reference (pointer) to the location that the value refers to. The memory location is also placed into a borrowed state for the duration of the reference. For a shared borrow (&), this implies that the place may not be mutated, but it may be read or shared again. For a mutable borrow (&mut), the place may not be accessed in any way until the borrow expires. &mut evaluates its operand in a mutable place expression context. If the & or &mut operators are applied to a value expression, then a temporary value is created.

These operators cannot be overloaded.


# #![allow(unused_variables)]
#fn main() {
{
    // a temporary with value 7 is created that lasts for this scope.
    let shared_reference = &7;
}
let mut array = [-2, 3, 9];
{
    // Mutably borrows `array` for this scope.
    // `array` may only be used through `mutable_reference`.
    let mutable_reference = &mut array;
}
#}

Even though && is a single token (the lazy 'and' operator), when used in the context of borrow expressions it works as two borrows:


# #![allow(unused_variables)]
#fn main() {
// same meanings:
let a = &&  10;
let a = & & 10;

// same meanings:
let a = &&&&  mut 10;
let a = && && mut 10;
let a = & & & & mut 10;
#}

The dereference operator

Syntax
DereferenceExpression :
   * Expression

The * (dereference) operator is also a unary prefix operator. When applied to a pointer it denotes the pointed-to location. If the expression is of type &mut T and *mut T, and is either a local variable, a (nested) field of a local variable or is a mutable place expression, then the resulting memory location can be assigned to. Dereferencing a raw pointer requires unsafe.

On non-pointer types *x is equivalent to *std::ops::Deref::deref(&x) in an immutable place expression context and *std::ops::Deref::deref_mut(&mut x) in a mutable place expression context.


# #![allow(unused_variables)]
#fn main() {
let x = &7;
assert_eq!(*x, 7);
let y = &mut 9;
*y = 11;
assert_eq!(*y, 11);
#}

The question mark operator

Syntax
ErrorPropagationExpression :
   Expression ?

The question mark operator (?) unwraps valid values or returns erroneous values, propagating them to the calling function. It is a unary postfix operator that can only be applied to the types Result<T, E> and Option<T>.

When applied to values of the Result<T, E> type, it propagates errors. If the value is Err(e), then it will return Err(From::from(e)) from the enclosing function or closure. If applied to Ok(x), then it will unwrap the value to evaluate to x.


# #![allow(unused_variables)]
#fn main() {
# use std::num::ParseIntError;
fn try_to_parse() -> Result<i32, ParseIntError> {
    let x: i32 = "123".parse()?; // x = 123
    let y: i32 = "24a".parse()?; // returns an Err() immediately
    Ok(x + y)                    // Doesn't run.
}

let res = try_to_parse();
println!("{:?}", res);
# assert!(res.is_err())
#}

When applied to values of the Option<T> type, it propagates Nones. If the value is None, then it will return None. If applied to Some(x), then it will unwrap the value to evaluate to x.


# #![allow(unused_variables)]
#fn main() {
fn try_option_some() -> Option<u8> {
    let val = Some(1)?;
    Some(val)
}
assert_eq!(try_option_some(), Some(1));

fn try_option_none() -> Option<u8> {
    let val = None?;
    Some(val)
}
assert_eq!(try_option_none(), None);
#}

? cannot be overloaded.

Negation operators

Syntax
NegationExpression :
      - Expression
   | ! Expression

These are the last two unary operators. This table summarizes the behavior of them on primitive types and which traits are used to overload these operators for other types. Remember that signed integers are always represented using two's complement. The operands of all of these operators are evaluated in value expression context so are moved or copied.

Symbol Integer bool Floating Point Overloading Trait
- Negation* Negation std::ops::Neg
! Bitwise NOT Logical NOT std::ops::Not

* Only for signed integer types.

Here are some example of these operators


# #![allow(unused_variables)]
#fn main() {
let x = 6;
assert_eq!(-x, -6);
assert_eq!(!x, -7);
assert_eq!(true, !false);
#}

Arithmetic and Logical Binary Operators

Syntax
ArithmeticOrLogicalExpression :
      Expression + Expression
   | Expression - Expression
   | Expression * Expression
   | Expression / Expression
   | Expression % Expression
   | Expression & Expression
   | Expression | Expression
   | Expression ^ Expression
   | Expression << Expression
   | Expression >> Expression

Binary operators expressions are all written with infix notation. This table summarizes the behavior of arithmetic and logical binary operators on primitive types and which traits are used to overload these operators for other types. Remember that signed integers are always represented using two's complement. The operands of all of these operators are evaluated in value expression context so are moved or copied.

Symbol Integer bool Floating Point Overloading Trait
+ Addition Addition std::ops::Add
- Subtraction Subtraction std::ops::Sub
* Multiplication Multiplication std::ops::Mul
/ Division* Division std::ops::Div
% Remainder Remainder std::ops::Rem
& Bitwise AND Logical AND std::ops::BitAnd
| Bitwise OR Logical OR std::ops::BitOr
^ Bitwise XOR Logical XOR std::ops::BitXor
<< Left Shift std::ops::Shl
>> Right Shift** std::ops::Shr

* Integer division rounds towards zero.

** Arithmetic right shift on signed integer types, logical right shift on unsigned integer types.

Here are examples of these operators being used.


