Operator expressions

OperatorExpression ->
      BorrowExpression
    | DereferenceExpression
    | TryPropagationExpression
    | 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:

Note

The exception for literal expressions behind unary - means that forms such as -128_i8 or let j: i8 = -(128) never cause a panic and have the expected value of -128.

In these cases, the literal expression already has the most negative value for its type (for example, 128_i8 has the value -128) because integer literals are truncated to their type per the description in Integer literal expressions.

Negation of these most negative values leaves the value unchanged due to two's complement overflow conventions.

In rustc, these most negative expressions are also ignored by the overflowing_literals lint check.

Borrow operators

BorrowExpression ->
      (`&`|`&&`) Expression
    | (`&`|`&&`) `mut` Expression
    | (`&`|`&&`) `raw` `const` Expression
    | (`&`|`&&`) `raw` `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.

{
    // 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:

// same meanings:
let a = &&  10;
let a = & & 10;

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

Raw borrow operators

&raw const and &raw mut are the raw borrow operators.

The operand expression of these operators is evaluated in place expression context.

&raw const expr then creates a const raw pointer of type *const T to the given place, and &raw mut expr creates a mutable raw pointer of type *mut T.

The raw borrow operators must be used instead of a borrow operator whenever the place expression could evaluate to a place that is not properly aligned or does not store a valid value as determined by its type, or whenever creating a reference would introduce incorrect aliasing assumptions. In those situations, using a borrow operator would cause undefined behavior by creating an invalid reference, but a raw pointer may still be constructed.

The following is an example of creating a raw pointer to an unaligned place through a packed struct:

#[repr(packed)]
struct Packed {
    f1: u8,
    f2: u16,
}

let packed = Packed { f1: 1, f2: 2 };
// `&packed.f2` would create an unaligned reference, and thus be undefined behavior!
let raw_f2 = &raw const packed.f2;
assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);

The following is an example of creating a raw pointer to a place that does not contain a valid value:

use std::mem::MaybeUninit;

struct Demo {
    field: bool,
}

let mut uninit = MaybeUninit::<Demo>::uninit();
// `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
// and thus be undefined behavior!
let f1_ptr = unsafe { &raw mut (*uninit.as_mut_ptr()).field };
unsafe { f1_ptr.write(true); }
let init = unsafe { uninit.assume_init() };

The dereference operator

DereferenceExpression -> `*` Expression

The * (dereference) operator is also a unary prefix operator.

When applied to a pointer or Box, it denotes the pointed-to location.

If the expression is of type &mut T, *mut T, or Box<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.

When applied to a Box, the resultant place may be moved from.

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::DerefMut::deref_mut(&mut x) in a mutable place expression context, except that when *x undergoes temporary lifetime extension, the dereferenced expression x also has its temporary scope extended.

# struct NoCopy;
let a = &7;
assert_eq!(*a, 7);
let b = &mut 9;
*b = 11;
assert_eq!(*b, 11);
let c = Box::new(NoCopy);
let d: NoCopy = *c;

// The temporary holding the result of `String::new()` is extended
// to live to the end of the block, so `x` may be used in subsequent
// statements.
let x = &*String::new();
# x;

// The temporary holding the result of `String::new()` is dropped at
// the end of the statement, so it's an error to use `y` after.
let y = &*std::ops::Deref::deref(&String::new()); // ERROR
# y;

The try propagation expression

TryPropagationExpression -> Expression `?`

The try propagation expression uses the value of the inner expression and the Try trait to decide whether to produce a value, and if so, what value to produce, or whether to return a value to the caller, and if so, what value to return.

Example

# use std::num::ParseIntError;
fn try_to_parse() -> Result<i32, ParseIntError> {
    let x: i32 = "123".parse()?; // `x` is `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())

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);

use std::ops::ControlFlow;

pub struct TreeNode<T> {
    value: T,
    left: Option<Box<TreeNode<T>>>,
    right: Option<Box<TreeNode<T>>>,
}

impl<T> TreeNode<T> {
    pub fn traverse_inorder<B>(&self, f: &mut impl FnMut(&T) -> ControlFlow<B>) -> ControlFlow<B> {
        if let Some(left) = &self.left {
            left.traverse_inorder(f)?;
        }
        f(&self.value)?;
        if let Some(right) = &self.right {
            right.traverse_inorder(f)?;
        }
        ControlFlow::Continue(())
    }
}
#
# fn main() {
#     let n = TreeNode {
#         value: 1,
#         left: Some(Box::new(TreeNode{value: 2, left: None, right: None})),
#         right: None,
#     };
#     let v = n.traverse_inorder(&mut |t| {
#         if *t == 2 {
#             ControlFlow::Break("found")
#         } else {
#             ControlFlow::Continue(())
#         }
#     });
#     assert_eq!(v, ControlFlow::Break("found"));
# }

Note

The Try trait is currently unstable, and thus cannot be implemented for user types.

