Fixing `fail_configured_limit.rs` UI Test Failure

by Alex Johnson 50 views

We've encountered a snag in our UI tests, specifically with fail_configured_limit.rs within the conditional_max_n_branches crate. This article dives into the specifics of the failure, its causes, and how to resolve it, ensuring our Windows CI pipeline runs smoothly.

Description

The UI test for fail_configured_limit.rs, found in the conditional_max_n_branches crate, is currently failing on our Windows Continuous Integration (CI) environment. This failure is due to a mismatch between the actual standard error (stderr) output generated by the test and the expected stderr output. Essentially, what the test is producing doesn't match what we anticipated, causing the test to fail. This article explores the reason for this discrepancy and how to resolve it.

Understanding the Issue

The core issue lies in the difference between the expected and actual error messages generated by the fail_configured_limit.rs test. The conditional_max_n_branches crate is designed to check for overly complex conditional branching in your Rust code. When the test encounters a situation where the branching exceeds a configured limit, it should emit a specific warning message. However, the format or content of this warning message has changed slightly, leading to the mismatch.

This type of failure is common when the Rust compiler or linter itself is updated, as these updates can sometimes alter the exact wording or structure of diagnostic messages. While the underlying functionality of the lint (detecting excessive branching) remains correct, the test expects a specific output string, and any deviation causes the test to fail. The key is to update the expected output to align with the new, current output.

Why This Matters

While the functionality itself is working correctly, a failing UI test can block the CI pipeline. This means that new code changes cannot be automatically integrated, preventing the smooth flow of development and deployment. Therefore, resolving this mismatch is essential for maintaining an efficient and reliable development workflow. This ensures that all changes are properly tested and validated before being merged into the main codebase.

Failure Details

Let's break down the specifics of the failure:

  • Test: conditional_max_n_branches::ui::ui
  • Fixture: fail_configured_limit.rs
  • Platform: Windows (nightly-2025-09-18-x86_64-pc-windows-msvc)

Error Message Breakdown

The error message we're seeing is quite informative:

The actual stderr differed from the expected stderr.
Actual stderr saved to C:\Users\RUNNER~1\AppData\Local\Temp\fail_configured_limit.stage-id.stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args fail_configured_limit.rs`

This tells us several things:

  1. The actual error output (stderr) doesn't match the expected output.
  2. The actual output has been saved to a temporary file for inspection.
  3. We can update the expected output using the --bless flag when running the tests. This is the key to resolving the issue. The --bless flag tells the test runner to overwrite the existing expected output with the current actual output.

Examining the Actual Diagnostic Output

Fortunately, the error message also provides the actual diagnostic output:

warning: Collapse the "if condition" to "1 branch" or fewer.
  --> $DIR/fail_configured_limit.rs:5:8
   |
LL |     if ready() && approved() {
   |        ^^^^^^^^^^^^^^^^^^^^^
   |
note: The "if condition" currently contains "2 branches".
  --> $DIR/fail_configured_limit.rs:5:8
   |
LL |     if ready() && approved() {
   |        ^^^^^^^^^^^^^^^^^^^^^
   = help: Extract helper functions or simplify the "if condition" to reduce branching.
   = note: `#[warn(conditional_max_n_branches)]` on by default

warning: 1 warning emitted

This output confirms that the lint is working as expected. It correctly identifies the overly complex conditional and provides suggestions for simplification. The issue is simply that the expected output file doesn't match this exact format.

Root Cause

The root cause of this failure is that the expected stderr file (crates/conditional_max_n_branches/ui/fail_configured_limit.stderr) is outdated. It needs to be updated to reflect the current diagnostic output format produced by the lint. This often happens after updates to the Rust toolchain or the lint itself, which may subtly change the wording or formatting of diagnostic messages. Regularly updating these expected output files is a crucial part of maintaining a healthy test suite.

Reproduction

To reproduce this failure, you can run the following command in your terminal:

cargo test -p conditional_max_n_branches --lib --features dylint-driver

This command specifically targets the conditional_max_n_branches crate, builds it as a library, and enables the dylint-driver feature, which is necessary for running the lint as part of the test suite.

Suggested Fix

The recommended solution is to update the expected stderr file. You can do this by running either of the following commands:

cargo test -p conditional_max_n_branches --lib --features dylint-driver -- --bless

This command updates all expected output files in the crate.

Or, to update only the fail_configured_limit.rs test:

cargo test -p conditional_max_n_branches --lib --features dylint-driver -- --bless --test-args fail_configured_limit.rs

This command is more specific, targeting only the failing test and minimizing the risk of accidentally updating other expected output files unnecessarily. Using the more specific command is generally preferred.

Important: After running the --bless command, you should review the changes to the expected output file to ensure that they are correct and that no unexpected changes have been introduced.

Impact

The impact of this failure is primarily on the CI pipeline. Specifically:

  • It blocks the CI pipeline on Windows, preventing automated integration of new code.
  • All other tests (66 common tests, 4 context tests, 24 i18n tests, 7 localizer tests, 7 conditional_max_n_branches unit tests) are passing, indicating that the core functionality of the lint and related components is working correctly.
  • The lint functionality itself is working correctly; the only issue is the outdated expected test output.

Context

This issue is related to PR #52 and has been discussed in this comment. This context provides further information about the surrounding changes and discussions related to this test failure.

Conclusion

By updating the expected stderr file for the fail_configured_limit.rs UI test, we can resolve the current failure and unblock the Windows CI pipeline. This ensures that our development workflow remains smooth and efficient. Remember to always review the changes introduced by the --bless command to ensure their correctness.

For more information about UI testing and updating expected output files, refer to the Rust documentation.