# #![allow(unused_variables)]
#fn main() {
assert_eq!(3 + 6, 9);
assert_eq!(5.5 - 1.25, 4.25);
assert_eq!(-5 * 14, -70);
assert_eq!(14 / 3, 4);
assert_eq!(100 % 7, 2);
assert_eq!(0b1010 & 0b1100, 0b1000);
assert_eq!(0b1010 | 0b1100, 0b1110);
assert_eq!(0b1010 ^ 0b1100, 0b110);
assert_eq!(13 << 3, 104);
assert_eq!(-10 >> 2, -3);
#}

Comparison Operators

Syntax
ComparisonExpression :
      Expression == Expression
   | Expression != Expression
   | Expression > Expression
   | Expression < Expression
   | Expression >= Expression
   | Expression <= Expression

Comparison operators are also defined both for primitive types and many type in the standard library. Parentheses are required when chaining comparison operators. For example, the expression a == b == c is invalid and may be written as (a == b) == c.

Unlike arithmetic and logical operators, the traits for overloading the operators the traits for these operators are used more generally to show how a type may be compared and will likely be assumed to define actual comparisons by functions that use these traits as bounds. Many functions and macros in the standard library can then use that assumption (although not to ensure safety). Unlike the arithmetic and logical operators above, these operators implicitly take shared borrows of their operands, evaluating them in place expression context:

a == b;
// is equivalent to
::std::cmp::PartialEq::eq(&a, &b);

This means that the operands don't have to be moved out of.

Symbol Meaning Overloading method
== Equal std::cmp::PartialEq::eq
!= Not equal std::cmp::PartialEq::ne
> Greater than std::cmp::PartialOrd::gt
< Less than std::cmp::PartialOrd::lt
>= Greater than or equal to std::cmp::PartialOrd::ge
<= Less than or equal to std::cmp::PartialOrd::le

Here are examples of the comparison operators being used.


# #![allow(unused_variables)]
#fn main() {
assert!(123 == 123);
assert!(23 != -12);
assert!(12.5 > 12.2);
assert!([1, 2, 3] < [1, 3, 4]);
assert!('A' <= 'B');
assert!("World" >= "Hello");
#}

Lazy boolean operators

Syntax
LazyBooleanExpression :
      Expression || Expression
   | Expression && Expression

The operators || and && may be applied to operands of boolean type. The || operator denotes logical 'or', and the && operator denotes logical 'and'. They differ from | and & in that the right-hand operand is only evaluated when the left-hand operand does not already determine the result of the expression. That is, || only evaluates its right-hand operand when the left-hand operand evaluates to false, and && only when it evaluates to true.


# #![allow(unused_variables)]
#fn main() {
let x = false || true; // true
let y = false && panic!(); // false, doesn't evaluate `panic!()`
#}

Type cast expressions

Syntax
TypeCastExpression :
   Expression as PathInExpression

A type cast expression is denoted with the binary operator as.

Executing an as expression casts the value on the left-hand side to the type on the right-hand side.

An example of an as expression:


# #![allow(unused_variables)]
#fn main() {
# fn sum(values: &[f64]) -> f64 { 0.0 }
# fn len(values: &[f64]) -> i32 { 0 }
fn average(values: &[f64]) -> f64 {
    let sum: f64 = sum(values);
    let size: f64 = len(values) as f64;
    sum / size
}
#}

as can be used to explicitly perform coercions, as well as the following additional casts. Here *T means either *const T or *mut T.

Type of e U Cast performed by e as U
Integer or Float type Integer or Float type Numeric cast
C-like enum Integer type Enum cast
bool or char Integer type Primitive to integer cast
u8 char u8 to char cast
*T *V where V: Sized * Pointer to pointer cast
*T where T: Sized Numeric type Pointer to address cast
Integer type *V where V: Sized Address to pointer cast
&[T; n] *const T Array to pointer cast
Function pointer *V where V: Sized Function pointer to pointer cast
Function pointer Integer Function pointer to address cast
Closure ** Function pointer Closure to function pointer cast

* or T and V are compatible unsized types, e.g., both slices, both the same trait object.

** only for closures that do capture (close over) any local variables

Semantics

Assignment expressions

Syntax
AssignmentExpression :
   | Expression = Expression

An assignment expression consists of a place expression followed by an equals sign (=) and a value expression. Such an expression always has the unit type.

Evaluating an assignment expression drops the left-hand operand, unless it's an uninitialized local variable or field of a local variable, and either copies or moves its right-hand operand to its left-hand operand. The left-hand operand must be a place expression: using a value expression results in a compiler error, rather than promoting it to a temporary.


# #![allow(unused_variables)]
#fn main() {
# let mut x = 0;
# let y = 0;
x = y;
#}

Compound assignment expressions

Syntax
CompoundAssignmentExpression :
      Expression += Expression
   | Expression -= Expression
   | Expression *= Expression
   | Expression /= Expression
   | Expression %= Expression
   | Expression &= Expression
   | Expression |= Expression
   | Expression ^= Expression
   | Expression <<= Expression
   | Expression >>= Expression

The +, -, *, /, %, &, |, ^, <<, and >> operators may be composed with the = operator. The expression place_exp OP= value is equivalent to place_expr = place_expr OP val. For example, x = x + 1 may be written as x += 1. Any such expression always has the unit type. These operators can all be overloaded using the trait with the same name as for the normal operation followed by 'Assign', for example, std::ops::AddAssign is used to overload +=. As with =, place_expr must be a place expression.


# #![allow(unused_variables)]
#fn main() {
let mut x = 10;
x += 4;
assert_eq!(x, 14);
#}