Appendix B: Operators

Unary operator expressions

Rust defines the following unary operators. They are all written as prefix operators, before the expression they apply to.

- : Negation. Signed integer types and floating-point types support negation. It is an error to apply negation to unsigned types; for example, the compiler rejects -1u32.
* : Dereference. When applied to a pointer, it denotes the pointed-to location. For pointers to mutable locations, the resulting value can be assigned to. On non-pointer types, it calls the deref method of the std::ops::Deref trait, or the deref_mut method of the std::ops::DerefMut trait (if implemented by the type and required for an outer expression that will or could mutate the dereference), and produces the result of dereferencing the & or &mut borrowed pointer returned from the overload method.
! : Logical negation. On the boolean type, this flips between true and false. On integer types, this inverts the individual bits in the two's complement representation of the value.
& and &mut : Borrowing. When applied to a value, these operators produce a reference (pointer) to that value. The value is also placed into a borrowed state for the duration of the reference. For a shared borrow (&), this implies that the value may not be mutated, but it may be read or shared again. For a mutable borrow (&mut), the value may not be accessed in any way until the borrow expires.

Binary operator expressions

Binary operators expressions are given in order of operator precedence.

Arithmetic operators

Binary arithmetic expressions are syntactic sugar for calls to built-in traits, defined in the std::ops module of the std library. This means arithmetic operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here.

+ : Addition and array/string concatenation. Calls the add method on the std::ops::Add trait.
- : Subtraction. Calls the sub method on the std::ops::Sub trait.
* : Multiplication. Calls the mul method on the std::ops::Mul trait.
/ : Quotient. Calls the div method on the std::ops::Div trait.
% : Remainder. Calls the rem method on the std::ops::Rem trait.

Note that Rust does not have a built-in operator for exponential (power) calculation; see the pow method on the numeric types.

Bitwise operators

Like the arithmetic operators, bitwise operators are syntactic sugar for calls to methods of built-in traits. This means bitwise operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here. Bitwise &, | and ^ applied to boolean arguments are equivalent to logical &&, || and != evaluated in non-lazy fashion.

& : Bitwise AND. Calls the bitand method of the std::ops::BitAnd trait.
| : Bitwise inclusive OR. Calls the bitor method of the std::ops::BitOr trait.
^ : Bitwise exclusive OR. Calls the bitxor method of the std::ops::BitXor trait.
<< : Left shift. Calls the shl method of the std::ops::Shl trait.
>> : Right shift (arithmetic). Calls the shr method of the std::ops::Shr trait.

Lazy boolean operators

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.

Comparison operators

Comparison operators are, like the arithmetic operators and bitwise operators, syntactic sugar for calls to built-in traits. This means that comparison operators can be overridden for user-defined types. The default meaning of the operators on standard types is given here.

== : Equal to. Calls the eq method on the std::cmp::PartialEq trait.
!= : Unequal to. Calls the ne method on the std::cmp::PartialEq trait.
< : Less than. Calls the lt method on the std::cmp::PartialOrd trait.
> : Greater than. Calls the gt method on the std::cmp::PartialOrd trait.
<= : Less than or equal. Calls the le method on the std::cmp::PartialOrd trait.
>= : Greater than or equal. Calls the ge method on the std::cmp::PartialOrd trait.

Type cast expressions

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:

# 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
}

Some of the conversions which can be done through the as operator can also be done implicitly at various points in the program, such as argument passing and assignment to a let binding with an explicit type. Implicit conversions are limited to "harmless" conversions that do not lose information and which have minimal or no risk of surprising side-effects on the dynamic execution semantics.

Assignment expressions

An assignment expression consists of a pattern followed by an equals sign (=) and an expression.

Evaluating an assignment expression either copies or moves its right-hand operand to its left-hand operand.

# let mut x = 0;
# let y = 0;
x = y;

Compound assignment expressions

The +, -, *, /, %, &, |, ^, <<, and >> operators may be composed with the = operator. The expression lval OP= val is equivalent to lval = lval OP val. For example, x = x + 1 may be written as x += 1.

Any such expression always has the unit type.

Operator precedence

The precedence of Rust binary operators is ordered as follows, going from strong to weak:

as :
* / %
+ -
<< >>
&
^
|
== != < > <= >=
&&
||
.. ...
<-
=

Operators at the same precedence level are evaluated left-to-right. Unary operators have the same precedence level and are stronger than any of the binary operators.

B - Operators