book/nostarch/chapter10.md

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[TOC]

# Generic Types, Traits, and Lifetimes

Every programming language has tools for effectively handling the duplication
of concepts. In Rust, one such tool is *generics*. Generics are abstract
stand-ins for concrete types or other properties. When we’re writing code, we
can express the behavior of generics or how they relate to other generics
without knowing what will be in their place when compiling and running the code.

Similar to the way a function takes parameters with unknown values to run the
same code on multiple concrete values, functions can take parameters of some
generic type instead of a concrete type, like `i32` or `String`. In fact, we’ve
already used generics in Chapter 6 with `Option<T>`, Chapter 8 with `Vec<T>`
and `HashMap<K, V>`, and Chapter 9 with `Result<T, E>`. In this chapter, you’ll
explore how to define your own types, functions, and methods with generics!

First, we’ll review how to extract a function to reduce code duplication. Next,
we’ll use the same technique to make a generic function from two functions that
differ only in the types of their parameters. We’ll also explain how to use
generic types in struct and enum definitions.

Then you’ll learn how to use *traits* to define behavior in a generic way. You
can combine traits with generic types to constrain a generic type to only
those types that have a particular behavior, as opposed to just any type.

Finally, we’ll discuss *lifetimes*, a variety of generics that give the
compiler information about how references relate to each other. Lifetimes allow
us to borrow values in many situations while still enabling the compiler to
check that the references are valid.

## Removing Duplication by Extracting a Function

Before diving into generics syntax, let’s first look at how to remove
duplication that doesn’t involve generic types by extracting a function. Then
we’ll apply this technique to extract a generic function! In the same way that
you recognize duplicated code to extract into a function, you’ll start to
recognize duplicated code that can use generics.

Consider a short program that finds the largest number in a list, as shown in
Listing 10-1.

Filename: src/main.rs

```
fn main() {
    let number_list = vec![34, 50, 25, 100, 65];

    let mut largest = number_list[0];

    for number in number_list {
        if number > largest {
            largest = number;
        }
    }

    println!("The largest number is {}", largest);
}
```

Listing 10-1: Code to find the largest number in a list of numbers

This code stores a list of integers in the variable `number_list` and places
the first number in the list in a variable named `largest`. Then it iterates
through all the numbers in the list, and if the current number is greater than
the number stored in `largest`, it replaces the number in that variable.
However, if the current number is less than or equal to the largest number seen
so far, the variable doesn’t change, and the code moves on to the next number
in the list. After considering all the numbers in the list, `largest` should
hold the largest number, which in this case is 100.

To find the largest number in two different lists of numbers, we can duplicate
the code in Listing 10-1 and use the same logic at two different places in the
program, as shown in Listing 10-2.

Filename: src/main.rs

```
fn main() {
    let number_list = vec![34, 50, 25, 100, 65];

    let mut largest = number_list[0];

    for number in number_list {
        if number > largest {
            largest = number;
        }
    }

    println!("The largest number is {}", largest);

    let number_list = vec![102, 34, 6000, 89, 54, 2, 43, 8];

    let mut largest = number_list[0];

    for number in number_list {
        if number > largest {
            largest = number;
        }
    }

    println!("The largest number is {}", largest);
}
```

Listing 10-2: Code to find the largest number in *two* lists of numbers

Although this code works, duplicating code is tedious and error prone. We also
have to update the code in multiple places when we want to change it.

To eliminate this duplication, we can create an abstraction by defining a
function that operates on any list of integers given to it in a parameter. This
solution makes our code clearer and lets us express the concept of finding the
largest number in a list abstractly.

In Listing 10-3, we extracted the code that finds the largest number into a
function named `largest`. Unlike the code in Listing 10-1, which can find the
largest number in only one particular list, this program can find the largest
number in two different lists.

Filename: src/main.rs

```
fn largest(list: &[i32]) -> i32 {
    let mut largest = list[0];

    for &item in list {
        if item > largest {
            largest = item;
        }
    }

    largest
}

fn main() {
    let number_list = vec![34, 50, 25, 100, 65];

    let result = largest(&number_list);
    println!("The largest number is {}", result);

    let number_list = vec![102, 34, 6000, 89, 54, 2, 43, 8];

    let result = largest(&number_list);
    println!("The largest number is {}", result);
}
```

Listing 10-3: Abstracted code to find the largest number in two lists

The `largest` function has a parameter called `list`, which represents any
concrete slice of `i32` values that we might pass into the function. As a
result, when we call the function, the code runs on the specific values that we
pass in. Don’t worry about the syntax of the `for` loop for now. We aren’t
referencing a reference to an `i32` here; we’re pattern matching and
destructuring each `&i32` that the `for` loop gets so that `item` will be an
`i32` inside the loop body. We’ll cover pattern matching in detail in Chapter
18.

In sum, here are the steps we took to change the code from Listing 10-2 to
Listing 10-3:

1. Identify duplicate code.
2. Extract the duplicate code into the body of the function and specify the
   inputs and return values of that code in the function signature.
3. Update the two instances of duplicated code to call the function instead.

Next, we’ll use these same steps with generics to reduce code duplication in
different ways. In the same way that the function body can operate on an
abstract `list` instead of specific values, generics allow code to operate on
abstract types.

For example, say we had two functions: one that finds the largest item in a
slice of `i32` values and one that finds the largest item in a slice of `char`
values. How would we eliminate that duplication? Let’s find out!

## Generic Data Types

We can use generics to create definitions for items like function signatures or
structs, which we can then use with many different concrete data types. Let’s
first look at how to define functions, structs, enums, and methods using
generics. Then we’ll discuss how generics affect code performance.

### In Function Definitions

When defining a function that uses generics, we place the generics in the
signature of the function where we would usually specify the data types of the
parameters and return value. Doing so makes our code more flexible and provides
more functionality to callers of our function while preventing code duplication.

Continuing with our `largest` function, Listing 10-4 shows two functions that
both find the largest value in a slice.

Filename: src/main.rs

```
fn largest_i32(list: &[i32]) -> i32 {
    let mut largest = list[0];

    for &item in list {
        if item > largest {
            largest = item;
        }
    }

    largest
}

fn largest_char(list: &[char]) -> char {
    let mut largest = list[0];

    for &item in list {
        if item > largest {
            largest = item;
        }
    }

    largest
}

fn main() {
    let number_list = vec![34, 50, 25, 100, 65];

    let result = largest_i32(&number_list);
    println!("The largest number is {}", result);

    let char_list = vec!['y', 'm', 'a', 'q'];

    let result = largest_char(&char_list);
    println!("The largest char is {}", result);
}
```

Listing 10-4: Two functions that differ only in their names and the types in
their signatures

The `largest_i32` function is the one we extracted in Listing 10-3 that finds
the largest `i32` in a slice. The `largest_char` function finds the largest
`char` in a slice. The function bodies have the same code, so let’s eliminate
the duplication by introducing a generic type parameter in a single function.

To parameterize the types in the new function we’ll define, we need to name the
type parameter, just as we do for the value parameters to a function. You can
use any identifier as a type parameter name. But we’ll use `T` because, by
convention, parameter names in Rust are short, often just a letter, and Rust’s
type-naming convention is CamelCase. Short for “type,” `T` is the default
choice of most Rust programmers.

When we use a parameter in the body of the function, we have to declare the
parameter name in the signature so the compiler knows what that name means.
Similarly, when we use a type parameter name in a function signature, we have
to declare the type parameter name before we use it. To define the generic
`largest` function, place type name declarations inside angle brackets, `<>`,
between the name of the function and the parameter list, like this:

```
fn largest<T>(list: &[T]) -> T {
```

We read this definition as: the function `largest` is generic over some type
`T`. This function has one parameter named `list`, which is a slice of values
of type `T`. The `largest` function will return a value of the
same type `T`.

Listing 10-5 shows the combined `largest` function definition using the generic
data type in its signature. The listing also shows how we can call the function
with either a slice of `i32` values or `char` values. Note that this code won’t
compile yet, but we’ll fix it later in this chapter.

