mdbook test fails with "error[E0463]: can't find crate for trpl"

[bordenc]

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

URL to the section(s) of the book with this problem: https://github.com/rust-lang/book/blob/main/README.md

Description of the problem: I followed the instructions on a local git-cloned copy of the book. After following instructions to install mdbook & mdbook-trpl, I ran mdbook test and encountered a number of errors to the effect of:

error[E0463]: can't find crate for `trpl`
 --> <chapter>.md:<line>:<position>
  |
2 | extern crate trpl; // for mdbook test
  | ^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to 1 previous error

Not sure why I'm running into this error. Either I'm actually missing a crate (so installation instructions should be added to the README), or there's an error in the software.

[nathany]

On macOS ~~there was a simple fix.~~

❯ cd packages/trpl 
                                                                                          
❯ mdbook test --library-path packages/trpl/target/debug/deps
2025-06-30 10:09:36 [ERROR] (mdbook::utils): Error: Couldn't open SUMMARY.md in "/Users/nathany/Development/rust/books/the-rust-programming-language-manuscript/packages/trpl/src" directory
2025-06-30 10:09:36 [ERROR] (mdbook::utils): 	Caused By: No such file or directory (os error 2)

But if I run it from the book folder on macOS (no change directory #4426), it runs fine ~~and all the tests pass.~~ Update: the tests fail on macOS too, I was just multi-tasking when the failures scrolled by.

---

On Windows 11 it's ~~a different~~ the same story. Testing in Git Bash from the packages/trpl folder is basically the same error

nathany@Owl MINGW64 ~/src/github.com/rust-lang/book/packages/trpl (readme-mdbook)
$ mdbook test --library-path packages/trpl/target/debug/deps
2025-06-30 14:16:47 [ERROR] (mdbook::utils): Error: Couldn't open SUMMARY.md in "C:\\Users\\nathany\\src\\github.com\\rust-lang\\book\\packages\\trpl\\src" directory
2025-06-30 14:16:47 [ERROR] (mdbook::utils):    Caused By: The system cannot find the file specified. (os error 2)

Running from the book/ folder gets through chapters 1-16, but then fails on chapter 17 with the errors @bordenc mentioned.

2025-06-30 14:18:37 [INFO] (mdbook::book): Testing chapter 'Futures and the Async Syntax': "ch17-01-futures-and-syntax.md"
2025-06-30 14:18:38 [ERROR] (mdbook::book): rustdoc returned an error:

--- stdout

running 8 tests
test ch17-01-futures-and-syntax.md - _::Determining_a_Single_Page_s_Title (line 243) ... ignored
test ch17-01-futures-and-syntax.md - _::Determining_a_Single_Page_s_Title (line 376) ... FAILED
test ch17-01-futures-and-syntax.md - _::Determining_a_Single_Page_s_Title (line 319) ... FAILED
test ch17-01-futures-and-syntax.md - _::Our_First_Async_Program::Defining_the_page_title_Function (line 162) ... FAILED
test ch17-01-futures-and-syntax.md - _::Our_First_Async_Program::Defining_the_page_title_Function (line 197) ... FAILED
test ch17-01-futures-and-syntax.md - _::Our_First_Async_Program::Defining_the_page_title_Function (line 84) ... FAILED
test ch17-01-futures-and-syntax.md - _::Determining_a_Single_Page_s_Title::Racing_Our_Two_URLs_Against_Each_Other (line 428) ... FAILED
test ch17-01-futures-and-syntax.md - _::Determining_a_Single_Page_s_Title::Racing_Our_Two_URLs_Against_Each_Other (line 490) ... ok

failures:

---- ch17-01-futures-and-syntax.md - _::Determining_a_Single_Page_s_Title (line 376) stdout ----
error[E0463]: can't find crate for `trpl`
 --> ch17-01-futures-and-syntax.md:376:1
  |
2 | extern crate trpl;
  | ^^^^^^^^^^^^^^^^^^ can't find crate

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0463`.
Couldn't compile the test.

~~The CI actions only run on ubuntu-latest, so I'm not sure if there's something different on Windows.~~

The same errors occur in Powershell.

[bordenc]

For further clarity, I'm getting the error on Debian Trixie, so this bug has officially scored the OS hat trick.

[nathany]

I think it's not OS-specific -- I was wrong about it passing on macOS in my original post. (Updated)

It looks like trpl is a support crate for the book, so hopefully there isn't anything wrong with Chapter 17 itself.

[nathany]

GitHub Actions is passing, so that provides a clue. From https://github.com/rust-lang/book/blob/main/.github/workflows/main.yml

# mdBook does not currently have particularly good support for “external”
    # crates. To make the test suite work correctly with `trpl`, we must first
    # build `trpl` itself (`mdbook` will not do it), and then explicitly pass
    # its `deps` path as a library search path for `mdbook test`. That will make
    # sure all the crates can be resolved when running the tests.
    - name: Build `trpl` crate
      run: |
        cd packages/trpl
        cargo build
    - name: Run tests
      run:
        mdbook test --library-path packages/trpl/target/debug/deps

[nathany]

I built the trpl crate successfully, but when I back out of the folder and run the tests, it is still failing locally with the same errors. 🤷‍♂

At least it works in CI.