Expressions - The Rust Reference

# 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 and rvalues

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]`), field references (`expr.f`) and parenthesized lvalue expressions. 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, and the operand of any implicit borrow. The discriminant or subject of a match expression and right side of a `let` binding may be an lvalue context, if ref bindings are made, but is otherwise an rvalue context. All other expression contexts are rvalue contexts.

### Moved and copied types

When an lvalue is evaluated in an rvalue context, it denotes the value held in that memory location. If value is of a type that implements `Copy`, then the value will be copied. In the remaining situations if the type of the value is `Sized` it may be possible to move the value. Only the following lvalues may be moved out of:

- Variables which are not currently borrowed.
- Temporary values.
- Fields of an lvalue 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 an lvalue deinitializes that location (if it comes from a local variable), so that it can't be read from again. In all other cases, trying to use an lvalue in an rvalue context is an error.

### Mutability

For an lvalue to be assigned to, mutably borrowed, implicitly mutably borrowed or bound to a pattern containing `ref mut` it must be mutable, we call these contexts mutable lvalue contexts, other lvalue contexts are called immutable.

The following expressions can create mutable lvalues:

- Mutable variables, which are not currently borrowed.
- Mutable `static` items.
- Temporary values.
- Fields, this evaluates the subexpression in a mutable lvalue 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 for the next rule.
- Dereferences of a type that implements `DerefMut`, this then requires that the value being dereferenced is evaluated is a mutable lvalue context.
- Indexing of a type that implements `DerefMut`, this then evaluates the value being indexed (but not the index) in mutable lvalue context.

### Temporary lifetimes

When using an rvalue in most lvalue contexts, a temporary unnamed 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.

### Implicit Borrows

Certain expressions will treat an expression as an lvalue 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 index expressions.
- Operand of the dereference (`*`) operator.
- Operands of comparison operators.
- Left operands of the compound assignment.

## Constant expressions

Certain types of expressions can be evaluated at compile time. These are called constant expressions. Certain places, such as in constants and statics, require a constant expression, and are always evaluated at compile time. In other places, such as in `let` statements, constant expressions may be evaluated at compile time. If errors, such as out of bounds array access or overflow occurs, then it is a compiler error if the value must be evaluated at compile time, otherwise it is just a warning, but the code will most likely panic when run.

The following expressions are constant expressions, so long as any operands are also constant expressions:

- Literals.
- Paths to functions and constants. Recursively defining constants is not allowed.
- Paths to statics, so long as only their address, not their value, is used. This includes using their value indirectly through a complicated expression. *
- Tuple expressions.
- Array expressions.
- Struct expressions, where the type does not implement `Drop`.
- Variant expressions, where the enumeration type does not implement `Drop`.
- Block expressions (and `unsafe` blocks) which contain only items and possibly a (constant) tail expression.
- Field expressions.
- Index expressions, indexing a array or slice with a `usize`.
- Range expressions.
- Closure expressions which don't capture variables from the environment.
- Built in negation, arithmetic, logical, comparison or lazy boolean operators used on integer and floating point types, `bool` and `char`.
- Shared borrow expressions.
- The dereference operator, but not to circumvent the rule on statics.
- Grouped expressions.
- Cast expressions, except pointer to address and function pointer to address casts.

* Only in static items.

## 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 here also exist in `core::ops` and `core::cmp` with the same names.

## 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 that resolve to local or static variables are lvalues, other paths are rvalues. Using a `static mut` variable requires an `unsafe` block.

    # #![allow(unused_variables)]
    #fn main() {
    # mod globals {
    #     pub static STATIC_VAR: i32 = 5;
    #     pub static mut STATIC_MUT_VAR: i32 = 7;
    # }
    # let local_var = 3;
    local_var;
    globals::STATIC_VAR;
    unsafe { globals::STATIC_MUT_VAR };
    let some_constructor = Some::<i32>;
    let push_integer = Vec::<i32>::push;
    let slice_reverse = <[i32]>::reverse;

    #}

## 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. In the case of a tuple struct the field names are numbers corresponding to the position of the field. The numbers must be written in decimal, containing no underscores and with no leading zeros or integer suffix. A value of a union type can also be created using this syntax, except that it must specify exactly one field.

