Chapter 15-05: Incorrect interpretation of compiler error

[kingh0730]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • Messenger
  • 15
  • &mut self
• I have checked the latest main branch to see if this has already been fixed, in this file:

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

    Line 195 in 2bd5d42

    signature of `send` wouldn’t match the signature in the `Messenger` trait

  • book/listings/ch15-smart-pointers/listing-15-21/output.txt

    Line 6 in 2bd5d42

    2 | fn send(&self, msg: &str);

URL to the section(s) of the book with this problem: https://doc.rust-lang.org/book/ch15-05-interior-mutability.html#a-use-case-for-interior-mutability-mock-objects

Description of the problem:

The following sentence is incorrect:

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

because the suggestion of the compiler is to modify Line 2, the signature in the Messenger trait definition itself, NOT Line 57 which is the implementation of send in MockMessenger.

Suggested fix:

We also can't take the suggestion from the error text to use &mut self instead, because we are testing an API and it's not a good idea to modify the API for the sole purpose of testing. Usually, the test engineers do not have permission to modify the API they're testing.

[chriskrycho]

As noted on the PR, the original text is fine here, actually. In particular, the reasons given by the book are accurate, and in this case the text assumes the person implementing the code is also testing it—nothing to do with test engineers!