| When writing functions that return references, lifetimes must be explicitly |
| annotated. These functions are generic and we must tell the compiler what is |
| the relationship between the lifetimes of the objects that appear in the |
| arguments and the output. |
| |
| Let's illustrate with an example: we want a function that returns a reference |
| to the title field of a Book struct. The most generic function that we could |
| write would look like this: |
| |
| {explicit.play} |
| |
| The compiler can't tell how `'a` and `'b` are related, so we must supply this |
| information. The answer here is that `'a = 'b`, the reason is that the title |
| field will be destroyed when the book gets destroyed (same way with the |
| creation time), therefore the title field has the same lifetime as the book. |
| |
