More icu4x testing

[sven-oly]

Are these still valid?

[gemini-code-assist]

Summary of Changes

Hello @sven-oly, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request integrates icu4x version 2.0.0 into the existing Rust testing framework. The primary goal is to validate the conformance test suite against this new major version, ensuring that all functionalities behave as expected with the updated library. This involves setting up a new Rust project for the 2.0.0 executor and adapting existing test logic to accommodate any API changes or new features introduced in icu4x 2.0.0.

Highlights

• New Rust Executor for ICU4X 2.0.0: This pull request introduces a dedicated Rust executor (executors/rust/2.0) to run conformance tests against icu4x version 2.0.0. This includes new Cargo.toml, Cargo.lock, build.rs, and main.rs files to support the new version.
• Version Compatibility Updates: Existing common Rust executor code has been updated to handle API changes and new behaviors in icu4x 2.0.0. This includes modifications to locale handling, ZonedDateTime parsing, and segmenter object borrowing, ensuring forward compatibility.
• Expanded Test Configuration: The run_config.json file has been updated to include a new rust2.0 entry, enabling the automated execution of various icu4x conformance tests (collation, datetime formatting, number formatting, etc.) against the newly integrated icu4x 2.0.0 executor.

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[gemini-code-assist]

Code Review

This pull request adds a new test executor for ICU4X 2.0, along with compatibility shims to handle API changes between versions. The changes are well-structured, using conditional compilation to manage version-specific code. I've found one potential issue in the handling of the backwards collation option for ICU4X 2.0+ which might cause tests for fr-CA to fail incorrectly. My detailed feedback is in the review comment.

[gemini-code-assist]

The current implementation unconditionally marks the backwards option as unsupported for ICU4X 2.0 and later. However, the comment at line 158 correctly notes that this behavior is available implicitly for the fr-CA locale. This implementation will cause tests for fr-CA that specify the backwards option to fail, even if the behavior is correct.

I suggest refactoring this logic to check for the fr-CA locale. For fr-CA, the test should be allowed to proceed to verify the implicit behavior. For other locales, it should be marked as unsupported. This will make the test executor more accurate for ICU4X 2.0+.

    // From 2.0, backward second level is available only via the fr-CA locale
    // <https://github.com/unicode-org/icu4x/pull/6291>
    #[cfg(any(
        ver = "1.3",
        ver = "1.4",
        ver = "1.5",
        ver = "2.0-beta1",
        ver = "2.0-beta2"
    ))]
    if let Some(backwards) = backwards_option {
        options.backward_second_level = match backwards {
            "off" => Some(BackwardSecondLevel::Off),
            "on" => Some(BackwardSecondLevel::On),
            _ => {
                return Ok(json!({
                    "label": label,
                    "error_detail": {"backwards": backwards},
                    "unsupported": "backwards",
                    "error_type": "unsupported",
                }));
            }
        }
    }

    #[cfg(not(any(
        ver = "1.3",
        ver = "1.4",
        ver = "1.5",
        ver = "2.0-beta1",
        ver = "2.0-beta2"
    )))]
    if let Some(backwards) = backwards_option {
        if langid.to_string() != "fr-CA" {
            return Ok(json!({
                "label": label,
                "error_detail": {"backwards": backwards},
                "unsupported": "backwards option is only supported implicitly for fr-CA in ICU4X 2.0+",
                "error_type": "unsupported",
            }));
        }
        // For fr-CA, the behavior is implicit. We can't set the option,
        // but we can let the test proceed to check the default behavior.
    }