Struct expressions can't be used directly in the head of a loop or an `if`, `if let` or `match` expression. But struct expressions can still be in used inside parentheses, for example.

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);
    TuplePoint { 0: 10.0, 1: 20.0 }; // Results in the same value as the above line
    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. Just as with all struct expressions, all of the fields of the struct must be visible, even those not explicitly named.

    # #![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 (but not numbered) 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 };

    #}

### Enumeration Variant expressions

Enumeration variants can be constructed similarly to structs, using a path to an enum variant instead of to a struct:

    # #![allow(unused_variables)]
    #fn main() {
    # enum Message {
    #     Quit,
    #     WriteString(String),
    #     Move { x: i32, y: i32 },
    # }
    let q = Message::Quit;
    let w = Message::WriteString("Some string".to_string());
    let m = Message::Move { x: 50, y: 200 };

    #}

## Block expressions

A block expression is similar to a module in terms of the declarations that are possible, but can also contain statements and end with an expression. 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 doesn't end in an expression, 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);

    #}

Blocks are always rvalues and evaluate the last expression in rvalue context. This can be used to force moving a value if really needed.

### `unsafe` blocks

See `unsafe` block for more information on when to use `unsafe`

A block of code can be prefixed with the `unsafe` keyword, to permit calling `unsafe` functions or dereferencing raw pointers within a safe function.

## 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. Method call expressions will automatically take a shared or mutable borrow of the receiver if needed.

    # #![allow(unused_variables)]
    #fn main() {
    let pi: Result<f32, _> = "3.14".parse();
    let log_pi = pi.unwrap_or(1.0).log(2.72);
    # assert!(1.14 < log_pi && log_pi < 1.15)

    #}

When resolving method calls on an expression of type `A`, Rust will use the following order:

1. Inherent methods, with receiver of type `A`, `&A`, `&mut A`.
2. Trait methods with receiver of type `A`.
3. Trait methods with receiver of type `&A`.
4. Trait methods with receiver of type `&mut A`.
5. If it's possible, Rust will then repeat steps 1-5 with `<A as std::ops::Deref>::Target`, and insert a dereference operator.
6. If `A` is now an array type, then repeat steps 1-4 with the corresponding slice type.

Note: that in steps 1-4 the receiver is used, not the type of `Self` nor the type of `A`. For example

    // `Self` is `&A`, receiver is `&A`.
    impl<'a> Trait for &'a A {
        fn method(self) {}
    }
    // If `A` is `&B`, then `Self` is `B` and the receiver is `A`.
    impl B {
        fn method(&self) {}
    }

Another note: this process does not use the mutability or lifetime of the receiver, or whether `unsafe` methods can currently be called to resolve methods. These constraints instead lead to compiler errors.

If a step is reached where there is more than one possible method (where generic methods or traits are considered the same), then it is a compiler error. These cases require a more specific syntax. for method and function invocation.

## 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 always a method call expression). A field expression denotes a field of a struct or union. To call a function stored in a struct parentheses are needed around the field expression

    mystruct.myfield;
    foo().x;
    (Struct {a: 10, b: 20}).a;
    mystruct.method();          // Method expression
    (mystruct.function_field)() // Call expression containing a field expression

A field access is an lvalue referring to the location of that field. When the subexpression is mutable, the field expression is also mutable.

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.

Finally the fields of a struct, a reference to a struct are treated as separate entities when borrowing. If the struct does not implement `Drop` this also applies to moving out of each of its fields where possible. This also does not apply if automatic dereferencing is done though user defined types.

    # #![allow(unused_variables)]
    #fn main() {
    # struct A { f1: String, f2: String, f3: String }
    # let mut x = A {
    #     f1: "f1".to_string(),
    #     f2: "f2".to_string(),
    #     f3: "f3".to_string()
    # };
    let a: &mut String = &mut x.f1; // x.f1 borrowed mutably
    let b: &String = &x.f2;         // x.f2 borrowed immutably
    let c: &String = &x.f2;         // Can borrow again
    let d: String = x.f3;           // Move out of x.f3

    #}

### Tuple indexing expressions

Tuples and struct tuples can be indexed using the number corresponding to the position of the field. The index must be written as a decimal literal with no underscores or suffix. Tuple indexing expressions also differ from field expressions in that they can unambiguously be called as a function. In all other aspects they have the same behavior.

