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 returns 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
};

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.

A special kind of function can be declared with a ! character where the output type would normally be. For example:

# #![allow(unused_variables)]
#fn main() {
fn my_err(s: &str) -> ! {
    println!("{}", s);
    panic!();
}
#}

We call such functions "diverging" because they never return a value to the caller. Every control path in a diverging function must end with a panic!(), a loop expression without an associated break expression, or a call to another diverging function on every control path. The ! annotation does not denote a type.

It might be necessary to declare a diverging function because as mentioned previously, the typechecker checks that every control path in a function ends with a return or diverging expression. So, if my_err were declared without the ! annotation, the following code would not typecheck:

# #![allow(unused_variables)]
#fn main() {
# fn my_err(s: &str) -> ! { panic!() }

fn f(i: i32) -> i32 {
    if i == 42 {
        return 42;
    }
    else {
        my_err("Bad number!");
    }
}
#}

This will not compile without the ! annotation on my_err, since the else branch of the conditional in f does not return an i32, as required by the signature of f. Adding the ! annotation to my_err informs the typechecker that, should control ever enter my_err, no further type judgments about f need to hold, since control will never resume in any context that relies on those judgments. Thus the return type on f only needs to reflect the if branch of the conditional.

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 modifier.

# #![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.