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1. Consider two traits

   1. From<String>

   2. From<&'static str>

2. The enum Cow<'static, str> Cow<str> implements both traits

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

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

3. Consider another trait

   1. Into<Cow<'static, str>>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<'static,str>> Into<Cow<str>> trait

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

   Consider the additional trait

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

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

   fn into(self) → Cow<a', str>

   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> { ... } }

8. 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>.

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

10. A string literal has a static lifetime.

11. When applying the method Therefore the result of calling the into(), the method on a string literal is dereferenced into a string slice strThen the method into() can be applied on the str. We have already seen that this method exists.a Cow<'static, str>

12. Also we can call the into() method on a String to get a Cow<'static, str> without any lifetime limitations on the String