Rustfmt cannot resolve submodules of integration tests

[walkie]

Overview

Rustfmt cannot find a submodule of an integration test located in a separate file.

Running cargo fmt on a project that contains a submodule test1/sub1 implemented by the file tests/test1/sub1.rs produces the following error message:

Error writing files: failed to resolve mod `sub1`: $PACKAGE/tests/test1/test1 does not exist

Submodules in integration tests can be created as follows:

In tests/test1.rs:

mod test1 {
    mod sub1;
    mod sub2;
}

The submodules sub1 and sub2 can be implemented in the files tests/test1/sub1.rs and tests/test1/sub2.rs.

This structure works with cargo build, cargo test, and cargo clippy, but fails for cargo fmt with the error message given above.

Steps to reproduce

Clone this repo containing a minimal example illustrating the problem: https://github.com/walkie/rustfmt-test-submodule-issue

From within the repo:

• Run cargo test and observe that the package successfully compiles and the tests in both submodules run and pass.
• Run cargo fmt and observe the following error message:

Error writing files: failed to resolve mod `sub1`: [...]/rustfmt-test-submodule-issue/tests/test1/test1 does not exist

The same error can also be produced by rustfmt directly with the command: rustfmt tests/test1.rs

Expected behavior

• No Rustfmt I/O errors. :-)

Other information

• Installed Rustfmt by rustup component add rustfmt
• Tested with versions 1.51, 1.57, and 1.59.0-nightly (48a5999fc 2021-12-01)
• Tested on Fedora Workstation 34 and MacOS 11.6.1

The error occurs in all combinations of the above.

[walkie]

As a workaround, you can tell Rustfmt to skip the submodules when formatting the entire package:

mod test1 {
    #[rustfmt::skip]
    mod sub1;
    #[rustfmt::skip]
    mod sub2;
}

This will enable cargo fmt to complete successfully. You can then apply Rustfmt to each submodule directly:

• rustfmt tests/test1/sub1.rs
• rustfmt tests/test1/sub2.rs

[ytmimi]

@walkie I wanted to thank you for putting together a great minimal example. It definitely made it a whole lot easier for me as I was working to address this issue. 🤞 the PR review goes smoothly.