# `From` and `Into`

Let's go back to where our string journey started:

```rust
let ticket = Ticket::new(
    "A title".into(), 
    "A description".into(), 
    "To-Do".into()
);
```

We now know enough to start unpacking what `.into()` is doing here.

## The problem

This is the signature of the `new` method:

```rust
impl Ticket {
    pub fn new(
        title: String, 
        description: String, 
        status: String
    ) -> Self {
        // [...]
    }
}
```

We've also seen that string literals (such as `"A title"`) are of type `&str`.\
We have a type mismatch here: a `String` is expected, but we have a `&str`.
No magical coercion will come to save us this time; we need **to perform a conversion**.

## `From` and `Into`

The Rust standard library defines two traits for **infallible conversions**: `From` and `Into`,
in the `std::convert` module.

```rust
pub trait From<T>: Sized {
    fn from(value: T) -> Self;
}

pub trait Into<T>: Sized {
    fn into(self) -> T;
}
```

These trait definitions showcase a few concepts that we haven't seen before: **supertraits** and **implicit trait bounds**.
Let's unpack those first.

### Supertrait / Subtrait

The `From: Sized` syntax implies that `From` is a **subtrait** of `Sized`: any type that
implements `From` must also implement `Sized`.
Alternatively, you could say that `Sized` is a **supertrait** of `From`.

### Implicit trait bounds

Every time you have a generic type parameter, the compiler implicitly assumes that it's `Sized`.

For example:

```rust
pub struct Foo<T> {
    inner: T,
}
```

is actually equivalent to:

```rust
pub struct Foo<T: Sized> 
{
    inner: T,
}
```

In the case of `From<T>`, the trait definition is equivalent to:

```rust
pub trait From<T: Sized>: Sized {
    fn from(value: T) -> Self;
}
```

In other words, _both_ `T` and the type implementing `From<T>` must be `Sized`, even
though the former bound is implicit.

### Negative trait bounds

You can opt out of the implicit `Sized` bound with a **negative trait bound**:

```rust
pub struct Foo<T: ?Sized> {
    //            ^^^^^^^
    //            This is a negative trait bound
    inner: T,
}
```

This syntax reads as "`T` may or may not be `Sized`", and it allows you to
bind `T` to a DST (e.g. `Foo<str>`). It is a special case, though: negative trait bounds are exclusive to `Sized`,
you can't use them with other traits.

## `&str` to `String`

In [`std`'s documentation](https://doc.rust-lang.org/std/convert/trait.From.html#implementors)
you can see which `std` types implement the `From` trait.\
You'll find that `String` implements `From<&str> for String`. Thus, we can write:

```rust
let title = String::from("A title");
```

We've been primarily using `.into()`, though.\
If you check out the [implementors of `Into`](https://doc.rust-lang.org/std/convert/trait.Into.html#implementors)
you won't find `Into<String> for &str`. What's going on?

`From` and `Into` are **dual traits**.\
In particular, `Into` is implemented for any type that implements `From` using a **blanket implementation**:

```rust
impl<T, U> Into<U> for T
where
    U: From<T>,
{
    fn into(self) -> U {
        U::from(self)
    }
}
```

If a type `U` implements `From<T>`, then `Into<U> for T` is automatically implemented. That's why
we can write `let title = "A title".into();`.

## `.into()`

Every time you see `.into()`, you're witnessing a conversion between types.\
What's the target type, though?

In most cases, the target type is either:

- Specified by the signature of a function/method (e.g. `Ticket::new` in our example above)
- Specified in the variable declaration with a type annotation (e.g. `let title: String = "A title".into();`)

`.into()` will work out of the box as long as the compiler can infer the target type from the context without ambiguity.