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point(f32, f32);
    let pair = (1, 2);
    assert_eq!(pair.1, 2);
    let unit_x = Point(1.0, 0.0);
    assert_eq!(unit_x.0, 1.0);

    #}

## Call expressions

A call expression consists of an expression followed by a parenthesized expression-list. It invokes a function, providing zero or more input variables. If the function eventually returns, then the expression completes. For non-function types, the expression f(...) uses the method on one of the `std::ops::Fn`, `std::ops::FnMut` or `std::ops::FnOnce` traits, which differ in whether they take the type by reference, mutable reference, or take ownership respectively. An automatic borrow will be taken if needed. Rust will also automatically dereference `f` as required. Some examples of call expressions:

    # #![allow(unused_variables)]
    #fn main() {
    # fn add(x: i32, y: i32) -> i32 { 0 }
    let three: i32 = add(1i32, 2i32);
    let name: &'static str = (|| "Rust")();

    #}

### Disambiguating Function Calls

Rust treats all function calls as sugar for a more explicit, fully-qualified syntax. Upon compilation, Rust will desugar all function calls into the explicit form. Rust may sometimes require you to qualify function calls with trait, depending on the ambiguity of a call in light of in-scope items.

> Note: In the past, the Rust community used the terms "Unambiguous Function Call Syntax", "Universal Function Call Syntax", or "UFCS", in documentation, issues, RFCs, and other community writings. However, the term lacks descriptive power and potentially confuses the issue at hand. We mention it here for searchability's sake.

Several situations often occur which result in ambiguities about the receiver or referent of method or associated function calls. These situations may include:

- Multiple in-scope traits define methods with the same name for the same types
- Auto-`deref` is undesirable; for example, distinguishing between methods on a smart pointer itself and the pointer's referent
- Methods which take no arguments, like `default()`, and return properties of a type, like `size_of()`

To resolve the ambiguity, the programmer may refer to their desired method or function using more specific paths, types, or traits.

For example,

    trait Pretty {
        fn print(&self);
    }

    trait Ugly {
      fn print(&self);
    }

    struct Foo;
    impl Pretty for Foo {
        fn print(&self) {}
    }

    struct Bar;
    impl Pretty for Bar {
        fn print(&self) {}
    }
    impl Ugly for Bar{
        fn print(&self) {}
    }

    fn main() {
        let f = Foo;
        let b = Bar;

        // we can do this because we only have one item called `print` for `Foo`s
        f.print();
        // more explicit, and, in the case of `Foo`, not necessary
        Foo::print(&f);
        // if you're not into the whole brevity thing
        <Foo as Pretty>::print(&f);

        // b.print(); // Error: multiple 'print' found
        // Bar::print(&b); // Still an error: multiple `print` found

        // necessary because of in-scope items defining `print`
        <Bar as Pretty>::print(&b);
    }

Refer to RFC 132 for further details and motivations.

## Closure expressions

A closure expression defines a closure and denotes it as a value, in a single expression. A closure expression is a pipe-symbol-delimited (`|`) list of patterns followed by an expression. Type annotations may optionally be added for the type of the parameters or for the return type. If there is a return type, the expression used for the body of the closure must be a normal block. A closure expression also may begin with the `move` keyword before the initial `|`.

A closure expression denotes a function that maps a list of parameters (`ident_list`) onto the expression that follows the `ident_list`. The patterns in the `ident_list` are the parameters to the closure. If a parameter's types is not specified, then the compiler infers it from context. Each closure expression has a unique anonymous type.

