Add cloning example for dot operator behaviour (#292)

Co-authored-by: Yuki Okushi <yuki.okushi@huawei.com>

Author: zeramorphic

src/dot-operator.md

@@ -1,6 +1,131 @@
 # 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 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.
 
-TODO: steal information from http://stackoverflow.com/questions/28519997/what-are-rusts-exact-auto-dereferencing-rules/28552082#28552082
+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/method-lookup.html
+[index]: ../std/ops/trait.Index.html
+[clone]: ../std/clone/trait.Clone.html#derivable