6.2. Expressions

Expressions

An expression may have two roles: it always produces a value, and it may have effects (otherwise known as "side effects"). An expression evaluates to a value, and has effects during evaluation. Many expressions contain sub-expressions (operands). The meaning of each kind of expression dictates several things:

• Whether or not to evaluate the sub-expressions when evaluating the expression
• The order in which to evaluate the sub-expressions
• How to combine the sub-expressions' values to obtain the value of the expression

In this way, the structure of expressions dictates the structure of execution. Blocks are just another kind of expression, so blocks, statements, expressions, and blocks again can recursively nest inside each other to an arbitrary depth.

Lvalues, rvalues and temporaries

Expressions are divided into two main categories: lvalues and rvalues. Likewise within each expression, sub-expressions may occur in lvalue context or rvalue context. The evaluation of an expression depends both on its own category and the context it occurs within.

An lvalue is an expression that represents a memory location. These expressions are paths (which refer to local variables, function and method arguments, or static variables), dereferences (*expr), indexing expressions (expr[expr]), and field references (expr.f). All other expressions are rvalues.

The left operand of an assignment or compound-assignment expression is an lvalue context, as is the single operand of a unary borrow. The discriminant or subject of a match expression may be an lvalue context, if ref bindings are made, but is otherwise an rvalue context. All other expression contexts are rvalue contexts.

When an lvalue is evaluated in an lvalue context, it denotes a memory location; when evaluated in an rvalue context, it denotes the value held in that memory location.

Temporary lifetimes

When an rvalue is used in an lvalue context, a temporary un-named lvalue is created and used instead. The lifetime of temporary values is typically the innermost enclosing statement; the tail expression of a block is considered part of the statement that encloses the block.

When a temporary rvalue is being created that is assigned into a let declaration, however, the temporary is created with the lifetime of the enclosing block instead, as using the enclosing statement (the let declaration) would be a guaranteed error (since a pointer to the temporary would be stored into a variable, but the temporary would be freed before the variable could be used). The compiler uses simple syntactic rules to decide which values are being assigned into a let binding, and therefore deserve a longer temporary lifetime.

Here are some examples:

• let x = foo(&temp()). The expression temp() is an rvalue. As it is being borrowed, a temporary is created which will be freed after the innermost enclosing statement (the let declaration, in this case).
• let x = temp().foo(). This is the same as the previous example, except that the value of temp() is being borrowed via autoref on a method-call. Here we are assuming that foo() is an &self method defined in some trait, say Foo. In other words, the expression temp().foo() is equivalent to Foo::foo(&temp()).
• let x = &temp(). Here, the same temporary is being assigned into x, rather than being passed as a parameter, and hence the temporary's lifetime is considered to be the enclosing block.
• let x = SomeStruct { foo: &temp() }. As in the previous case, the temporary is assigned into a struct which is then assigned into a binding, and hence it is given the lifetime of the enclosing block.
• let x = [ &temp() ]. As in the previous case, the temporary is assigned into an array which is then assigned into a binding, and hence it is given the lifetime of the enclosing block.
• let ref x = temp(). In this case, the temporary is created using a ref binding, but the result is the same: the lifetime is extended to the enclosing block.

Moved and copied types

When a local variable is used as an rvalue, the variable will be copied if its type implements Copy. All others are moved.

Literal expressions

A literal expression consists of one of the literal forms described earlier. It directly describes a number, character, string, boolean value, or the unit value.

# #![allow(unused_variables)]
#fn main() {
();        // unit type
"hello";   // string type
'5';       // character type
5;         // integer type

#}

Path expressions

A path used as an expression context denotes either a local variable or an item. Path expressions are lvalues.

Tuple expressions

Tuples are written by enclosing zero or more comma-separated expressions in parentheses. They are used to create tuple-typed values.