Filename: src/main.rs

```
fn largest<T>(list: &[T]) -> T {
    let mut largest = list[0];

    for &item in list {
        if item > largest {
            largest = item;
        }
    }

    largest
}

fn main() {
    let number_list = vec![34, 50, 25, 100, 65];

    let result = largest(&number_list);
    println!("The largest number is {}", result);

    let char_list = vec!['y', 'm', 'a', 'q'];

    let result = largest(&char_list);
    println!("The largest char is {}", result);
}
```

Listing 10-5: A definition of the `largest` function that uses generic type
parameters but doesn’t compile yet

If we compile this code right now, we’ll get this error:

```
error[E0369]: binary operation `>` cannot be applied to type `T`
 --> src/main.rs:5:17
  |
5 |         if item > largest {
  |            ---- ^ ------- T
  |            |
  |            T
  |
help: consider restricting type parameter `T`
  |
1 | fn largest<T: std::cmp::PartialOrd>(list: &[T]) -> T {
  |             ^^^^^^^^^^^^^^^^^^^^^^
```

The note mentions `std::cmp::PartialOrd`, which is a *trait*. We’ll talk about
traits in the next section. For now, this error states that the body of
`largest` won’t work for all possible types that `T` could be. Because we want
to compare values of type `T` in the body, we can only use types whose values
can be ordered. To enable comparisons, the standard library has the
`std::cmp::PartialOrd` trait that you can implement on types (see Appendix C
for more on this trait). You’ll learn how to specify that a generic type has a
particular trait in the “Traits as Parameters” section, but let’s first explore
other ways of using generic type parameters.

### In Struct Definitions

We can also define structs to use a generic type parameter in one or more
fields using the `<>` syntax. Listing 10-6 shows how to define a `Point<T>`
struct to hold `x` and `y` coordinate values of any type.

Filename: src/main.rs

```
struct Point<T> {
    x: T,
    y: T,
}

fn main() {
    let integer = Point { x: 5, y: 10 };
    let float = Point { x: 1.0, y: 4.0 };
}
```

Listing 10-6: A `Point<T>` struct that holds `x` and `y` values of type `T`

The syntax for using generics in struct definitions is similar to that used in
function definitions. First, we declare the name of the type parameter inside
angle brackets just after the name of the struct. Then we can use the generic
type in the struct definition where we would otherwise specify concrete data
types.

Note that because we’ve used only one generic type to define `Point<T>`, this
definition says that the `Point<T>` struct is generic over some type `T`, and
the fields `x` and `y` are *both* that same type, whatever that type may be. If
we create an instance of a `Point<T>` that has values of different types, as in
Listing 10-7, our code won’t compile.

Filename: src/main.rs

```
struct Point<T> {
    x: T,
    y: T,
}

fn main() {
    let wont_work = Point { x: 5, y: 4.0 };
}
```

Listing 10-7: The fields `x` and `y` must be the same type because both have
the same generic data type `T`.

In this example, when we assign the integer value 5 to `x`, we let the
compiler know that the generic type `T` will be an integer for this instance of
`Point<T>`. Then when we specify 4.0 for `y`, which we’ve defined to have the
same type as `x`, we’ll get a type mismatch error like this:

```
error[E0308]: mismatched types
 --> src/main.rs:7:38
  |
7 |     let wont_work = Point { x: 5, y: 4.0 };
  |                                      ^^^ expected integer, found floating-point number
```

To define a `Point` struct where `x` and `y` are both generics but could have
different types, we can use multiple generic type parameters. For example, in
Listing 10-8, we can change the definition of `Point` to be generic over types
`T` and `U` where `x` is of type `T` and `y` is of type `U`.

Filename: src/main.rs

```
struct Point<T, U> {
    x: T,
    y: U,
}

fn main() {
    let both_integer = Point { x: 5, y: 10 };
    let both_float = Point { x: 1.0, y: 4.0 };
    let integer_and_float = Point { x: 5, y: 4.0 };
}
```

Listing 10-8: A `Point<T, U>` generic over two types so that `x` and `y` can be
values of different types

Now all the instances of `Point` shown are allowed! You can use as many generic
type parameters in a definition as you want, but using more than a few makes
your code hard to read. When you need lots of generic types in your code, it
could indicate that your code needs restructuring into smaller pieces.

### In Enum Definitions

As we did with structs, we can define enums to hold generic data types in their
variants. Let’s take another look at the `Option<T>` enum that the standard
library provides, which we used in Chapter 6:

```
enum Option<T> {
    Some(T),
    None,
}
```

This definition should now make more sense to you. As you can see, `Option<T>`
is an enum that is generic over type `T` and has two variants: `Some`, which
holds one value of type `T`, and a `None` variant that doesn’t hold any value.
By using the `Option<T>` enum, we can express the abstract concept of having an
optional value, and because `Option<T>` is generic, we can use this abstraction
no matter what the type of the optional value is.

Enums can use multiple generic types as well. The definition of the `Result`
enum that we used in Chapter 9 is one example:

```
enum Result<T, E> {
    Ok(T),
    Err(E),
}
```

The `Result` enum is generic over two types, `T` and `E`, and has two variants:
`Ok`, which holds a value of type `T`, and `Err`, which holds a value of type
`E`. This definition makes it convenient to use the `Result` enum anywhere we
have an operation that might succeed (return a value of some type `T`) or fail
(return an error of some type `E`). In fact, this is what we used to open a
file in Listing 9-3, where `T` was filled in with the type `std::fs::File` when
the file was opened successfully and `E` was filled in with the type
`std::io::Error` when there were problems opening the file.

When you recognize situations in your code with multiple struct or enum
definitions that differ only in the types of the values they hold, you can
avoid duplication by using generic types instead.

### In Method Definitions

We can implement methods on structs and enums (as we did in Chapter 5) and use
generic types in their definitions, too. Listing 10-9 shows the `Point<T>`
struct we defined in Listing 10-6 with a method named `x` implemented on it.

Filename: src/main.rs

```
struct Point<T> {
    x: T,
    y: T,
}

impl<T> Point<T> {
    fn x(&self) -> &T {
        &self.x
    }
}

fn main() {
    let p = Point { x: 5, y: 10 };

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

Listing 10-9: Implementing a method named `x` on the `Point<T>` struct that
will return a reference to the `x` field of type `T`

Here, we’ve defined a method named `x` on `Point<T>` that returns a reference
to the data in the field `x`.

Note that we have to declare `T` just after `impl` so we can use it to specify
that we’re implementing methods on the type `Point<T>`. By declaring `T` as a
generic type after `impl`, Rust can identify that the type in the angle
brackets in `Point` is a generic type rather than a concrete type. Because this
is declaring the generic again, we could have chosen a different name for the
generic parameter than the generic parameter declared in the struct definition,
but using the same name is conventional. Methods written within an `impl` that
declares the generic type will be defined on any instance of the type, no
matter what concrete type ends up substituting for the generic type.

The other option we have is defining methods on the type with some constraint
on the generic type. We could, for example, implement methods only on
`Point<f32>` instances rather than on `Point<T>` instances with any generic
type. In Listing 10-10 we use the concrete type `f32`, meaning we don’t declare
any types after `impl`.

Filename: src/main.rs

```
impl Point<f32> {
    fn distance_from_origin(&self) -> f32 {
        (self.x.powi(2) + self.y.powi(2)).sqrt()
    }
}
```

Listing 10-10: An `impl` block that only applies to a struct with a particular
concrete type for the generic type parameter `T`

This code means the type `Point<f32>` will have a method named
`distance_from_origin` and other instances of `Point<T>` where `T` is not of
type `f32` will not have this method defined. The method measures how far our
point is from the point at coordinates (0.0, 0.0) and uses mathematical
operations that are available only for floating point types.

Generic type parameters in a struct definition aren’t always the same as those
you use in that struct’s method signatures. Listing 10-11 uses the generic
types `X1` and `Y1` for the `Point` struct and `X2` `Y2` for the `mixup` method
signature to make the example clearer. The method creates a new `Point`
instance with the `x` value from the `self` `Point` (of type `X1`) and the `y`
value from the passed-in `Point` (of type `Y2`).

Filename: src/main.rs