The try propagation expression is currently roughly equivalent to:

# #![ feature(try_trait_v2) ]
# fn example() -> Result<(), ()> {
# let expr = Ok(());
match core::ops::Try::branch(expr) {
    core::ops::ControlFlow::Continue(val) => val,
    core::ops::ControlFlow::Break(residual) =>
        return core::ops::FromResidual::from_residual(residual),
}
# Ok(())
# }

Note

The try propagation operator is sometimes called the question mark operator, the ? operator, or the try operator.

The try propagation operator can be applied to expressions with the type of:

Negation operators

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

let x = 6;
assert_eq!(-x, -6);
assert_eq!(!x, -7);
assert_eq!(true, !false);

Arithmetic and logical binary operators

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 Overloading Compound Assignment Trait
+ Addition Addition std::ops::Add std::ops::AddAssign
- Subtraction Subtraction std::ops::Sub std::ops::SubAssign
* Multiplication Multiplication std::ops::Mul std::ops::MulAssign
/ Division*† Division std::ops::Div std::ops::DivAssign
% Remainder**† Remainder std::ops::Rem std::ops::RemAssign
& Bitwise AND Logical AND std::ops::BitAnd std::ops::BitAndAssign
| Bitwise OR Logical OR std::ops::BitOr std::ops::BitOrAssign
^ Bitwise XOR Logical XOR std::ops::BitXor std::ops::BitXorAssign
<< Left Shift std::ops::Shl std::ops::ShlAssign
>> Right Shift*** std::ops::Shr std::ops::ShrAssign

* Integer division rounds towards zero.

** Rust uses a remainder defined with truncating division. Given remainder = dividend % divisor, the remainder will have the same sign as the dividend.

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

† For integer types, division by zero panics.

Here are examples of these operators being used.

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

ComparisonExpression ->
      Expression `==` Expression
    | Expression `!=` Expression
    | Expression `>` Expression
    | Expression `<` Expression
    | Expression `>=` Expression
    | Expression `<=` Expression

Comparison operators are also defined both for primitive types and many types 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 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:

# let a = 1;
# let b = 1;
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.

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

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.

let x = false || true; // true
let y = false && panic!(); // false, doesn't evaluate `panic!()`

Type cast expressions

TypeCastExpression -> Expression `as` TypeNoBounds

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
}

as can be used to explicitly perform coercions, as well as the following additional casts. Any cast that does not fit either a coercion rule or an entry in the table is a compiler error. Here *T means either *const T or *mut T. m stands for optional mut in reference types and mut or const in pointer types.

Type of e U Cast performed by e as U
Integer or Float type Integer or Float type Numeric cast
Enumeration Integer type Enum cast
bool or char Integer type Primitive to integer cast
u8 char u8 to char cast
*T *V (when compatible) Pointer to pointer cast
*T where T: Sized Integer type Pointer to address cast
Integer type *V where V: Sized Address to pointer cast
&m₁ [T; n] *m₂ T 1 Array to pointer cast
*m₁ [T; n] *m₂ T 1 Array to pointer cast
Function item Function pointer Function item to function pointer cast
Function item *V where V: Sized Function item to pointer cast
Function item Integer Function item to address cast
Function pointer *V where V: Sized Function pointer to pointer cast
Function pointer Integer Function pointer to address cast
Closure 2 Function pointer Closure to function pointer cast

Semantics

Numeric cast

* if integer-to-float casts with this rounding mode and overflow behavior are not supported natively by the hardware, these casts will likely be slower than expected.

** if f64-to-f32 casts with this rounding mode and overflow behavior are not supported natively by the hardware, these casts will likely be slower than expected.

*** as defined in IEEE 754-2008 §4.3.1: pick the nearest floating point number, preferring the one with an even least significant digit if exactly halfway between two floating point numbers.

Enum cast

Casts an enum to its discriminant, then uses a numeric cast if needed. Casting is limited to the following kinds of enumerations:

enum Enum { A, B, C }
assert_eq!(Enum::A as i32, 0);
assert_eq!(Enum::B as i32, 1);
assert_eq!(Enum::C as i32, 2);

Casting is not allowed if the enum implements [Drop].

Primitive to integer cast

assert_eq!(false as i32, 0);
assert_eq!(true as i32, 1);
assert_eq!('A' as i32, 65);
assert_eq!('Ö' as i32, 214);

u8 to char cast

Casts to the char with the corresponding code point.

assert_eq!(65u8 as char, 'A');
assert_eq!(214u8 as char, 'Ö');

Pointer to address cast

Casting from a raw pointer to an integer produces the machine address of the referenced memory. If the integer type is smaller than the pointer type, the address may be truncated; using usize avoids this.