# #![allow(unused_variables)]
#fn main() {
(0.0, 4.5);
("a", 4usize, true);

#}

You can disambiguate a single-element tuple from a value in parentheses with a comma:

# #![allow(unused_variables)]
#fn main() {
(0,); // single-element tuple
(0); // zero in parentheses

#}

Struct expressions

There are several forms of struct expressions. A struct expression consists of the path of a struct item, followed by a brace-enclosed list of zero or more comma-separated name-value pairs, providing the field values of a new instance of the struct. A field name can be any identifier, and is separated from its value expression by a colon. The location denoted by a struct field is mutable if and only if the enclosing struct is mutable.

A tuple struct expression consists of the path of a struct item, followed by a parenthesized list of one or more comma-separated expressions (in other words, the path of a struct item followed by a tuple expression). The struct item must be a tuple struct item.

A unit-like struct expression consists only of the path of a struct item.

The following are examples of struct expressions:

# #![allow(unused_variables)]
#fn main() {
# struct Point { x: f64, y: f64 }
# struct NothingInMe { }
# struct TuplePoint(f64, f64);
# mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: usize } }
# struct Cookie; fn some_fn<T>(t: T) {}
Point {x: 10.0, y: 20.0};
NothingInMe {};
TuplePoint(10.0, 20.0);
let u = game::User {name: "Joe", age: 35, score: 100_000};
some_fn::<Cookie>(Cookie);

#}

A struct expression forms a new value of the named struct type. Note that for a given unit-like struct type, this will always be the same value.

A struct expression can terminate with the syntax .. followed by an expression to denote a functional update. The expression following .. (the base) must have the same struct type as the new struct type being formed. The entire expression denotes the result of constructing a new struct (with the same type as the base expression) with the given values for the fields that were explicitly specified and the values in the base expression for all other fields.

# #![allow(unused_variables)]
#fn main() {
# struct Point3d { x: i32, y: i32, z: i32 }
let base = Point3d {x: 1, y: 2, z: 3};
Point3d {y: 0, z: 10, .. base};

#}

Struct field init shorthand

When initializing a data structure (struct, enum, union) with named fields, it is allowed to write fieldname as a shorthand for fieldname: fieldname. This allows a compact syntax with less duplication.

Example:

# #![allow(unused_variables)]
#fn main() {
# struct Point3d { x: i32, y: i32, z: i32 }
# let x = 0;
# let y_value = 0;
# let z = 0;
Point3d { x: x, y: y_value, z: z };
Point3d { x, y: y_value, z };

#}

Block expressions

A block expression is similar to a module in terms of the declarations that are possible. Each block conceptually introduces a new namespace scope. Use items can bring new names into scopes and declared items are in scope for only the block itself.

A block will execute each statement sequentially, and then execute the expression (if given). If the block ends in a statement, its value is ():

# #![allow(unused_variables)]
#fn main() {
let x: () = { println!("Hello."); };

#}

If it ends in an expression, its value and type are that of the expression:

# #![allow(unused_variables)]
#fn main() {
let x: i32 = { println!("Hello."); 5 };

assert_eq!(5, x);

#}

Method-call expressions

A method call consists of an expression followed by a single dot, an identifier, and a parenthesized expression-list. Method calls are resolved to methods on specific traits, either statically dispatching to a method if the exact self-type of the left-hand-side is known, or dynamically dispatching if the left-hand-side expression is an indirect trait object.

Field expressions

A field expression consists of an expression followed by a single dot and an identifier, when not immediately followed by a parenthesized expression-list (the latter is a method call expression). A field expression denotes a field of a struct.

mystruct.myfield;
foo().x;
(Struct {a: 10, b: 20}).a;

A field access is an lvalue referring to the value of that field. When the type providing the field inherits mutability, it can be assigned to.

Also, if the type of the expression to the left of the dot is a pointer, it is automatically dereferenced as many times as necessary to make the field access possible. In cases of ambiguity, we prefer fewer autoderefs to more.