```
struct Point<X1, Y1> {
    x: X1,
    y: Y1,
}

impl<X1, Y1> Point<X1, Y1> {
    fn mixup<X2, Y2>(self, other: Point<X2, Y2>) -> Point<X1, Y2> {
        Point {
            x: self.x,
            y: other.y,
        }
    }
}

fn main() {
    let p1 = Point { x: 5, y: 10.4 };
    let p2 = Point { x: "Hello", y: 'c' };

    let p3 = p1.mixup(p2);

    println!("p3.x = {}, p3.y = {}", p3.x, p3.y);
}
```

Listing 10-11: A method that uses different generic types from its struct’s
definition

In `main`, we’ve defined a `Point` that has an `i32` for `x` (with value `5`)
and an `f64` for `y` (with value `10.4`). The `p2` variable is a `Point` struct
that has a string slice for `x` (with value `"Hello"`) and a `char` for `y`
(with value `c`). Calling `mixup` on `p1` with the argument `p2` gives us `p3`,
which will have an `i32` for `x`, because `x` came from `p1`. The `p3` variable
will have a `char` for `y`, because `y` came from `p2`. The `println!` macro
call will print `p3.x = 5, p3.y = c`.

The purpose of this example is to demonstrate a situation in which some generic
parameters are declared with `impl` and some are declared with the method
definition. Here, the generic parameters `X1` and `Y1` are declared after
`impl` because they go with the struct definition. The generic parameters `X2`
and `Y2` are declared after `fn mixup`, because they’re only relevant to the
method.

### Performance of Code Using Generics

You might be wondering whether there is a runtime cost when you’re using
generic type parameters. The good news is that Rust implements generics in such
a way that your code doesn’t run any slower using generic types than it would
with concrete types.

Rust accomplishes this by performing monomorphization of the code that is using
generics at compile time. *Monomorphization* is the process of turning generic
code into specific code by filling in the concrete types that are used when
compiled.

In this process, the compiler does the opposite of the steps we used to create
the generic function in Listing 10-5: the compiler looks at all the places
where generic code is called and generates code for the concrete types the
generic code is called with.

Let’s look at how this works with an example that uses the standard library’s
`Option<T>` enum:

```
let integer = Some(5);
let float = Some(5.0);
```

When Rust compiles this code, it performs monomorphization. During that
process, the compiler reads the values that have been used in `Option<T>`
instances and identifies two kinds of `Option<T>`: one is `i32` and the other
is `f64`. As such, it expands the generic definition of `Option<T>` into
`Option_i32` and `Option_f64`, thereby replacing the generic definition with
the specific ones.

The monomorphized version of the code looks like the following. The generic
`Option<T>` is replaced with the specific definitions created by the compiler:

Filename: src/main.rs

```
enum Option_i32 {
    Some(i32),
    None,
}

enum Option_f64 {
    Some(f64),
    None,
}

fn main() {
    let integer = Option_i32::Some(5);
    let float = Option_f64::Some(5.0);
}
```

Because Rust compiles generic code into code that specifies the type in each
instance, we pay no runtime cost for using generics. When the code runs, it
performs just as it would if we had duplicated each definition by hand. The
process of monomorphization makes Rust’s generics extremely efficient at
runtime.

## Traits: Defining Shared Behavior

A *trait* tells the Rust compiler about functionality a particular type has and
can share with other types. We can use traits to define shared behavior in an
abstract way. We can use trait bounds to specify that a generic type can be any
type that has certain behavior.

> Note: Traits are similar to a feature often called *interfaces* in other
> languages, although with some differences.

### Defining a Trait

A type’s behavior consists of the methods we can call on that type. Different
types share the same behavior if we can call the same methods on all of those
types. Trait definitions are a way to group method signatures together to
define a set of behaviors necessary to accomplish some purpose.

For example, let’s say we have multiple structs that hold various kinds and
amounts of text: a `NewsArticle` struct that holds a news story filed in a
particular location and a `Tweet` that can have at most 280 characters along
with metadata that indicates whether it was a new tweet, a retweet, or a reply
to another tweet.

We want to make a media aggregator library crate named `aggregator` that can
display summaries of data that might be stored in a `NewsArticle` or `Tweet`
instance. To do this, we need a summary from each type, and we’ll request
that summary by calling a `summarize` method on an instance. Listing 10-12
shows the definition of a public `Summary` trait that expresses this behavior.

Filename: src/lib.rs

```
pub trait Summary {
    fn summarize(&self) -> String;
}
```

Listing 10-12: A `Summary` trait that consists of the behavior provided by a
`summarize` method

Here, we declare a trait using the `trait` keyword and then the trait’s name,
which is `Summary` in this case. We’ve also declared the trait as `pub` so that
crates depending on this crate can make use of this trait too, as we’ll see in
a few examples. Inside the curly brackets, we declare the method signatures
that describe the behaviors of the types that implement this trait, which in
this case is `fn summarize(&self) -> String`.

After the method signature, instead of providing an implementation within curly
brackets, we use a semicolon. Each type implementing this trait must provide
its own custom behavior for the body of the method. The compiler will enforce
that any type that has the `Summary` trait will have the method `summarize`
defined with this signature exactly.

A trait can have multiple methods in its body: the method signatures are listed
one per line and each line ends in a semicolon.

### Implementing a Trait on a Type

Now that we’ve defined the desired signatures of the `Summary` trait’s methods,
we can implement it on the types in our media aggregator. Listing 10-13 shows
an implementation of the `Summary` trait on the `NewsArticle` struct that uses
the headline, the author, and the location to create the return value of
`summarize`. For the `Tweet` struct, we define `summarize` as the username
followed by the entire text of the tweet, assuming that tweet content is
already limited to 280 characters.

Filename: src/lib.rs

```
pub struct NewsArticle {
    pub headline: String,
    pub location: String,
    pub author: String,
    pub content: String,
}

impl Summary for NewsArticle {
    fn summarize(&self) -> String {
        format!("{}, by {} ({})", self.headline, self.author, self.location)
    }
}

pub struct Tweet {
    pub username: String,
    pub content: String,
    pub reply: bool,
    pub retweet: bool,
}

impl Summary for Tweet {
    fn summarize(&self) -> String {
        format!("{}: {}", self.username, self.content)
    }
}
```

Listing 10-13: Implementing the `Summary` trait on the `NewsArticle` and
`Tweet` types

Implementing a trait on a type is similar to implementing regular methods. The
difference is that after `impl`, we put the trait name that we want to
implement, then use the `for` keyword, and then specify the name of the type we
want to implement the trait for. Within the `impl` block, we put the method
signatures that the trait definition has defined. Instead of adding a semicolon
after each signature, we use curly brackets and fill in the method body with
the specific behavior that we want the methods of the trait to have for the
particular type.

Now that the library has implemented the `Summary` trait on `NewsArticle` and
`Tweet`, users of the crate can call the trait methods on instances of
`NewsArticle` and `Tweet` in the same way we call regular methods. The only
difference is that the trait has to be brought into scope as well as the types
to get the additional trait methods. Here’s an example of how a binary crate
could use our `aggregator` library crate:

```
use aggregator::{Summary, Tweet};

fn main() {
    let tweet = Tweet {
        username: String::from("horse_ebooks"),
        content: String::from(
            "of course, as you probably already know, people",
        ),
        reply: false,
        retweet: false,
    };

    println!("1 new tweet: {}", tweet.summarize());
}
```

This code prints `1 new tweet: horse_ebooks: of course, as you probably already
know, people`.

Other crates that depend on the `aggregator` crate can also bring the `Summary`
trait into scope to implement the trait on their own types. One restriction to
note with trait implementations is that we can implement a trait on a type only
if at least one of the trait or the type is local to our crate. For example, we
can implement standard library traits like `Display` on a custom type like
`Tweet` as part of our `aggregator` crate functionality, because the type
`Tweet` is local to our `aggregator` crate. We can also implement `Summary` on
`Vec<T>` in our `aggregator` crate, because the trait `Summary` is local to our
`aggregator` crate.

But we can’t implement external traits on external types. For example, we can’t
implement the `Display` trait on `Vec<T>` within our `aggregator` crate,
because `Display` and `Vec<T>` are defined in the standard library and aren’t
local to our `aggregator` crate. This restriction is part of a property of
programs called *coherence*, and more specifically the *orphan rule*, so named
because the parent type is not present. This rule ensures that other people’s
code can’t break your code and vice versa. Without the rule, two crates could
implement the same trait for the same type, and Rust wouldn’t know which
implementation to use.

### Default Implementations

Sometimes it’s useful to have default behavior for some or all of the methods
in a trait instead of requiring implementations for all methods on every type.
Then, as we implement the trait on a particular type, we can keep or override
each method’s default behavior.

Listing 10-14 shows how to specify a default string for the `summarize` method
of the `Summary` trait instead of only defining the method signature, as we did
in Listing 10-12.

Filename: src/lib.rs

