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A helpful resource for better understanding the Cow enum is The Secret Life of Cows. Be sure to click on the “playground” links in the discussion there on the serde crate.

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Step-by-step, how to understand the describe() function example

(assuming I understand this)

Part A

We can implement methods directly on shared reference types themselves. There is no deref coercion then when called on a shared reference that is of that shared reference type (playground)

In a sense then the method implemented, when called on a shared reference, takes ownership of the shared reference, per se, not the value the shared reference points to.

use std::borrow::Cow;

trait CustomInto<T> {
    fn custom_into(self) -> T;
}

impl<'a> CustomInto<Cow<'a, str>> for &'a str {
    fn custom_into(self) -> Cow<'a, str> {
        Cow::Borrowed(self)
    }
}

fn main(){
    let _cow = "hello there".custom_into();    
}

Note how the implementation is for the ref string slice type, not the primitive string slice type.

Part B

1. Consider two traits

   1. From<String>

   2. From<&str>

2. The enum Cow<str> implements both traits

   1. Cow has the associated function
      fn from(s: String) → Cow<str>

   2. Cow has another associated function
      fn from(s: &str) → Cow<str>

3. Consider another trait

   1. Into<Cow<str>>

4. A From implementation implies that there is a corresponding Into implementation. There is a blanket implementation of Into in the standard library. My guess is that the following code is the blanket implementation

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

5. Therefore String implements the Into<Cow<str>> trait

   Code Block
   language rust
   impl<'a> Into<Cow<'a, str>> for String { fn into(self) -> Cow<'a, str> { ... } }

6. Also &str implements the Into<Cow<str>> trait

   Code Block
   impl<'a> Into<Cow<'a, str>> for &'a str { fn into(self) -> Cow<'a, str> { ... } }

7. In the first four arms of the match expression, there are string literals upon which the method into() is called. We know these method calls must return a Cow<'static, str>

8. A string literal is of type &str, a shared reference to a string slice

9. A string literal has a static lifetime.

10. Therefore the result of calling the into() method on a string literal is a Cow<'static, str>

11. So, we see now how the first four arms of the match expression work.

12. In the last arm of the match expression, there is a String upon which the method into() is called. This method call must return a Cow<'static, str>

13. We can call the into() method on a String to get a Cow<str> without any lifetime limitations between the String and the Cow<str>

14. So the returned Cow<str> can be annotated with a static lifetime, without requiring that the String had to have one, too.

15. So, we see now how the fifth arm of the match expression works.