Array expressions

An array expression is written by enclosing zero or more comma-separated expressions of uniform type in square brackets.

In the [expr ';' expr] form, the expression after the ';' must be a constant expression that can be evaluated at compile time, such as a literal or a static item.

# #![allow(unused_variables)]
#fn main() {
[1, 2, 3, 4];
["a", "b", "c", "d"];
[0; 128];              // array with 128 zeros
[0u8, 0u8, 0u8, 0u8];

#}

Index expressions

Array-typed expressions can be indexed by writing a square-bracket-enclosed expression (the index) after them. When the array is mutable, the resulting lvalue can be assigned to.

Indices are zero-based, and may be of any integral type. Vector access is bounds-checked at compile-time for constant arrays being accessed with a constant index value. Otherwise a check will be performed at run-time that will put the thread in a panicked state if it fails.

# #![allow(unused_variables)]
#fn main() {
([1, 2, 3, 4])[0];

let x = (["a", "b"])[10]; // compiler error: const index-expr is out of bounds

let n = 10;
let y = (["a", "b"])[n]; // panics

let arr = ["a", "b"];
arr[10]; // panics

#}

Also, if the type of the expression to the left of the brackets is a pointer, it is automatically dereferenced as many times as necessary to make the indexing possible. In cases of ambiguity, we prefer fewer autoderefs to more.

Range expressions

The .. operator will construct an object of one of the std::ops::Range variants.

# #![allow(unused_variables)]
#fn main() {
1..2;   // std::ops::Range
3..;    // std::ops::RangeFrom
..4;    // std::ops::RangeTo
..;     // std::ops::RangeFull

#}

The following expressions are equivalent.

# #![allow(unused_variables)]
#fn main() {
let x = std::ops::Range {start: 0, end: 10};
let y = 0..10;

assert_eq!(x, y);

#}

Unary operator expressions

Rust defines the following unary operators. With the exception of ?, 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 lvalue 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 an lvalue, these operators produce a reference (pointer) to the lvalue. 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. 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.
• ? : Propagating errors if applied to Err(_) and unwrapping if applied to Ok(_). Only works on the Result<T, E> type, and written in postfix notation.

Binary operator expressions

Binary operators expressions are given in terms 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 that 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.

Bitwise operators

Like the arithmetic operators, bitwise operators are syntactic sugar for calls to methods of built-in traits. This means that 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.

Parentheses are required when chaining comparison operators. For example, the expression a == b == c is invalid and may be written as (a == b) == c.

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
}

#}

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 an lvalue expression followed by an equals sign (=) and an rvalue expression.

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

# #![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.

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.

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;

#}

Call expressions

A call expression invokes a function, providing zero or more input variables and an optional location to move the function's output into. If the function eventually returns, then the expression completes.

Some examples of call expressions:

# #![allow(unused_variables)]
#fn main() {
# fn add(x: i32, y: i32) -> i32 { 0 }

let x: i32 = add(1i32, 2i32);
let pi: Result<f32, _> = "3.14".parse();

#}

Lambda expressions

A lambda expression (sometimes called an "anonymous function expression") defines a function and denotes it as a value, in a single expression. A lambda expression is a pipe-symbol-delimited (|) list of identifiers followed by an expression.

A lambda expression denotes a function that maps a list of parameters (ident_list) onto the expression that follows the ident_list. The identifiers in the ident_list are the parameters to the function. These parameters' types need not be specified, as the compiler infers them from context.

Lambda expressions are most useful when passing functions as arguments to other functions, as an abbreviation for defining and capturing a separate function.

Significantly, lambda expressions capture their environment, which regular function definitions do not. The exact type of capture depends on the function type inferred for the lambda expression. In the simplest and least-expensive form (analogous to a || { } expression), the lambda expression captures its environment by reference, effectively borrowing pointers to all outer variables mentioned inside the function. Alternately, the compiler may infer that a lambda expression should copy or move values (depending on their type) from the environment into the lambda expression's captured environment. A lambda can be forced to capture its environment by moving values by prefixing it with the move keyword.

