# Functions

Ignoring elision, function signatures with lifetimes have a few constraints:

• any reference must have an annotated lifetime.
• any reference being returned must have the same lifetime as an input or be `static`.

Additionally, note that returning references without input is banned if it would result in returning references to invalid data. The following example shows off some valid forms of functions with lifetimes:

``````// One input reference with lifetime `'a` which must live
// at least as long as the function.
fn print_one<'a>(x: &'a i32) {
println!("`print_one`: x is {}", x);
}

// Mutable references are possible with lifetimes as well.
fn add_one<'a>(x: &'a mut i32) {
*x += 1;
}

// Multiple elements with different lifetimes. In this case, it
// would be fine for both to have the same lifetime `'a`, but
// in more complex cases, different lifetimes may be required.
fn print_multi<'a, 'b>(x: &'a i32, y: &'b i32) {
println!("`print_multi`: x is {}, y is {}", x, y);
}

// Returning references that have been passed in is acceptable.
// However, the correct lifetime must be returned.
fn pass_x<'a, 'b>(x: &'a i32, _: &'b i32) -> &'a i32 { x }

//fn invalid_output<'a>() -> &'a String { &String::from("foo") }
// The above is invalid: `'a` must live longer than the function.
// Here, `&String::from("foo")` would create a `String`, followed by a
// reference. Then the data is dropped upon exiting the scope, leaving
// a reference to invalid data to be returned.

fn main() {
let x = 7;
let y = 9;

print_one(&x);
print_multi(&x, &y);

let z = pass_x(&x, &y);
print_one(z);

let mut t = 3;