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

Significantly, closure expressions capture their environment, which regular function definitions do not. Without the `move` keyword, the closure expression infers how it captures each variable from its environment, preferring to capture by shared reference, effectively borrowing all outer variables mentioned inside the closure's body. If needed the compiler will infer that instead mutable references should be taken, or that the values should be moved or copied (depending on their type) from the environment. A closure can be forced to capture its environment by copying or moving values by prefixing it with the `move` keyword. This is often used to ensure that the closure's type is `'static`.

The compiler will determine which of the closure traits the closure's type will implement by how it acts on its captured variables. The closure will also implement `Send` and/or `Sync` if all of its captured types do. These traits allow functions to accept closures using generics, even though the exact types can't be named.

In this example, we define a function `ten_times` that takes a higher-order function argument, and we then call it with a closure expression as an argument, followed by a closure 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));
    // With type annotations
    ten_times(|j: i32| -> () { println!("hello, {}", j) });

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

    #}

## Array expressions

An array expression can be written by enclosing zero or more comma-separated expressions of uniform type in square brackets. This produces and array containing each of these values in the order they are written.

Alternatively there can be exactly two expressions inside the brackets, separated by a semi-colon. The expression after the `;` must be a have type `usize` and be a constant expression, such as a literal or a constant item. `[a; b]` creates an array containing `b` copies of the value of `a`. If the expression after the semi-colon has a value greater than 1 then this requires that the type of `a` is `Copy`.

    # #![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 and slice-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. For other types an index expression `a[b]` is equivalent to `*std::ops::Index::index(&a, b)`, or `*std::opsIndexMut::index_mut(&mut a, b)` in a mutable lvalue context. Just as with methods, Rust will also insert dereference operations on `a` repeatedly to find an implementation.

Indices are zero-based, and are of type `usize` for arrays and slices. Array access is a constant expression, so bounds can be checked at compile-time for constant arrays 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])[2];        // Evaluates to 3

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

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

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

    #}

## Range expressions

The `..` operator will construct an object of one of the `std::ops::Range` (or `core::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);

    #}

## 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.

### 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 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);

    #}

### Operator precedence

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

Operator | Associativity
--- | ---
`?` |
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
`=` `+=` `-=` `*=` `/=` `%=` `&=` `|=` `^=` `<<=` `>>=` | right to left

## 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);

    #}

## Loops

Rust supports three loop expressions:

- A `loop` expression denotes an infinite loop.
- A `while` expression loops until a predicate is false.
- A `for` expression extracts values from an iterator, looping until the iterator is empty.

All three types of loop support `break` expressions, `continue` expressions, and labels. Only `loop` supports evaluation to non-trivial values.

### Infinite loops

A `loop` expression repeats execution of its body continuously: `loop { println!("I live."); }`.

A `loop` expression without an associated `break` expression is diverging, and doesn't return anything. A `loop` expression containing associated `break` expression(s) may terminate, and must have type compatible with the value of the `break` expression(s).

### Predicate 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, then 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;
    }

    #}

### Iterator loops

A `for` expression is a syntactic construct for looping over elements provided by an implementation of `std::iter::IntoIterator`. If the iterator yields a value, that value is given the specified name and the body of the loop is executed, then control returns to the head of the `for` loop. If the iterator is empty, the `for` expression completes.

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

    # #![allow(unused_variables)]
    #fn main() {
    let v = &["apples", "cake", "coffee"];

    for text in v {
        println!("I like {}.", text);
    }

    #}

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

    # #![allow(unused_variables)]
    #fn main() {
    let mut sum = 0;
    for n in 1..11 {
        sum += n;
    }
    assert_eq!(sum, 55);

    #}

### Loop labels

A loop expression may optionally have a label. The label is written as a lifetime preceding the loop expression, as in `'foo: loop { break 'foo; }`, `'bar: while false {}`, `'humbug: for _ in 0..0 {}`. 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

When `break` is encountered, execution of the associated loop body is immediately terminated, for example:

    # #![allow(unused_variables)]
    #fn main() {
    let mut last = 0;
    for x in 1..100 {
        if x > 12 {
            break;
        }
        last = x;
    }
    assert_eq!(last, 12);

    #}

A `break` expression is normally associated with the innermost `loop`, `for` or `while` loop enclosing the `break` expression, but a label can be used to specify which enclosing loop is affected. Example:

    # #![allow(unused_variables)]
    #fn main() {
    'outer: loop {
        while true {
            break 'outer;
        }
    }

    #}

A `break` expression is only permitted in the body of a loop, and has one of the forms `break`, `break 'label` or (see below) `break EXPR` or `break 'label EXPR`.