Address to pointer cast

Casting from an integer to a raw pointer interprets the integer as a memory address and produces a pointer referencing that memory.

Warning

This interacts with the Rust memory model, which is still under development. A pointer obtained from this cast may suffer additional restrictions even if it is bitwise equal to a valid pointer. Dereferencing such a pointer may be undefined behavior if aliasing rules are not followed.

A trivial example of sound address arithmetic:

let mut values: [i32; 2] = [1, 2];
let p1: *mut i32 = values.as_mut_ptr();
let first_address = p1 as usize;
let second_address = first_address + 4; // 4 == size_of::<i32>()
let p2 = second_address as *mut i32;
unsafe {
    *p2 += 1;
}
assert_eq!(values[1], 3);

Pointer-to-pointer cast

*const T / *mut T can be cast to *const U / *mut U with the following behavior:

Assignment expressions

AssignmentExpression -> Expression `=` Expression

An assignment expression moves a value into a specified place.

An assignment expression consists of a mutable assignee expression, the assignee operand, followed by an equals sign (=) and a value expression, the assigned value operand.

In its most basic form, an assignee expression is a place expression, and we discuss this case first.

The more general case of destructuring assignment is discussed below, but this case always decomposes into sequential assignments to place expressions, which may be considered the more fundamental case.

Basic assignments

Evaluating assignment expressions begins by evaluating its operands. The assigned value operand is evaluated first, followed by the assignee expression.

For destructuring assignment, subexpressions of the assignee expression are evaluated left-to-right.

Note

This is different than other expressions in that the right operand is evaluated before the left one.

It then has the effect of first dropping the value at the assigned place, unless the place is an uninitialized local variable or an uninitialized field of a local variable.

Next it either copies or moves the assigned value to the assigned place.

An assignment expression always produces the unit value.

Example:

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

Destructuring assignments

Destructuring assignment is a counterpart to destructuring pattern matches for variable declaration, permitting assignment to complex values, such as tuples or structs. For instance, we may swap two mutable variables:

let (mut a, mut b) = (0, 1);
// Swap `a` and `b` using destructuring assignment.
(b, a) = (a, b);

In contrast to destructuring declarations using let, patterns may not appear on the left-hand side of an assignment due to syntactic ambiguities. Instead, a group of expressions that correspond to patterns are designated to be assignee expressions, and permitted on the left-hand side of an assignment. Assignee expressions are then desugared to pattern matches followed by sequential assignment.

The desugared patterns must be irrefutable: in particular, this means that only slice patterns whose length is known at compile-time, and the trivial slice [..], are permitted for destructuring assignment.

The desugaring method is straightforward, and is illustrated best by example.

# struct Struct { x: u32, y: u32 }
# let (mut a, mut b) = (0, 0);
(a, b) = (3, 4);

[a, b] = [3, 4];

Struct { x: a, y: b } = Struct { x: 3, y: 4};

// desugars to:

{
    let (_a, _b) = (3, 4);
    a = _a;
    b = _b;
}

{
    let [_a, _b] = [3, 4];
    a = _a;
    b = _b;
}

{
    let Struct { x: _a, y: _b } = Struct { x: 3, y: 4};
    a = _a;
    b = _b;
}

Identifiers are not forbidden from being used multiple times in a single assignee expression.

Underscore expressions and empty range expressions may be used to ignore certain values, without binding them.

Note that default binding modes do not apply for the desugared expression.

Note

The desugaring restricts the temporary scope of the assigned value operand (the RHS) of a destructuring assignment.

In a basic assignment, the temporary is dropped at the end of the enclosing temporary scope. Below, that's the statement. Therefore, the assignment and use is allowed.

# fn temp() {}
fn f<T>(x: T) -> T { x }
let x;
(x = f(&temp()), x); // OK

Conversely, in a destructuring assignment, the temporary is dropped at the end of the let statement in the desugaring. As that happens before we try to assign to x, below, it fails.

# fn temp() {}
# fn f<T>(x: T) -> T { x }
# let x;
[x] = [f(&temp())]; // ERROR

This desugars to:

# fn temp() {}
# fn f<T>(x: T) -> T { x }
# let x;
{
    let [_x] = [f(&temp())];
    //                     ^
    //      The temporary is dropped here.
    x = _x; // ERROR
}

Note

Due to the desugaring, the assigned value operand (the RHS) of a destructuring assignment is an extending expression within a newly-introduced block.

Below, because the temporary scope is extended to the end of this introduced block, the assignment is allowed.

