src/conversion/from_into.md

# `From` and `Into`

The [`From`] and [`Into`] traits are inherently linked, and this is actually part of
its implementation. If you are able to convert type A from type B, then it
should be easy to believe that we should be able to convert type B to type A.

## `From`

The [`From`] trait allows for a type to define how to create itself from another
type, hence providing a very simple mechanism for converting between several
types. There are numerous implementations of this trait within the standard
library for conversion of primitive and common types.

For example we can easily convert a `str` into a `String`

```rust
let my_str = "hello";
let my_string = String::from(my_str);
```

We can do something similar for defining a conversion for our own type.

```rust,editable
use std::convert::From;

#[derive(Debug)]
struct Number {
    value: i32,
}

impl From<i32> for Number {
    fn from(item: i32) -> Self {
        Number { value: item }
    }
}

fn main() {
    let num = Number::from(30);
    println!("My number is {:?}", num);
}
```

## `Into`

The [`Into`] trait is simply the reciprocal of the `From` trait. It
defines how to convert a type into another type.

Calling `into()` typically requires us to specify the result type as the compiler is unable to determine this most of the time.

```rust,editable
use std::convert::Into;

#[derive(Debug)]
struct Number {
    value: i32,
}

impl Into<Number> for i32 {
    fn into(self) -> Number {
        Number { value: self }
    }
}

fn main() {
    let int = 5;
    // Try removing the type annotation
    let num: Number = int.into();
    println!("My number is {:?}", num);
}
```

## `From` and `Into` are interchangeable

`From` and `Into` are designed to be complementary.
We do not need to provide an implementation for both traits.
If you have implemented the `From` trait for your type, `Into` will call it
when necessary. Note, however, that the converse is not true: implementing `Into` for your type will not automatically provide it with an implementation of `From`.

```rust,editable
use std::convert::From;

#[derive(Debug)]
struct Number {
    value: i32,
}

// Define `From`
impl From<i32> for Number {
    fn from(item: i32) -> Self {
        Number { value: item }
    }
}

fn main() {
    let int = 5;
    // use `Into`
    let num: Number = int.into();
    println!("My number is {:?}", num);
}
```

[`From`]: https://doc.rust-lang.org/std/convert/trait.From.html
[`Into`]: https://doc.rust-lang.org/std/convert/trait.Into.html