Operator expressions

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.

Grouped expressions

An expression enclosed in parentheses evaluates to the result of the enclosed expression. Parentheses can be used to explicitly specify evaluation order within an expression.

An example of a parenthesized expression:

# #![allow(unused_variables)]
#fn main() {
let x: i32 = 2 + 3 * 4;
let y: i32 = (2 + 3) * 4;
assert_eq!(x, 14);
assert_eq!(y, 20);
#}

Borrow operators

The & (shared borrow) and &mut (mutable borrow) operators are unary prefix operators. When applied to an lvalue produce a reference (pointer) to the location that the value refers to. The lvalue is also placed into a borrowed state for the duration of the reference. For a shared borrow (&), this implies that the lvalue may not be mutated, but it may be read or shared again. For a mutable borrow (&mut), the lvalue may not be accessed in any way until the borrow expires. &mut evaluates its operand in a mutable lvalue context. If the & or &mut operators are applied to an rvalue, a temporary value is created; the lifetime of this temporary value is defined by syntactic rules. 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;
}
#}

The dereference operator

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 variance or is a mutable lvalue, then the resulting lvalue 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 lvalue context and *std::ops::Deref::deref_mut(&mut x) in a mutable lvalue context.

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

The ? operator.

The ? ("question mark") operator can be applied to values of the Result<T, E> type to propagate errors. If applied to Err(e) it will return Err(From::from(e)) from the enclosing function or closure. If applied to Ok(x) it will unwrap the value to return x. Unlike other unary operators ? is written in postfix notation. ? cannot be overloaded.

# #![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())
#}

Negation operators

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 rvalue 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

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 rvalue 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

* 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

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 lvalue 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

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

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

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

Semantics

• Numeric cast
  • Casting between two integers of the same size (e.g. i32 -> u32) is a no-op
  • Casting from a larger integer to a smaller integer (e.g. u32 -> u8) will truncate
  • Casting from a smaller integer to a larger integer (e.g. u8 -> u32) will
    • zero-extend if the source is unsigned
    • sign-extend if the source is signed
  • Casting from a float to an integer will round the float towards zero
    • NOTE: currently this will cause Undefined Behavior if the rounded value cannot be represented by the target integer type. This includes Inf and NaN. This is a bug and will be fixed.
  • Casting from an integer to float will produce the floating point representation of the integer, rounded if necessary (rounding strategy unspecified)
  • Casting from an f32 to an f64 is perfect and lossless
  • Casting from an f64 to an f32 will produce the closest possible value (rounding strategy unspecified)
    • NOTE: currently this will cause Undefined Behavior if the value is finite but larger or smaller than the largest or smallest finite value representable by f32. This is a bug and will be fixed.
• Enum cast
  • Casts an enum to its discriminant, then uses a numeric cast if needed.
• Primitive to integer cast
  • false casts to 0, true casts to 1
  • char casts to the value of the code point, then uses a numeric cast if needed.
• u8 to char cast
  • Casts to the char with the corresponding code point.

Assignment expressions

An assignment expression consists of an lvalue expression followed by an equals sign (=) and an rvalue expression.

Evaluating an assignment expression drops the left-hand operand, unless it's an unitialized 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 an lvalue: using an rvalue 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

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. 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 =, lval must be an lvalue.

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