In this example, we define a function ten_times that takes a higher-order function argument, and we then call it with a lambda expression as an argument, followed by a lambda expression that moves values from its environment.

# #![allow(unused_variables)]
#fn main() {
fn ten_times<F>(f: F) where F: Fn(i32) {
    for index in 0..10 {
        f(index);
    }
}

ten_times(|j| println!("hello, {}", j));

let word = "konnichiwa".to_owned();
ten_times(move |j| println!("{}, {}", word, j));

#}

Infinite loops

A loop expression denotes an infinite loop.

A loop expression may optionally have a label. The label is written as a lifetime preceding the loop expression, as in 'foo: loop{ }. If a label is present, then labeled break and continue expressions nested within this loop may exit out of this loop or return control to its head. See break expressions and continue expressions.

break expressions

A break expression has an optional label. If the label is absent, then executing a break expression immediately terminates the innermost loop enclosing it. It is only permitted in the body of a loop. If the label is present, then break 'foo terminates the loop with label 'foo, which need not be the innermost label enclosing the break expression, but must enclose it.

continue expressions

A continue expression has an optional label. If the label is absent, then executing a continue expression immediately terminates the current iteration of the innermost loop enclosing it, returning control to the loop head. In the case of a while loop, the head is the conditional expression controlling the loop. In the case of a for loop, the head is the call-expression controlling the loop. If the label is present, then continue 'foo returns control to the head of the loop with label 'foo, which need not be the innermost label enclosing the continue expression, but must enclose it.

A continue expression is only permitted in the body of a loop.

while loops

A while loop begins by evaluating the boolean loop conditional expression. If the loop conditional expression evaluates to true, the loop body block executes and control returns to the loop conditional expression. If the loop conditional expression evaluates to false, the while expression completes.

An example:

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

while i < 10 {
    println!("hello");
    i = i + 1;
}

#}

Like loop expressions, while loops can be controlled with break or continue, and may optionally have a label. See infinite loops, break expressions, and continue expressions for more information.

for expressions

A for expression is a syntactic construct for looping over elements provided by an implementation of std::iter::IntoIterator.

An example of a for loop over the contents of an array:

# #![allow(unused_variables)]
#fn main() {
# type Foo = i32;
# fn bar(f: &Foo) { }
# let a = 0;
# let b = 0;
# let c = 0;

let v: &[Foo] = &[a, b, c];

for e in v {
    bar(e);
}

#}

An example of a for loop over a series of integers:

# #![allow(unused_variables)]
#fn main() {
# fn bar(b:usize) { }
for i in 0..256 {
    bar(i);
}

#}

Like loop expressions, for loops can be controlled with break or continue, and may optionally have a label. See infinite loops, break expressions, and continue expressions for more information.

if expressions

An if expression is a conditional branch in program control. The form of an if expression is a condition expression, followed by a consequent block, any number of else if conditions and blocks, and an optional trailing else block. The condition expressions must have type bool. If a condition expression evaluates to true, the consequent block is executed and any subsequent else if or else block is skipped. If a condition expression evaluates to false, the consequent block is skipped and any subsequent else if condition is evaluated. If all if and else if conditions evaluate to false then any else block is executed.

match expressions

A match expression branches on a pattern. The exact form of matching that occurs depends on the pattern. Patterns consist of some combination of literals, destructured arrays or enum constructors, structs and tuples, variable binding specifications, wildcards (..), and placeholders (_). A match expression has a head expression, which is the value to compare to the patterns. The type of the patterns must equal the type of the head expression.

In a pattern whose head expression has an enum type, a placeholder (_) stands for a single data field, whereas a wildcard .. stands for all the fields of a particular variant.