```
pub trait Summary {
    fn summarize(&self) -> String {
        String::from("(Read more...)")
    }
}
```

Listing 10-14: Definition of a `Summary` trait with a default implementation of
the `summarize` method

To use a default implementation to summarize instances of `NewsArticle` instead
of defining a custom implementation, we specify an empty `impl` block with
`impl Summary for NewsArticle {}`.

Even though we’re no longer defining the `summarize` method on `NewsArticle`
directly, we’ve provided a default implementation and specified that
`NewsArticle` implements the `Summary` trait. As a result, we can still call
the `summarize` method on an instance of `NewsArticle`, like this:

```
let article = NewsArticle {
    headline: String::from("Penguins win the Stanley Cup Championship!"),
    location: String::from("Pittsburgh, PA, USA"),
    author: String::from("Iceburgh"),
    content: String::from(
        "The Pittsburgh Penguins once again are the best \
         hockey team in the NHL.",
    ),
};

println!("New article available! {}", article.summarize());
```

This code prints `New article available! (Read more...)`.

Creating a default implementation for `summarize` doesn’t require us to change
anything about the implementation of `Summary` on `Tweet` in Listing 10-13. The
reason is that the syntax for overriding a default implementation is the same
as the syntax for implementing a trait method that doesn’t have a default
implementation.

Default implementations can call other methods in the same trait, even if those
other methods don’t have a default implementation. In this way, a trait can
provide a lot of useful functionality and only require implementors to specify
a small part of it. For example, we could define the `Summary` trait to have a
`summarize_author` method whose implementation is required, and then define a
`summarize` method that has a default implementation that calls the
`summarize_author` method:

```
pub trait Summary {
    fn summarize_author(&self) -> String;

    fn summarize(&self) -> String {
        format!("(Read more from {}...)", self.summarize_author())
    }
}
```

To use this version of `Summary`, we only need to define `summarize_author`
when we implement the trait on a type:

```
impl Summary for Tweet {
    fn summarize_author(&self) -> String {
        format!("@{}", self.username)
    }
}
```

After we define `summarize_author`, we can call `summarize` on instances of the
`Tweet` struct, and the default implementation of `summarize` will call the
definition of `summarize_author` that we’ve provided. Because we’ve implemented
`summarize_author`, the `Summary` trait has given us the behavior of the
`summarize` method without requiring us to write any more code.

```
let tweet = Tweet {
    username: String::from("horse_ebooks"),
    content: String::from(
        "of course, as you probably already know, people",
    ),
    reply: false,
    retweet: false,
};

println!("1 new tweet: {}", tweet.summarize());
```

This code prints `1 new tweet: (Read more from @horse_ebooks...)`.

Note that it isn’t possible to call the default implementation from an
overriding implementation of that same method.

### Traits as Parameters

Now that you know how to define and implement traits, we can explore how to use
traits to define functions that accept many different types.

For example, in Listing 10-13, we implemented the `Summary` trait on the
`NewsArticle` and `Tweet` types. We can define a `notify` function that calls
the `summarize` method on its `item` parameter, which is of some type that
implements the `Summary` trait. To do this, we can use the `impl Trait`
syntax, like this:

```
pub fn notify(item: &impl Summary) {
    println!("Breaking news! {}", item.summarize());
}
```

Instead of a concrete type for the `item` parameter, we specify the `impl`
keyword and the trait name. This parameter accepts any type that implements the
specified trait. In the body of `notify`, we can call any methods on `item`
that come from the `Summary` trait, such as `summarize`. We can call `notify`
and pass in any instance of `NewsArticle` or `Tweet`. Code that calls the
function with any other type, such as a `String` or an `i32`, won’t compile
because those types don’t implement `Summary`.

#### Trait Bound Syntax

The `impl Trait` syntax works for straightforward cases but is actually
syntax sugar for a longer form, which is called a *trait bound*; it looks like
this:

```
pub fn notify<T: Summary>(item: &T) {
    println!("Breaking news! {}", item.summarize());
}
```

This longer form is equivalent to the example in the previous section but is
more verbose. We place trait bounds with the declaration of the generic type
parameter after a colon and inside angle brackets.

The `impl Trait` syntax is convenient and makes for more concise code in simple
cases. The trait bound syntax can express more complexity in other cases. For
example, we can have two parameters that implement `Summary`. Using the `impl
Trait` syntax looks like this:

```
pub fn notify(item1: &impl Summary, item2: &impl Summary) {
```

If we wanted this function to allow `item1` and `item2` to have different
types, using `impl Trait` would be appropriate (as long as both types implement
`Summary`). If we wanted to force both parameters to have the same type, that’s
only possible to express using a trait bound, like this:

```
pub fn notify<T: Summary>(item1: &T, item2: &T) {
```

The generic type `T` specified as the type of the `item1` and `item2`
parameters constrains the function such that the concrete type of the value
passed as an argument for `item1` and `item2` must be the same.

#### Specifying Multiple Trait Bounds with the `+` Syntax

We can also specify more than one trait bound. Say we wanted `notify` to use
display formatting on `item` as well as the `summarize` method: we specify in
the `notify` definition that `item` must implement both `Display` and
`Summary`. We can do so using the `+` syntax:

```
pub fn notify(item: &(impl Summary + Display)) {
```

The `+` syntax is also valid with trait bounds on generic types:

```
pub fn notify<T: Summary + Display>(item: &T) {
```

With the two trait bounds specified, the body of `notify` can call `summarize`
and use `{}` to format `item`.

#### Clearer Trait Bounds with `where` Clauses

Using too many trait bounds has its downsides. Each generic has its own trait
bounds, so functions with multiple generic type parameters can contain lots of
trait bound information between the function’s name and its parameter list,
making the function signature hard to read. For this reason, Rust has alternate
syntax for specifying trait bounds inside a `where` clause after the function
signature. So instead of writing this:

```
fn some_function<T: Display + Clone, U: Clone + Debug>(t: &T, u: &U) -> i32 {
```

we can use a `where` clause, like this:

```
fn some_function<T, U>(t: &T, u: &U) -> i32
    where T: Display + Clone,
          U: Clone + Debug
{
```

This function’s signature is less cluttered: the function name, parameter list,
and return type are close together, similar to a function without lots of trait
bounds.

### Returning Types that Implement Traits

We can also use the `impl Trait` syntax in the return position to return a
value of some type that implements a trait, as shown here:

```
fn returns_summarizable() -> impl Summary {
    Tweet {
        username: String::from("horse_ebooks"),
        content: String::from(
            "of course, as you probably already know, people",
        ),
        reply: false,
        retweet: false,
    }
}
```

By using `impl Summary` for the return type, we specify that the
`returns_summarizable` function returns some type that implements the `Summary`
trait without naming the concrete type. In this case, `returns_summarizable`
returns a `Tweet`, but the code calling this function doesn’t know that.

The ability to return a type that is only specified by the trait it implements
is especially useful in the context of closures and iterators, which we cover
in Chapter 13. Closures and iterators create types that only the compiler knows
or types that are very long to specify. The `impl Trait` syntax lets you
concisely specify that a function returns some type that implements the
`Iterator` trait without needing to write out a very long type.

However, you can only use `impl Trait` if you’re returning a single type. For
example, this code that returns either a `NewsArticle` or a `Tweet` with the
return type specified as `impl Summary` wouldn’t work:

```
fn returns_summarizable(switch: bool) -> impl Summary {
    if switch {
        NewsArticle {
            headline: String::from(
                "Penguins win the Stanley Cup Championship!",
            ),
            location: String::from("Pittsburgh, PA, USA"),
            author: String::from("Iceburgh"),
            content: String::from(
                "The Pittsburgh Penguins once again are the best \
                 hockey team in the NHL.",
            ),
        }
    } else {
        Tweet {
            username: String::from("horse_ebooks"),
            content: String::from(
                "of course, as you probably already know, people",
            ),
            reply: false,
            retweet: false,
        }
    }
}
```

Returning either a `NewsArticle` or a `Tweet` isn’t allowed due to restrictions
around how the `impl Trait` syntax is implemented in the compiler. We’ll cover
how to write a function with this behavior in the “Using Trait Objects That
Allow for Values of Different
Types” section of Chapter 17.

### Fixing the `largest` Function with Trait Bounds

Now that you know how to specify the behavior you want to use using the generic
type parameter’s bounds, let’s return to Listing 10-5 to fix the definition of
the `largest` function that uses a generic type parameter! Last time we tried
to run that code, we received this error:

```
error[E0369]: binary operation `>` cannot be applied to type `T`
 --> src/main.rs:5:17
  |
5 |         if item > largest {
  |            ---- ^ ------- T
  |            |
  |            T
  |
help: consider restricting type parameter `T`
  |
1 | fn largest<T: std::cmp::PartialOrd>(list: &[T]) -> T {
  |             ^^^^^^^^^^^^^^^^^^^^^^
```

In the body of `largest` we wanted to compare two values of type `T` using the
greater than (`>`) operator. Because that operator is defined as a default
method on the standard library trait `std::cmp::PartialOrd`, we need to specify
`PartialOrd` in the trait bounds for `T` so the `largest` function can work on
slices of any type that we can compare. We don’t need to bring `PartialOrd`
into scope because it’s in the prelude. Change the signature of `largest` to
look like this:

```
fn largest<T: PartialOrd>(list: &[T]) -> T {
```

This time when we compile the code, we get a different set of errors:

```
error[E0508]: cannot move out of type `[T]`, a non-copy slice
 --> src/main.rs:2:23
  |
2 |     let mut largest = list[0];
  |                       ^^^^^^^
  |                       |
  |                       cannot move out of here
  |                       move occurs because `list[_]` has type `T`, which does not implement the `Copy` trait
  |                       help: consider borrowing here: `&list[0]`

error[E0507]: cannot move out of a shared reference
 --> src/main.rs:4:18
  |
4 |     for &item in list {
  |         -----    ^^^^
  |         ||
  |         |data moved here
  |         |move occurs because `item` has type `T`, which does not implement the `Copy` trait
  |         help: consider removing the `&`: `item`
```

The key line in this error is `cannot move out of type [T], a non-copy slice`.
With our non-generic versions of the `largest` function, we were only trying to
find the largest `i32` or `char`. As discussed in the “Stack-Only Data: Copy”
section in Chapter 4, types like `i32` and `char` that have a known size can be
stored on the stack, so they implement the `Copy` trait. But when we made the
`largest` function generic, it became possible for the `list` parameter to have
types in it that don’t implement the `Copy` trait. Consequently, we wouldn’t be
able to move the value out of `list[0]` and into the `largest` variable,
resulting in this error.

To call this code with only those types that implement the `Copy` trait, we can
add `Copy` to the trait bounds of `T`! Listing 10-15 shows the complete code of
a generic `largest` function that will compile as long as the types of the
values in the slice that we pass into the function implement the `PartialOrd`
*and* `Copy` traits, like `i32` and `char` do.

Filename: src/main.rs

```
fn largest<T: PartialOrd + Copy>(list: &[T]) -> T {
    let mut largest = list[0];

    for &item in list {
        if item > largest {
            largest = item;
        }
    }

    largest
}

fn main() {
    let number_list = vec![34, 50, 25, 100, 65];

    let result = largest(&number_list);
    println!("The largest number is {}", result);

    let char_list = vec!['y', 'm', 'a', 'q'];

    let result = largest(&char_list);
    println!("The largest char is {}", result);
}
```

Listing 10-15: A working definition of the `largest` function that works on any
generic type that implements the `PartialOrd` and `Copy` traits

If we don’t want to restrict the `largest` function to the types that implement
the `Copy` trait, we could specify that `T` has the trait bound `Clone` instead
of `Copy`. Then we could clone each value in the slice when we want the
`largest` function to have ownership. Using the `clone` function means we’re
potentially making more heap allocations in the case of types that own heap
data like `String`, and heap allocations can be slow if we’re working with
large amounts of data.

Another way we could implement `largest` is for the function to return a
reference to a `T` value in the slice. If we change the return type to `&T`
instead of `T`, thereby changing the body of the function to return a
reference, we wouldn’t need the `Clone` or `Copy` trait bounds and we could
avoid heap allocations. Try implementing these alternate solutions on your own!
If you get stuck with errors having to do with lifetimes, keep reading: the
“Validating References with Lifetimes” section coming up will explain, but
lifetimes aren’t required to solve these challenges.

### Using Trait Bounds to Conditionally Implement Methods

By using a trait bound with an `impl` block that uses generic type parameters,
we can implement methods conditionally for types that implement the specified
traits. For example, the type `Pair<T>` in Listing 10-16 always implements the
`new` function to return a new instance of `Pair<T>` (recall from the ”Defining
Methods” section of Chapter 5 that `Self` is a type alias for the type of the
`impl` block, which in this case is `Pair<T>`). But in the next `impl` block,
`Pair<T>` only implements the `cmp_display` method if its inner type `T`
implements the `PartialOrd` trait that enables comparison *and* the `Display`
trait that enables printing.

Filename: src/lib.rs

```
use std::fmt::Display;

struct Pair<T> {
    x: T,
    y: T,
}

impl<T> Pair<T> {
    fn new(x: T, y: T) -> Self {
        Self { x, y }
    }
}

impl<T: Display + PartialOrd> Pair<T> {
    fn cmp_display(&self) {
        if self.x >= self.y {
            println!("The largest member is x = {}", self.x);
        } else {
            println!("The largest member is y = {}", self.y);
        }
    }
}
```

Listing 10-16: Conditionally implement methods on a generic type depending on
trait bounds

We can also conditionally implement a trait for any type that implements
another trait. Implementations of a trait on any type that satisfies the trait
bounds are called *blanket implementations* and are extensively used in the
Rust standard library. For example, the standard library implements the
`ToString` trait on any type that implements the `Display` trait. The `impl`
block in the standard library looks similar to this code:

```
impl<T: Display> ToString for T {
    // --snip--
}
```

Because the standard library has this blanket implementation, we can call the
`to_string` method defined by the `ToString` trait on any type that implements
the `Display` trait. For example, we can turn integers into their corresponding
`String` values like this because integers implement `Display`:

```
let s = 3.to_string();
```

Blanket implementations appear in the documentation for the trait in the
“Implementors” section.

Traits and trait bounds let us write code that uses generic type parameters to
reduce duplication but also specify to the compiler that we want the generic
type to have particular behavior. The compiler can then use the trait bound
information to check that all the concrete types used with our code provide the
correct behavior. In dynamically typed languages, we would get an error at
runtime if we called a method on a type which didn’t define the method. But Rust
moves these errors to compile time so we’re forced to fix the problems before
our code is even able to run. Additionally, we don’t have to write code that
checks for behavior at runtime because we’ve already checked at compile time.
Doing so improves performance without having to give up the flexibility of
generics.

Another kind of generic that we’ve already been using is called *lifetimes*.
Rather than ensuring that a type has the behavior we want, lifetimes ensure
that references are valid as long as we need them to be. Let’s look at how
lifetimes do that.

## Validating References with Lifetimes

One detail we didn’t discuss in the “References and Borrowing” section in
Chapter 4 is that every reference in Rust has a *lifetime*, which is the scope
for which that reference is valid. Most of the time, lifetimes are implicit and
inferred, just like most of the time, types are inferred. We must annotate
types when multiple types are possible. In a similar way, we must annotate
lifetimes when the lifetimes of references could be related in a few different
ways. Rust requires us to annotate the relationships using generic lifetime
parameters to ensure the actual references used at runtime will definitely be
valid.

Annotating lifetimes is not even a concept most other programming languages
have, so this is going to feel unfamiliar. Although we won’t cover lifetimes in
their entirety in this chapter, we’ll discuss common ways you might encounter
lifetime syntax so you can get introduced to the concept.

### Preventing Dangling References with Lifetimes

The main aim of lifetimes is to prevent dangling references, which cause a
program to reference data other than the data it’s intended to reference.
Consider the program in Listing 10-17, which has an outer scope and an inner
scope.

