Functions - The Rust Reference

# Functions

> Syntax
> Function :
> FunctionQualifiers `fn` IDENTIFIER Generics?
> `(` FunctionParameters? `)`
> FunctionReturnType? WhereClause?
> BlockExpression

> FunctionQualifiers :
> `const`? `unsafe`? (`extern` Abi?)?

> Abi :
> STRING_LITERAL | RAW_STRING_LITERAL

> FunctionParameters :
> FunctionParam (`,` FunctionParam)* `,`?

> FunctionParam :
> Pattern `:` Type

> FunctionReturnType :
> `->` Type

A function consists of a block, along with a name and a set of parameters. Other than a name, all these are optional. Functions are declared with the keyword `fn`. Functions may declare a set of input variables as parameters, through which the caller passes arguments into the function, and the output type of the value the function will return to its caller on completion.

When referred to, a function yields a first-class value of the corresponding zero-sized function item type, which when called evaluates to a direct call to the function.

For example, this is a simple function:

    # #![allow(unused_variables)]
    #fn main() {
    fn answer_to_life_the_universe_and_everything() -> i32 {
        return 42;
    }
    #}

As with `let` bindings, function arguments are irrefutable patterns, so any pattern that is valid in a let binding is also valid as an argument:

    # #![allow(unused_variables)]
    #fn main() {
    fn first((value, _): (i32, i32)) -> i32 { value }
    #}

The block of a function is conceptually wrapped in a block that binds the argument patterns and then `return`s the value of the function's block. This means that the tail expression of the block, if evaluated, ends up being returned to the caller. As usual, an explicit return expression within the body of the function will short-cut that implicit return, if reached.

For example, the function above behaves as if it was written as:

    // argument_0 is the actual first argument passed from the caller
    let (value, _) = argument_0;
    return {
        value
    };

## Generic functions

A generic function allows one or more parameterized types to appear in its signature. Each type parameter must be explicitly declared in an angle-bracket-enclosed and comma-separated list, following the function name.

    # #![allow(unused_variables)]
    #fn main() {
    // foo is generic over A and B

    fn foo<A, B>(x: A, y: B) {
    # }
    #}

Inside the function signature and body, the name of the type parameter can be used as a type name. Trait bounds can be specified for type parameters to allow methods with that trait to be called on values of that type. This is specified using the `where` syntax:

    # #![allow(unused_variables)]
    #fn main() {
    # use std::fmt::Debug;
    fn foo<T>(x: T) where T: Debug {
    # }
    #}

When a generic function is referenced, its type is instantiated based on the context of the reference. For example, calling the `foo` function here:

    # #![allow(unused_variables)]
    #fn main() {
    use std::fmt::Debug;

    fn foo<T>(x: &[T]) where T: Debug {
        // details elided
    }

    foo(&[1, 2]);
    #}

will instantiate type parameter `T` with `i32`.

The type parameters can also be explicitly supplied in a trailing path component after the function name. This might be necessary if there is not sufficient context to determine the type parameters. For example, `mem::size_of::<u32>() == 4`.

## Extern functions

Extern functions are part of Rust's foreign function interface, providing the opposite functionality to external blocks. Whereas external blocks allow Rust code to call foreign code, extern functions with bodies defined in Rust code can be called by foreign code. They are defined in the same way as any other Rust function, except that they have the `extern` qualifier.

    # #![allow(unused_variables)]
    #fn main() {
    // Declares an extern fn, the ABI defaults to "C"
    extern fn new_i32() -> i32 { 0 }

    // Declares an extern fn with "stdcall" ABI
    # #[cfg(target_arch = "x86_64")]
    extern "stdcall" fn new_i32_stdcall() -> i32 { 0 }
    #}

Unlike normal functions, extern fns have type `extern "ABI" fn()`. This is the same type as the functions declared in an extern block.

    # #![allow(unused_variables)]
    #fn main() {
    # extern fn new_i32() -> i32 { 0 }
    let fptr: extern "C" fn() -> i32 = new_i32;
    #}

As non-Rust calling conventions do not support unwinding, unwinding past the end of an extern function will cause the process to abort. In LLVM, this is implemented by executing an illegal instruction.

## Const functions

Functions qualified with the `const` keyword are const functions. Const functions can be called from within const contexts. When called from a const context, the function is interpreted by the compiler at compile time. The interpretation happens in the environment of the compilation target and not the host. So `usize` is `32` bits if you are compiling against a `32` bit system, irrelevant of whether you are building on a `64` bit or a `32` bit system.

If a const function is called outside a [const context], it is indistinguishable from any other function. You can freely do anything with a const function that you can do with a regular function.

Const functions have various restrictions to makes sure that they can't be evaluated at compile-time. It is, for example, not possible to write a random number generator as a const function. Calling a const function at compile-time will always yield the same result as calling it at runtime, even when called multiple times. There's one exception to this rule: if you are doing complex floating point operations in extreme situations, then you might get (very slightly) different results. It is advisable to not make array lengths and enum discriminants depend on floating point computations.

Exhaustive list of permitted structures in const functions:

> Note: this list is more restrictive than what you can write in regular constants

- Type parameters where the parameters only have any trait bounds of the following kind:

  - lifetimes
  - `Sized` or `?Sized`

  This means that `<T: 'a + ?Sized>`, `<T: 'b + Sized>` and `<T>` are all permitted.

  This rule also applies to type parameters of impl blocks that contain const methods

- Arithmetic and comparison operators on integers

- All boolean operators except for `&&` and `||` which are banned since they are short-circuiting.

- Any kind of aggregate constructor (array, `struct`, `enum`, tuple, ...)

- Calls to other safe const functions (whether by function call or method call)

- Index expressions on arrays and slices

- Field accesses on structs and tuples

- Reading from constants (but not statics, not even taking a reference to a static)

- `&` and `*` (only dereferencing of references, not raw pointers)

- Casts except for raw pointer to integer casts

- `const unsafe fn` is allowed, but the body must consist of safe operations only and you won't be able to call the `const unsafe fn` from within another const function even if you use `unsafe`

## Attributes on functions

Outer attributes are allowed on functions. Inner attributes are allowed directly after the `{` inside its block.

This example shows an inner attribute on a function. The function will only be available while running tests.

    fn test_only() {
        #![test]
    }

> Note: Except for lints, it is idiomatic to only use outer attributes on function items.

The attributes that have meaning on a function are `cfg`, `deprecated`, `doc`, `export_name`, `link_section`, `no_mangle`, the lint check attributes, `must_use`, the procedural macro attributes, the testing attributes, and the optimization hint attributes. Functions also accept attributes macros.