A match behaves differently depending on whether or not the head expression is an lvalue or an rvalue. If the head expression is an rvalue, it is first evaluated into a temporary location, and the resulting value is sequentially compared to the patterns in the arms until a match is found. The first arm with a matching pattern is chosen as the branch target of the match, any variables bound by the pattern are assigned to local variables in the arm's block, and control enters the block.

When the head expression is an lvalue, the match does not allocate a temporary location (however, a by-value binding may copy or move from the lvalue). When possible, it is preferable to match on lvalues, as the lifetime of these matches inherits the lifetime of the lvalue, rather than being restricted to the inside of the match.

An example of a match expression:

# #![allow(unused_variables)]
#fn main() {
let x = 1;

match x {
    1 => println!("one"),
    2 => println!("two"),
    3 => println!("three"),
    4 => println!("four"),
    5 => println!("five"),
    _ => println!("something else"),
}

#}

Patterns that bind variables default to binding to a copy or move of the matched value (depending on the matched value's type). This can be changed to bind to a reference by using the ref keyword, or to a mutable reference using ref mut.

Subpatterns can also be bound to variables by the use of the syntax variable @ subpattern. For example:

# #![allow(unused_variables)]
#fn main() {
let x = 1;

match x {
    e @ 1 ... 5 => println!("got a range element {}", e),
    _ => println!("anything"),
}

#}

Patterns can also dereference pointers by using the &, &mut and box symbols, as appropriate. For example, these two matches on x: &i32 are equivalent:

# #![allow(unused_variables)]
#fn main() {
# let x = &3;
let y = match *x { 0 => "zero", _ => "some" };
let z = match x { &0 => "zero", _ => "some" };

assert_eq!(y, z);

#}

Multiple match patterns may be joined with the | operator. A range of values may be specified with .... For example:

# #![allow(unused_variables)]
#fn main() {
# let x = 2;

let message = match x {
    0 | 1  => "not many",
    2 ... 9 => "a few",
    _      => "lots"
};

#}

Range patterns only work on scalar types (like integers and characters; not like arrays and structs, which have sub-components). A range pattern may not be a sub-range of another range pattern inside the same match.

Finally, match patterns can accept pattern guards to further refine the criteria for matching a case. Pattern guards appear after the pattern and consist of a bool-typed expression following the if keyword. A pattern guard may refer to the variables bound within the pattern they follow.

# #![allow(unused_variables)]
#fn main() {
# let maybe_digit = Some(0);
# fn process_digit(i: i32) { }
# fn process_other(i: i32) { }

let message = match maybe_digit {
    Some(x) if x < 10 => process_digit(x),
    Some(x) => process_other(x),
    None => panic!(),
};

#}

if let expressions

An if let expression is semantically identical to an if expression but in place of a condition expression it expects a let statement with a refutable pattern. If the value of the expression on the right hand side of the let statement matches the pattern, the corresponding block will execute, otherwise flow proceeds to the first else block that follows.

# #![allow(unused_variables)]
#fn main() {
let dish = ("Ham", "Eggs");

// this body will be skipped because the pattern is refuted
if let ("Bacon", b) = dish {
    println!("Bacon is served with {}", b);
}

// this body will execute
if let ("Ham", b) = dish {
    println!("Ham is served with {}", b);
}

#}

while let loops

A while let loop is semantically identical to a while loop but in place of a condition expression it expects let statement with a refutable pattern. If the value of the expression on the right hand side of the let statement matches the pattern, the loop body block executes and control returns to the pattern matching statement. Otherwise, the while expression completes.

return expressions

Return expressions are denoted with the keyword return. Evaluating a return expression moves its argument into the designated output location for the current function call, destroys the current function activation frame, and transfers control to the caller frame.

An example of a return expression:

# #![allow(unused_variables)]
#fn main() {
fn max(a: i32, b: i32) -> i32 {
    if a > b {
        return a;
    }
    return b;
}

#}