```
{
    let r;

    {
        let x = 5;
        r = &x;
    }

    println!("r: {}", r);
}
```

Listing 10-17: An attempt to use a reference whose value has gone out of scope

> Note: The examples in Listings 10-17, 10-18, and 10-24 declare variables
> without giving them an initial value, so the variable name exists in the
> outer scope. At first glance, this might appear to be in conflict with Rust’s
> having no null values. However, if we try to use a variable before giving it
> a value, we’ll get a compile-time error, which shows that Rust indeed does
> not allow null values.

The outer scope declares a variable named `r` with no initial value, and the
inner scope declares a variable named `x` with the initial value of 5. Inside
the inner scope, we attempt to set the value of `r` as a reference to `x`. Then
the inner scope ends, and we attempt to print the value in `r`. This code won’t
compile because the value `r` is referring to has gone out of scope before we
try to use it. Here is the error message:

```
error[E0597]: `x` does not live long enough
  --> src/main.rs:7:17
   |
7  |             r = &x;
   |                 ^^ borrowed value does not live long enough
8  |         }
   |         - `x` dropped here while still borrowed
9  |
10 |         println!("r: {}", r);
   |                           - borrow later used here
```

The variable `x` doesn’t “live long enough.” The reason is that `x` will be out
of scope when the inner scope ends on line 7. But `r` is still valid for the
outer scope; because its scope is larger, we say that it “lives longer.” If
Rust allowed this code to work, `r` would be referencing memory that was
deallocated when `x` went out of scope, and anything we tried to do with `r`
wouldn’t work correctly. So how does Rust determine that this code is invalid?
It uses a borrow checker.

### The Borrow Checker

The Rust compiler has a *borrow checker* that compares scopes to determine
whether all borrows are valid. Listing 10-18 shows the same code as Listing
10-17 but with annotations showing the lifetimes of the variables.

```
{
    let r;                // ---------+-- 'a
                          //          |
    {                     //          |
        let x = 5;        // -+-- 'b  |
        r = &x;           //  |       |
    }                     // -+       |
                          //          |
    println!("r: {}", r); //          |
}                         // ---------+
```

Listing 10-18: Annotations of the lifetimes of `r` and `x`, named `'a` and
`'b`, respectively

Here, we’ve annotated the lifetime of `r` with `'a` and the lifetime of `x`
with `'b`. As you can see, the inner `'b` block is much smaller than the outer
`'a` lifetime block. At compile time, Rust compares the size of the two
lifetimes and sees that `r` has a lifetime of `'a` but that it refers to memory
with a lifetime of `'b`. The program is rejected because `'b` is shorter than
`'a`: the subject of the reference doesn’t live as long as the reference.

Listing 10-19 fixes the code so it doesn’t have a dangling reference and
compiles without any errors.

```
{
    let x = 5;            // ----------+-- 'b
                          //           |
    let r = &x;           // --+-- 'a  |
                          //   |       |
    println!("r: {}", r); //   |       |
                          // --+       |
}                         // ----------+
```

Listing 10-19: A valid reference because the data has a longer lifetime than
the reference

Here, `x` has the lifetime `'b`, which in this case is larger than `'a`. This
means `r` can reference `x` because Rust knows that the reference in `r` will
always be valid while `x` is valid.

Now that you know where the lifetimes of references are and how Rust analyzes
lifetimes to ensure references will always be valid, let’s explore generic
lifetimes of parameters and return values in the context of functions.

### Generic Lifetimes in Functions

Let’s write a function that returns the longer of two string slices. This
function will take two string slices and return a string slice. After we’ve
implemented the `longest` function, the code in Listing 10-20 should print `The
longest string is abcd`.

Filename: src/main.rs

```
fn main() {
    let string1 = String::from("abcd");
    let string2 = "xyz";

    let result = longest(string1.as_str(), string2);
    println!("The longest string is {}", result);
}
```

Listing 10-20: A `main` function that calls the `longest` function to find the
longer of two string slices

Note that we want the function to take string slices, which are references,
because we don’t want the `longest` function to take ownership of its
parameters. Refer to the “String Slices as
Parameters” section in Chapter 4
for more discussion about why the parameters we use in Listing 10-20 are the
ones we want.

If we try to implement the `longest` function as shown in Listing 10-21, it
won’t compile.

Filename: src/main.rs

```
fn longest(x: &str, y: &str) -> &str {
    if x.len() > y.len() {
        x
    } else {
        y
    }
}
```

Listing 10-21: An implementation of the `longest` function that returns the
longer of two string slices but does not yet compile

Instead, we get the following error that talks about lifetimes:

```
error[E0106]: missing lifetime specifier
 --> src/main.rs:9:33
  |
9 | fn longest(x: &str, y: &str) -> &str {
  |               ----     ----     ^ expected named lifetime parameter
  |
  = help: this function's return type contains a borrowed value, but the signature does not say whether it is borrowed from `x` or `y`
help: consider introducing a named lifetime parameter
  |
9 | fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
  |           ^^^^    ^^^^^^^     ^^^^^^^     ^^^
```

The help text reveals that the return type needs a generic lifetime parameter
on it because Rust can’t tell whether the reference being returned refers to
`x` or `y`. Actually, we don’t know either, because the `if` block in the body
of this function returns a reference to `x` and the `else` block returns a
reference to `y`!

When we’re defining this function, we don’t know the concrete values that will
be passed into this function, so we don’t know whether the `if` case or the
`else` case will execute. We also don’t know the concrete lifetimes of the
references that will be passed in, so we can’t look at the scopes as we did in
Listings 10-18 and 10-19 to determine whether the reference we return will
always be valid. The borrow checker can’t determine this either, because it
doesn’t know how the lifetimes of `x` and `y` relate to the lifetime of the
return value. To fix this error, we’ll add generic lifetime parameters that
define the relationship between the references so the borrow checker can
perform its analysis.

### Lifetime Annotation Syntax

Lifetime annotations don’t change how long any of the references live. Just
as functions can accept any type when the signature specifies a generic type
parameter, functions can accept references with any lifetime by specifying a
generic lifetime parameter. Lifetime annotations describe the relationships of
the lifetimes of multiple references to each other without affecting the
lifetimes.

Lifetime annotations have a slightly unusual syntax: the names of lifetime
parameters must start with an apostrophe (`'`) and are usually all lowercase and
very short, like generic types. Most people use the name `'a`. We place
lifetime parameter annotations after the `&` of a reference, using a space to
separate the annotation from the reference’s type.

Here are some examples: a reference to an `i32` without a lifetime parameter, a
reference to an `i32` that has a lifetime parameter named `'a`, and a mutable
reference to an `i32` that also has the lifetime `'a`.

```
&i32        // a reference
&'a i32     // a reference with an explicit lifetime
&'a mut i32 // a mutable reference with an explicit lifetime
```

One lifetime annotation by itself doesn’t have much meaning, because the
annotations are meant to tell Rust how generic lifetime parameters of multiple
references relate to each other. For example, let’s say we have a function with
the parameter `first` that is a reference to an `i32` with lifetime `'a`. The
function also has another parameter named `second` that is another reference to
an `i32` that also has the lifetime `'a`. The lifetime annotations indicate
that the references `first` and `second` must both live as long as that generic
lifetime.

### Lifetime Annotations in Function Signatures

Now let’s examine lifetime annotations in the context of the `longest`
function. As with generic type parameters, we need to declare generic lifetime
parameters inside angle brackets between the function name and the parameter
list. The constraint we want to express in this signature is that the lifetimes
of both of the parameters and the lifetime of the returned reference are
related such that the returned reference will be valid as long as both the
parameters are. We’ll name the lifetime `'a` and then add it to each reference,
as shown in Listing 10-22.

Filename: src/main.rs

