Rollup merge of #25614 - parir:patch-2, r=alexcrichton

[rust.git] / src / doc / trpl / lifetimes.md

% Lifetimes

This guide is one of three presenting Rust’s ownership system. This is one of
Rust’s most unique and compelling features, with which Rust developers should
become quite acquainted. Ownership is how Rust achieves its largest goal,
memory safety. There are a few distinct concepts, each with its own chapter:

* [ownership][ownership], the key concept
* [borrowing][borrowing], and their associated feature ‘references’
* lifetimes, which you’re reading now

These three chapters are related, and in order. You’ll need all three to fully
understand the ownership system.

[ownership]: ownership.html
[borrowing]: references-and-borrowing.html

# Meta

Before we get to the details, two important notes about the ownership system.

Rust has a focus on safety and speed. It accomplishes these goals through many
‘zero-cost abstractions’, which means that in Rust, abstractions cost as little
as possible in order to make them work. The ownership system is a prime example
of a zero-cost abstraction. All of the analysis we’ll talk about in this guide
is _done at compile time_. You do not pay any run-time cost for any of these
features.

However, this system does have a certain cost: learning curve. Many new users
to Rust experience something we like to call ‘fighting with the borrow
checker’, where the Rust compiler refuses to compile a program that the author
thinks is valid. This often happens because the programmer’s mental model of
how ownership should work doesn’t match the actual rules that Rust implements.
You probably will experience similar things at first. There is good news,
however: more experienced Rust developers report that once they work with the
rules of the ownership system for a period of time, they fight the borrow
checker less and less.

With that in mind, let’s learn about lifetimes.

# Lifetimes

Lending out a reference to a resource that someone else owns can be
complicated. For example, imagine this set of operations:

- I acquire a handle to some kind of resource.
- I lend you a reference to the resource.
- I decide I’m done with the resource, and deallocate it, while you still have
  your reference.
- You decide to use the resource.

Uh oh! Your reference is pointing to an invalid resource. This is called a
dangling pointer or ‘use after free’, when the resource is memory.

To fix this, we have to make sure that step four never happens after step
three. The ownership system in Rust does this through a concept called
lifetimes, which describe the scope that a reference is valid for.

When we have a function that takes a reference by argument, we can be implicit
or explicit about the lifetime of the reference:

```rust
// implicit
fn foo(x: &i32) {
}

// explicit
fn bar<'a>(x: &'a i32) {
}
```

The `'a` reads ‘the lifetime a’. Technically, every reference has some lifetime
associated with it, but the compiler lets you elide them in common cases.
Before we get to that, though, let’s break the explicit example down:

```rust,ignore
fn bar<'a>(...)
```

This part declares our lifetimes. This says that `bar` has one lifetime, `'a`.
If we had two reference parameters, it would look like this:

```rust,ignore
fn bar<'a, 'b>(...)
```

Then in our parameter list, we use the lifetimes we’ve named:

```rust,ignore
...(x: &'a i32)
```

If we wanted an `&mut` reference, we’d do this:

```rust,ignore
...(x: &'a mut i32)
```

If you compare `&mut i32` to `&'a mut i32`, they’re the same, it’s just that
the lifetime `'a` has snuck in between the `&` and the `mut i32`. We read `&mut
i32` as ‘a mutable reference to an i32’ and `&'a mut i32` as ‘a mutable
reference to an `i32` with the lifetime `'a`’.

You’ll also need explicit lifetimes when working with [`struct`][structs]s:

```rust
struct Foo<'a> {
    x: &'a i32,
}

fn main() {
    let y = &5; // this is the same as `let _y = 5; let y = &_y;`
    let f = Foo { x: y };

    println!("{}", f.x);
}
```

[structs]: structs.html

As you can see, `struct`s can also have lifetimes. In a similar way to functions,

```rust
struct Foo<'a> {
# x: &'a i32,
# }
```

declares a lifetime, and

```rust
# struct Foo<'a> {
x: &'a i32,
# }
```

uses it. So why do we need a lifetime here? We need to ensure that any reference
to a `Foo` cannot outlive the reference to an `i32` it contains.