### `continue` expressions

When `continue` is encountered, the current iteration of the associated loop body is immediately terminated, 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.

Like `break`, `continue` is normally associated with the innermost enclosing loop, but `continue 'label` may be used to specify the loop affected. A `continue` expression is only permitted in the body of a loop.

### `break` and loop values

When associated with a `loop`, a break expression may be used to return a value from that loop, via one of the forms `break EXPR` or `break 'label EXPR`, where `EXPR` is an expression whose result is returned from the `loop`. For example:

    # #![allow(unused_variables)]
    #fn main() {
    let (mut a, mut b) = (1, 1);
    let result = loop {
        if b > 10 {
            break b;
        }
        let c = a + b;
        a = b;
        b = c;
    };
    // first number in Fibonacci sequence over 10:
    assert_eq!(result, 13);

    #}

In the case a `loop` has an associated `break`, it is not considered diverging, and the `loop` must have a type compatible with each `break` expression. `break` without an expression is considered identical to `break` with expression `()`.

## `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. An if expression evaluates to the same value as the executed block, or `()` if no block is evaluated. An `if` expression must have the same type in all situations.

    # #![allow(unused_variables)]
    #fn main() {
    # let x = 3;
    if x == 4 {
        println!("x is four");
    } else if x == 3 {
        println!("x is three");
    } else {
        println!("x is something else");
    }

    let y = if 12 * 15 > 150 {
        "Bigger"
    } else {
        "Smaller"
    };
    assert_eq!(y, "Bigger");

    #}

## `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.

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`.

Patterns can be used to destructure structs, enums, and tuples. Destructuring breaks a value up into its component pieces. The syntax used is the same as when creating such values. When destructing a data structure with named (but not numbered) fields, it is allowed to write `fieldname` as a shorthand for `fieldname: fieldname`. In a pattern whose head expression has a `struct`, `enum` or `tupl` type, a placeholder (`_`) stands for a single data field, whereas a wildcard `..` stands for all the fields of a particular variant.

    # #![allow(unused_variables)]
    #fn main() {
    # enum Message {
    #     Quit,
    #     WriteString(String),
    #     Move { x: i32, y: i32 },
    #     ChangeColor(u8, u8, u8),
    # }
    # let message = Message::Quit;
    match message {
        Message::Quit => println!("Quit"),
        Message::WriteString(write) => println!("{}", &write),
        Message::Move{ x, y: 0 } => println!("move {} horizontally", x),
        Message::Move{ .. } => println!("other move"),
        Message::ChangeColor { 0: red, 1: green, 2: _ } => {
            println!("color change, red: {}, green: {}", red, green);
        }
    };

    #}

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);

    #}

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"),
    }

    #}

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 similar to an `if` expression but in place of a condition expression it expects the keyword `let` followed by a refutable pattern, an `=` and an expression. If the value of the expression on the right hand side of the `=` matches the pattern, the corresponding block will execute, otherwise flow proceeds to the following `else` block if it exists. Like `if` expressions, `if let` expressions have a value determined by the block that is evaluated.

    # #![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);
    } else {
        // This block is evaluated instead.
        println!("No bacon will be served");
    }

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

    #}

## `while let` loops

A `while let` loop is semantically similar to a `while` loop but in place of a condition expression it expects the keyword `let` followed by a refutable pattern, an `=` and an expression. If the value of the expression on the right hand side of the `=` matches the pattern, the loop body block executes then control returns to the pattern matching statement. Otherwise, the while expression completes.

    # #![allow(unused_variables)]
    #fn main() {
    let mut x = vec![1, 2, 3];

    while let Some(y) = x.pop() {
        println!("y = {}", y);
    }

    #}

## `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;
    }

    #}