src/dot-operator.md

# The Dot Operator

The dot operator will perform a lot of magic to convert types.
It will perform auto-referencing, auto-dereferencing, and coercion until types
match.
The detailed mechanics of method lookup are defined [here][method_lookup],
but here is a brief overview that outlines the main steps.

Suppose we have a function `foo` that has a receiver (a `self`, `&self` or
`&mut self` parameter).
If we call `value.foo()`, the compiler needs to determine what type `Self` is before
it can call the correct implementation of the function.
For this example, we will say that `value` has type `T`.

We will use [fully-qualified syntax][fqs] to be more clear about exactly which
type we are calling a function on.

- First, the compiler checks if it can call `T::foo(value)` directly.
This is called a "by value" method call.
- If it can't call this function (for example, if the function has the wrong type
or a trait isn't implemented for `Self`), then the compiler tries to add in an
automatic reference.
This means that the compiler tries `<&T>::foo(value)` and `<&mut T>::foo(value)`.
This is called an "autoref" method call.
- If none of these candidates worked, it dereferences `T` and tries again.
This uses the `Deref` trait - if `T: Deref<Target = U>` then it tries again with
type `U` instead of `T`.
If it can't dereference `T`, it can also try _unsizing_ `T`.
This just means that if `T` has a size parameter known at compile time, it "forgets"
it for the purpose of resolving methods.
For instance, this unsizing step can convert `[i32; 2]` into `[i32]` by "forgetting"
the size of the array.

Here is an example of the method lookup algorithm:

```rust,ignore
let array: Rc<Box<[T; 3]>> = ...;
let first_entry = array[0];
```

How does the compiler actually compute `array[0]` when the array is behind so
many indirections?
First, `array[0]` is really just syntax sugar for the [`Index`][index] trait -
the compiler will convert `array[0]` into `array.index(0)`.
Now, the compiler checks to see if `array` implements `Index`, so that it can call
the function.

Then, the compiler checks if `Rc<Box<[T; 3]>>` implements `Index`, but it
does not, and neither do `&Rc<Box<[T; 3]>>` or `&mut Rc<Box<[T; 3]>>`.
Since none of these worked, the compiler dereferences the `Rc<Box<[T; 3]>>` into
`Box<[T; 3]>` and tries again.
`Box<[T; 3]>`, `&Box<[T; 3]>`, and `&mut Box<[T; 3]>` do not implement `Index`,
so it dereferences again.
`[T; 3]` and its autorefs also do not implement `Index`.
It can't dereference `[T; 3]`, so the compiler unsizes it, giving `[T]`.
Finally, `[T]` implements `Index`, so it can now call the actual `index` function.

Consider the following more complicated example of the dot operator at work:

```rust
fn do_stuff<T: Clone>(value: &T) {
    let cloned = value.clone();
}
```

What type is `cloned`?
First, the compiler checks if it can call by value.
The type of `value` is `&T`, and so the `clone` function has signature
`fn clone(&T) -> T`.
It knows that `T: Clone`, so the compiler finds that `cloned: T`.

What would happen if the `T: Clone` restriction was removed? It would not be able
to call by value, since there is no implementation of `Clone` for `T`.
So the compiler tries to call by autoref.
In this case, the function has the signature `fn clone(&&T) -> &T` since
`Self = &T`.
The compiler sees that `&T: Clone`, and then deduces that `cloned: &T`.

Here is another example where the autoref behavior is used to create some subtle
effects:

```rust
# use std::sync::Arc;
#
#[derive(Clone)]
struct Container<T>(Arc<T>);

fn clone_containers<T>(foo: &Container<i32>, bar: &Container<T>) {
    let foo_cloned = foo.clone();
    let bar_cloned = bar.clone();
}
```

What types are `foo_cloned` and `bar_cloned`?
We know that `Container<i32>: Clone`, so the compiler calls `clone` by value to give
`foo_cloned: Container<i32>`.
However, `bar_cloned` actually has type `&Container<T>`.
Surely this doesn't make sense - we added `#[derive(Clone)]` to `Container`, so it
must implement `Clone`!
Looking closer, the code generated by the `derive` macro is (roughly):

```rust,ignore
impl<T> Clone for Container<T> where T: Clone {
    fn clone(&self) -> Self {
        Self(Arc::clone(&self.0))
    }
}
```

The derived `Clone` implementation is [only defined where `T: Clone`][clone],
so there is no implementation for `Container<T>: Clone` for a generic `T`.
The compiler then looks to see if `&Container<T>` implements `Clone`, which it does.
So it deduces that `clone` is called by autoref, and so `bar_cloned` has type
`&Container<T>`.

We can fix this by implementing `Clone` manually without requiring `T: Clone`:

```rust,ignore
impl<T> Clone for Container<T> {
    fn clone(&self) -> Self {
        Self(Arc::clone(&self.0))
    }
}
```

Now, the type checker deduces that `bar_cloned: Container<T>`.

[fqs]: ../book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
[method_lookup]: https://rustc-dev-guide.rust-lang.org/hir-typeck/method-lookup.html
[index]: ../std/ops/trait.Index.html
[clone]: ../std/clone/trait.Clone.html#derivable