```
fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() > y.len() {
        x
    } else {
        y
    }
}
```

Listing 10-22: The `longest` function definition specifying that all the
references in the signature must have the same lifetime `'a`

This code should compile and produce the result we want when we use it with the
`main` function in Listing 10-20.

The function signature now tells Rust that for some lifetime `'a`, the function
takes two parameters, both of which are string slices that live at least as
long as lifetime `'a`. The function signature also tells Rust that the string
slice returned from the function will live at least as long as lifetime `'a`.
In practice, it means that the lifetime of the reference returned by the
`longest` function is the same as the smaller of the lifetimes of the
references passed in. These relationships are what we want Rust to use when
analyzing this code.

Remember, when we specify the lifetime parameters in this function signature,
we’re not changing the lifetimes of any values passed in or returned. Rather,
we’re specifying that the borrow checker should reject any values that don’t
adhere to these constraints. Note that the `longest` function doesn’t need to
know exactly how long `x` and `y` will live, only that some scope can be
substituted for `'a` that will satisfy this signature.

When annotating lifetimes in functions, the annotations go in the function
signature, not in the function body. The lifetime annotations become part of
the contract of the function, much like the types in the signature are. Having
function signatures contain the lifetime contract means the analysis the Rust
compiler does can be simpler. If there’s a problem with the way a function is
annotated or the way it is called, the compiler errors can point to the part of
our code and the constraints more precisely. If, instead, the Rust compiler
made more inferences about what we intended the relationships of the lifetimes
to be, the compiler might only be able to point to a use of our code many steps
away from the cause of the problem.

When we pass concrete references to `longest`, the concrete lifetime that is
substituted for `'a` is the part of the scope of `x` that overlaps with the
scope of `y`. In other words, the generic lifetime `'a` will get the concrete
lifetime that is equal to the smaller of the lifetimes of `x` and `y`. Because
we’ve annotated the returned reference with the same lifetime parameter `'a`,
the returned reference will also be valid for the length of the smaller of the
lifetimes of `x` and `y`.

Let’s look at how the lifetime annotations restrict the `longest` function by
passing in references that have different concrete lifetimes. Listing 10-23 is
a straightforward example.

Filename: src/main.rs

```
fn main() {
    let string1 = String::from("long string is long");

    {
        let string2 = String::from("xyz");
        let result = longest(string1.as_str(), string2.as_str());
        println!("The longest string is {}", result);
    }
}
```

Listing 10-23: Using the `longest` function with references to `String` values
that have different concrete lifetimes

In this example, `string1` is valid until the end of the outer scope, `string2`
is valid until the end of the inner scope, and `result` references something
that is valid until the end of the inner scope. Run this code, and you’ll see
that the borrow checker approves of this code; it will compile and print `The
longest string is long string is long`.

Next, let’s try an example that shows that the lifetime of the reference in
`result` must be the smaller lifetime of the two arguments. We’ll move the
declaration of the `result` variable outside the inner scope but leave the
assignment of the value to the `result` variable inside the scope with
`string2`. Then we’ll move the `println!` that uses `result` outside the inner
scope, after the inner scope has ended. The code in Listing 10-24 will not
compile.

Filename: src/main.rs

```
fn main() {
    let string1 = String::from("long string is long");
    let result;
    {
        let string2 = String::from("xyz");
        result = longest(string1.as_str(), string2.as_str());
    }
    println!("The longest string is {}", result);
}
```

Listing 10-24: Attempting to use `result` after `string2` has gone out of scope

When we try to compile this code, we’ll get this error:

```
error[E0597]: `string2` does not live long enough
 --> src/main.rs:6:44
  |
6 |         result = longest(string1.as_str(), string2.as_str());
  |                                            ^^^^^^^ borrowed value does not live long enough
7 |     }
  |     - `string2` dropped here while still borrowed
8 |     println!("The longest string is {}", result);
  |                                          ------ borrow later used here
```

The error shows that for `result` to be valid for the `println!` statement,
`string2` would need to be valid until the end of the outer scope. Rust knows
this because we annotated the lifetimes of the function parameters and return
values using the same lifetime parameter `'a`.

As humans, we can look at this code and see that `string1` is longer than
`string2` and therefore `result` will contain a reference to `string1`.
Because `string1` has not gone out of scope yet, a reference to `string1` will
still be valid for the `println!` statement. However, the compiler can’t see
that the reference is valid in this case. We’ve told Rust that the lifetime of
the reference returned by the `longest` function is the same as the smaller of
the lifetimes of the references passed in. Therefore, the borrow checker
disallows the code in Listing 10-24 as possibly having an invalid reference.

Try designing more experiments that vary the values and lifetimes of the
references passed in to the `longest` function and how the returned reference
is used. Make hypotheses about whether or not your experiments will pass the
borrow checker before you compile; then check to see if you’re right!

### Thinking in Terms of Lifetimes

The way in which you need to specify lifetime parameters depends on what your
function is doing. For example, if we changed the implementation of the
`longest` function to always return the first parameter rather than the longest
string slice, we wouldn’t need to specify a lifetime on the `y` parameter. The
following code will compile:

Filename: src/main.rs

```
fn longest<'a>(x: &'a str, y: &str) -> &'a str {
    x
}
```

In this example, we’ve specified a lifetime parameter `'a` for the parameter
`x` and the return type, but not for the parameter `y`, because the lifetime of
`y` does not have any relationship with the lifetime of `x` or the return value.

When returning a reference from a function, the lifetime parameter for the
return type needs to match the lifetime parameter for one of the parameters. If
the reference returned does *not* refer to one of the parameters, it must refer
to a value created within this function, which would be a dangling reference
because the value will go out of scope at the end of the function. Consider
this attempted implementation of the `longest` function that won’t compile:

Filename: src/main.rs

```
fn longest<'a>(x: &str, y: &str) -> &'a str {
    let result = String::from("really long string");
    result.as_str()
}
```

Here, even though we’ve specified a lifetime parameter `'a` for the return
type, this implementation will fail to compile because the return value
lifetime is not related to the lifetime of the parameters at all. Here is the
error message we get:

```
error[E0515]: cannot return value referencing local variable `result`
  --> src/main.rs:11:5
   |
11 |     result.as_str()
   |     ------^^^^^^^^^
   |     |
   |     returns a value referencing data owned by the current function
   |     `result` is borrowed here
```

The problem is that `result` goes out of scope and gets cleaned up at the end
of the `longest` function. We’re also trying to return a reference to `result`
from the function. There is no way we can specify lifetime parameters that
would change the dangling reference, and Rust won’t let us create a dangling
reference. In this case, the best fix would be to return an owned data type
rather than a reference so the calling function is then responsible for
cleaning up the value.

Ultimately, lifetime syntax is about connecting the lifetimes of various
parameters and return values of functions. Once they’re connected, Rust has
enough information to allow memory-safe operations and disallow operations that
would create dangling pointers or otherwise violate memory safety.

### Lifetime Annotations in Struct Definitions

So far, we’ve only defined structs to hold owned types. It’s possible for
structs to hold references, but in that case we would need to add a lifetime
annotation on every reference in the struct’s definition. Listing 10-25 has a
struct named `ImportantExcerpt` that holds a string slice.

Filename: src/main.rs

```
struct ImportantExcerpt<'a> {
    part: &'a str,
}

fn main() {
    let novel = String::from("Call me Ishmael. Some years ago...");
    let first_sentence = novel.split('.').next().expect("Could not find a '.'");
    let i = ImportantExcerpt {
        part: first_sentence,
    };
}
```

Listing 10-25: A struct that holds a reference, so its definition needs a
lifetime annotation

This struct has one field, `part`, that holds a string slice, which is a
reference. As with generic data types, we declare the name of the generic
lifetime parameter inside angle brackets after the name of the struct so we can
use the lifetime parameter in the body of the struct definition. This
annotation means an instance of `ImportantExcerpt` can’t outlive the reference
it holds in its `part` field.

The `main` function here creates an instance of the `ImportantExcerpt` struct
that holds a reference to the first sentence of the `String` owned by the
variable `novel`. The data in `novel` exists before the `ImportantExcerpt`
instance is created. In addition, `novel` doesn’t go out of scope until after
the `ImportantExcerpt` goes out of scope, so the reference in the
`ImportantExcerpt` instance is valid.

### Lifetime Elision

You’ve learned that every reference has a lifetime and that you need to specify
lifetime parameters for functions or structs that use references. However, in
Chapter 4 we had a function in Listing 4-9, which is shown again in Listing
10-26, that compiled without lifetime annotations.

Filename: src/lib.rs

```
fn first_word(s: &str) -> &str {
    let bytes = s.as_bytes();

    for (i, &item) in bytes.iter().enumerate() {
        if item == b' ' {
            return &s[0..i];
        }
    }

    &s[..]
}
```

Listing 10-26: A function we defined in Listing 4-9 that compiled without
lifetime annotations, even though the parameter and return type are references

The reason this function compiles without lifetime annotations is historical:
in early versions (pre-1.0) of Rust, this code wouldn’t have compiled because
every reference needed an explicit lifetime. At that time, the function
signature would have been written like this:

