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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

1. Consider two traits

   1. From<String>

   2. From<&'a str>

2. Cow<'a, str> implements both traits

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

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

3. Consider the additional trait

   1. Into<'a, Cow<'a str>>

4. String automatically implements Into, as does str, given that the From traits are implemented

5. Both String and str implement Into with a method of the following form

   1. fn into(self) → Cow<a' str>

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

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

8. When applying the method into(), the string literal is dereferenced into a string slice str

9. Then the method into() can be applied on the str. We have already seen that this method exists.