Reference to compiler warning is slightly outdated in chapter 15.5

[matthias-t]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • milestone:ch15
• I have checked the latest main branch to see if this has already been fixed, in this file:
  • src/ch15-05-interior-mutability.md

URL to the section(s) of the book with this problem:

book/src/ch15-05-interior-mutability.md

Lines 190 to 194 in 139538d

	We can’t modify the `MockMessenger` to keep track of the messages, because the
	`send` method takes an immutable reference to `self`. We also can’t take the
	suggestion from the error text to use `&mut self` instead, because then the
	signature of `send` wouldn’t match the signature in the `Messenger` trait
	definition (feel free to try and see what error message you get).

Description of the problem: Rust 1.82.0 (and possibly earlier) suggests changing the trait definition as well as the impl method, as below. This still would not compile, but the reason now becomes that messenger is contained as an immutable reference in LimitTracker, and so cannot be passed to send.

error[E0596]: cannot borrow `self.sent_messages` as mutable, as it is behind a `&` reference
  --> src/sec_5.rs:62:13
   |
62 |             self.sent_messages.push(message.to_string());
   |             ^^^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
   |
help: consider changing this to be a mutable reference in the `impl` method and the `trait` definition
   |
6  ~     fn send(&mut self, message: &str);
7  | }
...
60 |     impl Messenger for MockMessenger {
61 ~         fn send(&mut self, message: &str) {
   |

For more information about this error, try `rustc --explain E0596`.

Suggested fix: Perhaps change the second sentence to: "We also can’t take the suggestion from the error text to use &mut self instead, because our LimitTracker only holds an immutable reference to a Messenger instance (feel free to try and see what error message you get)." Or, to adress the concern that this might be changed in turn: "We could follow the compiler's suggestions to work with mutable references instead, but here we'll assume that we want the send method in our library to accept immutable references."

[chriskrycho]

It looks like this change landed in 1.79. I think the sentence could indeed use a tiny bit of tweaking, but I think in keeping with the actual flow of the text here, the point should be that we shouldn't be changing the trait definition for the sake of test code. I’ll open a small PR to that effect!

[matthias-t]

Thanks! 🙂 Looks good to me.