Expressions - The Rust Reference

# Expressions

> Syntax
> Expression :
> ExpressionWithoutBlock
> | ExpressionWithBlock

> ExpressionWithoutBlock :
> OuterAttribute*†
> (
> LiteralExpression
> | PathExpression
> | OperatorExpression
> | GroupedExpression
> | ArrayExpression
> | IndexExpression
> | TupleExpression
> | TupleIndexingExpression
> | StructExpression
> | EnumerationVariantExpression
> | CallExpression
> | MethodCallExpression
> | FieldExpression
> | ClosureExpression
> | ContinueExpression
> | BreakExpression
> | RangeExpression
> | ReturnExpression
> | MacroInvocation
> )

> ExpressionWithBlock :
> OuterAttribute*†
> (
> BlockExpression
> | UnsafeBlockExpression
> | LoopExpression
> | IfExpression
> | IfLetExpression
> | MatchExpression
> )

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.

## Expression precedence

The precedence of Rust operators and expressions is ordered as follows, going from strong to weak. Binary Operators at the same precedence level are grouped in the order given by their associativity.

Operator/Expression | Associativity
--- | ---
Paths |
Method calls |
Field expressions | left to right
Function calls, array indexing |
`?` |
Unary `-` `*` `!` `&` `&mut` |
`as` | left to right
`*` `/` `%` | left to right
`+` `-` | left to right
`<<` `>>` | left to right
`&` | left to right
`^` | left to right
`|` | left to right
`==` `!=` `<` `>` `<=` `>=` | Require parentheses
`&&` | left to right
`||` | left to right
`..` `..=` | Require parentheses
`=` `+=` `-=` `*=` `/=` `%=` `&=` `|=` `^=` `<<=` `>>=` | right to left
`return` `break` closures |

## Place Expressions and Value Expressions

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

A place expression is an expression that represents a memory location. These expressions are paths which refer to local variables, static variables, dereferences (`*expr`), array indexing expressions (`expr[expr]`), field references (`expr.f`) and parenthesized place expressions. All other expressions are value expressions.

A value expression is an expression that represents an actual value.

The following contexts are place expression contexts:

- The left operand of an assignment or compound assignment expression.
- The operand of a unary borrow or dereference operator.
- The operand of a field expression.
- The indexed operand of an array indexing expression.
- The operand of any implicit borrow.
- The initializer of a let statement.
- The scrutinee of an `if let`, `match`, or `while let` expression.
- The base of a functional update struct expression.

> Note: Historically, place expressions were called lvalues and value expressions were called rvalues.

### Moved and copied types

When a place expression is evaluated in a value expression context, or is bound by value in a pattern, it denotes the value held in that memory location. If the type of that value implements `Copy`, then the value will be copied. In the remaining situations if that type is `Sized`, then it may be possible to move the value. Only the following place expressions may be moved out of:

- Variables which are not currently borrowed.
- Temporary values.
- Fields of a place expression which can be moved out of and doesn't implement `Drop`.
- The result of dereferencing an expression with type `Box<T>` and that can also be moved out of.

Moving out of a place expression that evaluates to a local variable, the location is deinitialized and cannot be read from again until it is reinitialized. In all other cases, trying to use a place expression in a value expression context is an error.

### Mutability

For a place expression to be assigned to, mutably borrowed, implicitly mutably borrowed, or bound to a pattern containing `ref mut` it must be mutable. We call these mutable place expressions. In contrast, other place expressions are called immutable place expressions.

The following expressions can be mutable place expression contexts:

- Mutable variables, which are not currently borrowed.
- Mutable `static` items.
- Temporary values.
- Fields, this evaluates the subexpression in a mutable place expression context.
- Dereferences of a `*mut T` pointer.
- Dereference of a variable, or field of a variable, with type `&mut T`. Note: This is an exception to the requirement of the next rule.
- Dereferences of a type that implements `DerefMut`, this then requires that the value being dereferenced is evaluated is a mutable place expression context.
- Array indexing of a type that implements `DerefMut`, this then evaluates the value being indexed, but not the index, in mutable place expression context.

### Temporary lifetimes

When using a value expression in most place expression contexts, a temporary unnamed memory location is created initialized to that value and the expression evaluates to that location instead, except if promoted to `'static`. Promotion of a value expression to a `'static` slot occurs when the expression could be written in a constant, borrowed, and dereferencing that borrow where the expression was originally written, without changing the runtime behavior. That is, the promoted expression can be evaluated at compile-time and the resulting value does not contain interior mutability or destructors (these properties are determined based on the value where possible, e.g. `&None` always has the type `&'static Option<_>`, as it contains nothing disallowed). Otherwise, 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, or
- the condition expression or the loop conditional expression if the temporary is created in the condition expression of an `if` or in the loop conditional expression of a `while` expression.

When a temporary value expression 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 `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 a value expression. As it is being borrowed, a temporary is created which will be freed after the innermost enclosing statement; in this case, the `let` declaration.
- `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 = if foo(&temp()) {bar()} else {baz()};`. The expression `temp()` is a value expression. As the temporary is created in the condition expression of an `if`, it will be freed at the end of the condition expression; in this example before the call to `bar` or `baz` is made.
- `let x = if temp().must_run_bar {bar()} else {baz()};`. Here we assume the type of `temp()` is a struct with a boolean field `must_run_bar`. As the previous example, the temporary corresponding to `temp()` will be freed at the end of the condition expression.
- `while foo(&temp()) {bar();}`. The temporary containing the return value from the call to `temp()` is created in the loop conditional expression. Hence it will be freed at the end of the loop conditional expression; in this example before the call to `bar` if the loop body is executed.
- `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.

### Implicit Borrows

Certain expressions will treat an expression as a place expression by implicitly borrowing it. For example, it is possible to compare two unsized slices for equality directly, because the `==` operator implicitly borrows it's operands:

    # #![allow(unused_variables)]
    #fn main() {
    # let c = [1, 2, 3];
    # let d = vec![1, 2, 3];
    let a: &[i32];
    let b: &[i32];
    # a = &c;
    # b = &d;
    // ...
    *a == *b;
    // Equivalent form:
    ::std::cmp::PartialEq::eq(&*a, &*b);
    #}

Implicit borrows may be taken in the following expressions:

- Left operand in method-call expressions.
- Left operand in field expressions.
- Left operand in call expressions.
- Left operand in array indexing expressions.
- Operand of the dereference operator (`*`).
- Operands of comparison.
- Left operands of the compound assignment.

## Overloading Traits

Many of the following operators and expressions can also be overloaded for other types using traits in `std::ops` or `std::cmp`. These traits also exist in `core::ops` and `core::cmp` with the same names.

## Expression Attributes

Outer attributes before an expression are allowed only in a few specific cases:

- Before an expression used as a statement.
- Elements of array expressions, tuple expressions, call expressions, tuple-style struct and enum variant expressions.
- The tail expression of block expressions.

They are never allowed before:

- `if` and `if let` expressions.
- Range expressions.
- Binary operator expressions (ArithmeticOrLogicalExpression, ComparisonExpression, LazyBooleanExpression, TypeCastExpression, AssignmentExpression, CompoundAssignmentExpression).