Make it clear that default traits do not require a separate empty impl block in Section 10.2

[zkkv]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:

  • impl
  • 10.2
  • default

• I have checked the latest main branch to see if this has already been fixed, in this file:

  • src/ch10-02-traits.md

URL to the section(s) of the book with this problem: https://rust-book.cs.brown.edu/ch10-02-traits.html#default-implementations

Description of the problem:
I think the sentence below could be worded better.

To use a default implementation to summarize instances of NewsArticle, we specify an empty impl block with impl Summary for NewsArticle {}.

It can be helpful to emphasize that a trait containing both default and non-implemented methods doesn't require two impl blocks, one of which is empty.

Suggested fix:
The phrasing that could be used:

To use a default implementation to summarize instances of NewsArticle, we must specify an empty impl block with impl Summary for NewsArticle {}, unless there's an existing impl block already.

[chriskrycho]

This is actually covered implicitly just a little ways below, starting at the paragraph which begins “Creating a default implementation doesn’t require us to change anything…” In that section, you will see an impl block for a non-default method which is for the trait. I can imagine ways we could make that more explicit, but I think the text does in fact already cover it. The key is one of sequencing for introducing those ideas, and any sequence we pick will confuse someone. 😉 In any case, I am glad you worked it out!