## Thinking in scopes

A way to think about lifetimes is to visualize the scope that a reference is
valid for. For example:

```rust
fn main() {
    let y = &5;     // -+ y goes into scope
                    //  |
    // stuff        //  |
                    //  |
}                   // -+ y goes out of scope
```

Adding in our `Foo`:

```rust
struct Foo<'a> {
    x: &'a i32,
}

fn main() {
    let y = &5;           // -+ y goes into scope
    let f = Foo { x: y }; // -+ f goes into scope
    // stuff              //  |
                          //  |
}                         // -+ f and y go out of scope
```

Our `f` lives within the scope of `y`, so everything works. What if it didn’t?
This code won’t work:

```rust,ignore
struct Foo<'a> {
    x: &'a i32,
}

fn main() {
    let x;                    // -+ x goes into scope
                              //  |
    {                         //  |
        let y = &5;           // ---+ y goes into scope
        let f = Foo { x: y }; // ---+ f goes into scope
        x = &f.x;             //  | | error here
    }                         // ---+ f and y go out of scope
                              //  |
    println!("{}", x);        //  |
}                             // -+ x goes out of scope
```

Whew! As you can see here, the scopes of `f` and `y` are smaller than the scope
of `x`. But when we do `x = &f.x`, we make `x` a reference to something that’s
about to go out of scope.

Named lifetimes are a way of giving these scopes a name. Giving something a
name is the first step towards being able to talk about it.

## 'static

The lifetime named ‘static’ is a special lifetime. It signals that something
has the lifetime of the entire program. Most Rust programmers first come across
`'static` when dealing with strings:

```rust
let x: &'static str = "Hello, world.";
```

String literals have the type `&'static str` because the reference is always
alive: they are baked into the data segment of the final binary. Another
example are globals:

```rust
static FOO: i32 = 5;
let x: &'static i32 = &FOO;
```

This adds an `i32` to the data segment of the binary, and `x` is a reference
to it.

## Lifetime Elision

Rust supports powerful local type inference in function bodies, but it’s
forbidden in item signatures to allow reasoning about the types based on
the item signature alone. However, for ergonomic reasons a very restricted
secondary inference algorithm called “lifetime elision” applies in function
signatures. It infers only based on the signature components themselves and not
based on the body of the function, only infers lifetime parameters, and does
this with only three easily memorizable and unambiguous rules. This makes
lifetime elision a shorthand for writing an item signature, while not hiding
away the actual types involved as full local inference would if applied to it.

When talking about lifetime elision, we use the term *input lifetime* and
*output lifetime*. An *input lifetime* is a lifetime associated with a parameter
of a function, and an *output lifetime* is a lifetime associated with the return
value of a function. For example, this function has an input lifetime:

```rust,ignore
fn foo<'a>(bar: &'a str)
```

This one has an output lifetime:

```rust,ignore
fn foo<'a>() -> &'a str
```

This one has a lifetime in both positions:

```rust,ignore
fn foo<'a>(bar: &'a str) -> &'a str
```

Here are the three rules:

* Each elided lifetime in a function’s arguments becomes a distinct lifetime
  parameter.

* If there is exactly one input lifetime, elided or not, that lifetime is
  assigned to all elided lifetimes in the return values of that function.

* If there are multiple input lifetimes, but one of them is `&self` or `&mut
  self`, the lifetime of `self` is assigned to all elided output lifetimes.

Otherwise, it is an error to elide an output lifetime.

### Examples

Here are some examples of functions with elided lifetimes.  We’ve paired each
example of an elided lifetime with its expanded form.

```rust,ignore
fn print(s: &str); // elided
fn print<'a>(s: &'a str); // expanded

fn debug(lvl: u32, s: &str); // elided
fn debug<'a>(lvl: u32, s: &'a str); // expanded

// In the preceding example, `lvl` doesn’t need a lifetime because it’s not a
// reference (`&`). Only things relating to references (such as a `struct`
// which contains a reference) need lifetimes.

fn substr(s: &str, until: u32) -> &str; // elided
fn substr<'a>(s: &'a str, until: u32) -> &'a str; // expanded

fn get_str() -> &str; // ILLEGAL, no inputs

fn frob(s: &str, t: &str) -> &str; // ILLEGAL, two inputs
fn frob<'a, 'b>(s: &'a str, t: &'b str) -> &str; // Expanded: Output lifetime is unclear

fn get_mut(&mut self) -> &mut T; // elided
fn get_mut<'a>(&'a mut self) -> &'a mut T; // expanded

fn args<T:ToCStr>(&mut self, args: &[T]) -> &mut Command // elided
fn args<'a, 'b, T:ToCStr>(&'a mut self, args: &'b [T]) -> &'a mut Command // expanded

fn new(buf: &mut [u8]) -> BufWriter; // elided
fn new<'a>(buf: &'a mut [u8]) -> BufWriter<'a> // expanded
```