```
fn first_word<'a>(s: &'a str) -> &'a str {
```

After writing a lot of Rust code, the Rust team found that Rust programmers
were entering the same lifetime annotations over and over in particular
situations. These situations were predictable and followed a few deterministic
patterns. The developers programmed these patterns into the compiler’s code so
the borrow checker could infer the lifetimes in these situations and wouldn’t
need explicit annotations.

This piece of Rust history is relevant because it’s possible that more
deterministic patterns will emerge and be added to the compiler. In the future,
even fewer lifetime annotations might be required.

The patterns programmed into Rust’s analysis of references are called the
*lifetime elision rules*. These aren’t rules for programmers to follow; they’re
a set of particular cases that the compiler will consider, and if your code
fits these cases, you don’t need to write the lifetimes explicitly.

The elision rules don’t provide full inference. If Rust deterministically
applies the rules but there is still ambiguity as to what lifetimes the
references have, the compiler won’t guess what the lifetime of the remaining
references should be. In this case, instead of guessing, the compiler will give
you an error that you can resolve by adding the lifetime annotations that
specify how the references relate to each other.

Lifetimes on function or method parameters are called *input lifetimes*, and
lifetimes on return values are called *output lifetimes*.

The compiler uses three rules to figure out what lifetimes references have when
there aren’t explicit annotations. The first rule applies to input lifetimes,
and the second and third rules apply to output lifetimes. If the compiler gets
to the end of the three rules and there are still references for which it can’t
figure out lifetimes, the compiler will stop with an error. These rules apply
to `fn` definitions as well as `impl` blocks.

The first rule is that each parameter that is a reference gets its own lifetime
parameter. In other words, a function with one parameter gets one lifetime
parameter: `fn foo<'a>(x: &'a i32)`; a function with two parameters gets two
separate lifetime parameters: `fn foo<'a, 'b>(x: &'a i32, y: &'b i32)`; and so
on.

The second rule is if there is exactly one input lifetime parameter, that
lifetime is assigned to all output lifetime parameters: `fn foo<'a>(x: &'a i32)
-> &'a i32`.

The third rule is if there are multiple input lifetime parameters, but one of
them is `&self` or `&mut self` because this is a method, the lifetime of `self`
is assigned to all output lifetime parameters. This third rule makes methods
much nicer to read and write because fewer symbols are necessary.

Let’s pretend we’re the compiler. We’ll apply these rules to figure out what
the lifetimes of the references in the signature of the `first_word` function
in Listing 10-26 are. The signature starts without any lifetimes associated
with the references:

```
fn first_word(s: &str) -> &str {
```

Then the compiler applies the first rule, which specifies that each parameter
gets its own lifetime. We’ll call it `'a` as usual, so now the signature is
this:

```
fn first_word<'a>(s: &'a str) -> &str {
```

The second rule applies because there is exactly one input lifetime. The second
rule specifies that the lifetime of the one input parameter gets assigned to
the output lifetime, so the signature is now this:

```
fn first_word<'a>(s: &'a str) -> &'a str {
```

Now all the references in this function signature have lifetimes, and the
compiler can continue its analysis without needing the programmer to annotate
the lifetimes in this function signature.

Let’s look at another example, this time using the `longest` function that had
no lifetime parameters when we started working with it in Listing 10-21:

```
fn longest(x: &str, y: &str) -> &str {
```

Let’s apply the first rule: each parameter gets its own lifetime. This time we
have two parameters instead of one, so we have two lifetimes:

```
fn longest<'a, 'b>(x: &'a str, y: &'b str) -> &str {
```

You can see that the second rule doesn’t apply because there is more than one
input lifetime. The third rule doesn’t apply either, because `longest` is a
function rather than a method, so none of the parameters are `self`. After
working through all three rules, we still haven’t figured out what the return
type’s lifetime is. This is why we got an error trying to compile the code in
Listing 10-21: the compiler worked through the lifetime elision rules but still
couldn’t figure out all the lifetimes of the references in the signature.

Because the third rule really only applies in method signatures, we’ll look at
lifetimes in that context next to see why the third rule means we don’t have to
annotate lifetimes in method signatures very often.

### Lifetime Annotations in Method Definitions

When we implement methods on a struct with lifetimes, we use the same syntax as
that of generic type parameters shown in Listing 10-11. Where we declare and
use the lifetime parameters depends on whether they’re related to the struct
fields or the method parameters and return values.

Lifetime names for struct fields always need to be declared after the `impl`
keyword and then used after the struct’s name, because those lifetimes are part
of the struct’s type.

In method signatures inside the `impl` block, references might be tied to the
lifetime of references in the struct’s fields, or they might be independent. In
addition, the lifetime elision rules often make it so that lifetime annotations
aren’t necessary in method signatures. Let’s look at some examples using the
struct named `ImportantExcerpt` that we defined in Listing 10-25.

First, we’ll use a method named `level` whose only parameter is a reference to
`self` and whose return value is an `i32`, which is not a reference to anything:

```
impl<'a> ImportantExcerpt<'a> {
    fn level(&self) -> i32 {
        3
    }
}
```

The lifetime parameter declaration after `impl` and its use after the type name
are required, but we’re not required to annotate the lifetime of the reference
to `self` because of the first elision rule.

Here is an example where the third lifetime elision rule applies:

```
impl<'a> ImportantExcerpt<'a> {
    fn announce_and_return_part(&self, announcement: &str) -> &str {
        println!("Attention please: {}", announcement);
        self.part
    }
}
```

There are two input lifetimes, so Rust applies the first lifetime elision rule
and gives both `&self` and `announcement` their own lifetimes. Then, because
one of the parameters is `&self`, the return type gets the lifetime of `&self`,
and all lifetimes have been accounted for.

### The Static Lifetime

One special lifetime we need to discuss is `'static`, which means that this
reference *can* live for the entire duration of the program. All string
literals have the `'static` lifetime, which we can annotate as follows:

```
let s: &'static str = "I have a static lifetime.";
```

The text of this string is stored directly in the program’s binary, which
is always available. Therefore, the lifetime of all string literals is
`'static`.

You might see suggestions to use the `'static` lifetime in error messages. But
before specifying `'static` as the lifetime for a reference, think about
whether the reference you have actually lives the entire lifetime of your
program or not. You might consider whether you want it to live that long, even
if it could. Most of the time, the problem results from attempting to create a
dangling reference or a mismatch of the available lifetimes. In such cases, the
solution is fixing those problems, not specifying the `'static` lifetime.

## Generic Type Parameters, Trait Bounds, and Lifetimes Together

Let’s briefly look at the syntax of specifying generic type parameters, trait
bounds, and lifetimes all in one function!

```
use std::fmt::Display;

fn longest_with_an_announcement<'a, T>(
    x: &'a str,
    y: &'a str,
    ann: T,
) -> &'a str
where
    T: Display,
{
    println!("Announcement! {}", ann);
    if x.len() > y.len() {
        x
    } else {
        y
    }
}
```

This is the `longest` function from Listing 10-22 that returns the longer of
two string slices. But now it has an extra parameter named `ann` of the generic
type `T`, which can be filled in by any type that implements the `Display`
trait as specified by the `where` clause. This extra parameter will be printed
using `{}`, which is why the `Display` trait bound is necessary. Because
lifetimes are a type of generic, the declarations of the lifetime parameter
`'a` and the generic type parameter `T` go in the same list inside the angle
brackets after the function name.

## Summary

We covered a lot in this chapter! Now that you know about generic type
parameters, traits and trait bounds, and generic lifetime parameters, you’re
ready to write code without repetition that works in many different situations.
Generic type parameters let you apply the code to different types. Traits and
trait bounds ensure that even though the types are generic, they’ll have the
behavior the code needs. You learned how to use lifetime annotations to ensure
that this flexible code won’t have any dangling references. And all of this
analysis happens at compile time, which doesn’t affect runtime performance!

Believe it or not, there is much more to learn on the topics we discussed in
this chapter: Chapter 17 discusses trait objects, which are another way to use
traits. There are also more complex scenarios involving lifetime annotations
that you will only need in very advanced scenarios; for those, you should read
the Rust Reference at *https://doc.rust-lang.org/reference/trait-bounds.html*.
But next, you’ll learn how to write tests in Rust so you can make sure your
code is working the way it should.