# fn temp() {}
# let x;
[x] = [&temp()]; // OK

This desugars to:

# fn temp() {}
# let x;
{ let [_x] = [&temp()]; x = _x; } // OK

However, if we try to use x, even within the same statement, we'll get an error because the temporary is dropped at the end of this introduced block.

# fn temp() {}
# let x;
([x] = [&temp()], x); // ERROR

This desugars to:

# fn temp() {}
# let x;
(
    {
        let [_x] = [&temp()];
        x = _x;
    }, // <-- The temporary is dropped here.
    x, // ERROR
);

Compound assignment expressions

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

Compound assignment expressions combine arithmetic and logical binary operators with assignment expressions.

For example:

let mut x = 5;
x += 1;
assert!(x == 6);

The syntax of compound assignment is a mutable place expression, the assigned operand, then one of the operators followed by an = as a single token (no whitespace), and then a value expression, the modifying operand.

Unlike other place operands, the assigned place operand must be a place expression.

Attempting to use a value expression is a compiler error rather than promoting it to a temporary.

Evaluation of compound assignment expressions depends on the types of the operands.

If the types of both operands are known, prior to monomorphization, to be primitive, the right hand side is evaluated first, the left hand side is evaluated next, and the place given by the evaluation of the left hand side is mutated by applying the operator to the values of both sides.

# use core::{num::Wrapping, ops::AddAssign};
#
trait Equate {}
impl<T> Equate for (T, T) {}

fn f1(x: (u8,)) {
    let mut order = vec![];
    // The RHS is evaluated first as both operands are of primitive
    // type.
    { order.push(2); x }.0 += { order.push(1); x }.0;
    assert!(order.is_sorted());
}

fn f2(x: (Wrapping<u8>,)) {
    let mut order = vec![];
    // The LHS is evaluated first as `Wrapping<_>` is not a primitive
    // type.
    { order.push(1); x }.0 += { order.push(2); (0u8,) }.0;
    assert!(order.is_sorted());
}

fn f3<T: AddAssign<u8> + Copy>(x: (T,)) where (T, u8): Equate {
    let mut order = vec![];
    // The LHS is evaluated first as one of the operands is a generic
    // parameter, even though that generic parameter can be unified
    // with a primitive type due to the where clause bound.
    { order.push(1); x }.0 += { order.push(2); (0u8,) }.0;
    assert!(order.is_sorted());
}

fn main() {
    f1((0u8,));
    f2((Wrapping(0u8),));
    // We supply a primitive type as the generic argument, but this
    // does not affect the evaluation order in `f3` when
    // monomorphized.
    f3::<u8>((0u8,));
}

Note

This is unusual. Elsewhere left to right evaluation is the norm.

See the eval order test for more examples.

Otherwise, this expression is syntactic sugar for using the corresponding trait for the operator (see expr.arith-logic.behavior) and calling its method with the left hand side as the receiver and the right hand side as the next argument.

For example, the following two statements are equivalent:

# use std::ops::AddAssign;
fn f<T: AddAssign + Copy>(mut x: T, y: T) {
    x += y; // Statement 1.
    x.add_assign(y); // Statement 2.
}

Note

Surprisingly, desugaring this further to a fully qualified method call is not equivalent, as there is special borrow checker behavior when the mutable reference to the first operand is taken via autoref.

# use std::ops::AddAssign;
fn f<T: AddAssign + Copy>(mut x: T) {
    // Here we used `x` as both the LHS and the RHS. Because the
    // mutable borrow of the LHS needed to call the trait method
    // is taken implicitly by autoref, this is OK.
    x += x; //~ OK
    x.add_assign(x); //~ OK
}

# use std::ops::AddAssign;
fn f<T: AddAssign + Copy>(mut x: T) {
    // We can't desugar the above to the below, as once we take the
    // mutable borrow of `x` to pass the first argument, we can't
    // pass `x` by value in the second argument because the mutable
    // reference is still live.
    <T as AddAssign>::add_assign(&mut x, x);
    //~^ ERROR cannot use `x` because it was mutably borrowed
}

# use std::ops::AddAssign;
fn f<T: AddAssign + Copy>(mut x: T) {
    // As above.
    (&mut x).add_assign(x);
    //~^ ERROR cannot use `x` because it was mutably borrowed
}

As with normal assignment expressions, compound assignment expressions always produce the unit value.

Warning

Avoid writing code that depends on the evaluation order of operands in compound assignments as it can be unusual and surprising.

  1. Only when m₁ is mut or m₂ is const. Casting mut reference/pointer to const pointer is allowed. ↩ ↩2

  2. Only closures that do not capture (close over) any local variables can be cast to function pointers. ↩