# 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.

TODO: steal information from http://stackoverflow.com/questions/28519997/what-are-rusts-exact-auto-dereferencing-rules/28552082#28552082

Consider the following example of the dot operator at work.
```rust.ignore
fn do_stuff<T: Clone>(value: &T) {
    let cloned = value.clone();
}
```
What type is `cloned`? First, the compiler checks if we can call by value.
The type of `value` is `&T`, and so the `clone` function has signature
`fn clone(&T) -> T`. We know that `T: Clone`, so the compiler finds that
`cloned: T`.

What would happen if the `T: Clone` restriction was removed? We 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 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 behaviour is used to create some subtle
effects.
```rust.ignore
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`](https://doc.rust-lang.org/std/clone/trait.Clone.html#derivable),
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>`.