[saranshhalwai]

Summary

Chapter 13.3 suggests returning an iterator from search() instead of collecting the results into a vector. Sounds simple enough, except following that suggestion breaks the existing run() implementation.

I followed the instructions and promptly got screwed over by E0308.

The problem is that run() uses an if/else expression to choose between search() and search_case_insensitive(). Changing only one function leaves the branches returning different types. Changing both to return impl Iterator doesn't help either, since Rust treats each opaque return type as distinct.

There's also the lifetime issue introduced by capturing query in the returned iterator.

Changes

• Explain why the existing if/else expression stops compiling.
• Clarify why two functions returning impl Iterator<Item = &'a str> don't necessarily return compatible types.
• Suggest moving the iteration into the individual branches.
• Mention the lifetime adjustment needed for query.

This is a documentation-only change. No existing examples or functionality have been modified.

Motivation

The book explicitly encourages readers to try this optimization, but doesn't mention that doing so requires changes elsewhere in the project.

I ran into this while working through the book, and it took me a while to figure out why two seemingly identical iterator return types weren't compatible.

Figured the next person shouldn't have to discover this through a compiler error.

Fixes #4518