Add -lint=suggest to include replacement in --format=json output

[Silic0nS0ldier]

This PR adds a new CLI flag value for -lint, suggest. When used in conjunction with --format=json the output populates the new field .files[].warnings[].replacement.

Note that start and end are inconsistent with the existing fields.

• .files[].warnings[].start and .files[].warnings[].end
  • .line is a 1-indexed line number.
  • .column is a 1-indexed UTF-16 code unit.
• .files[].warnings[].replacement.start and .files[].warnings[].replacement.end
  • . is a 0-indexed byte positions.

Replacement positions are derived from []byte arrays, so conversion (to the correct encoding) would be necessary. But also;

• Correctly detecting line endings in a manner consistent with the parser.
• Correctly handling multi-byte sequences (and potentially Unicode grapheme clusters) in a manner consistent with the parser.

As a result normalisation is difficult, bug prone, and unless []byte usage is eliminated close to it's source, liable to carry a (minor) performance and memory penalty.

Output sample;

❯ buildifier -lint=suggest -mode=check -format json -v ___/BUILD.bazel
{
    "success": false,
    "files": [
        {
            "filename": "___/BUILD.bazel",
            "formatted": false,
            "valid": true,
            "warnings": [
                {
                    "start": {
                        "line": 259,
                        "column": 1
                    },
                    "end": {
                        "line": 259,
                        "column": 10
                    },
                    "category": "native-java-test",
                    "actionable": true,
                    "autoFixable": true,
                    "message": "Function \"java_test\" is not global anymore and needs to be loaded from \"@rules_java//java:java_test.bzl\".",
                    "url": "https://github.com/bazelbuild/buildtools/blob/main/WARNINGS.md#native-java-test",
                    "replacement": {
                        "start": 57,
                        "end": 57,
                        "content": "@rules_java//java:java_test.bzl\", \"java_test\")\nload(\""
                    }
                }
            ]
        }
    ]
}

The use case for this change is integrating suggested fixes with an internal tool.

[gemini-code-assist]

Summary of Changes

Hello @Silic0nS0ldier, 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 enhances the buildifier tool by introducing a new suggest lint mode. This mode, when used with JSON output, provides detailed code replacement suggestions directly within the warning objects. The primary purpose is to facilitate the integration of buildifier's linting capabilities with external automation tools that can consume these structured suggestions to apply fixes programmatically. The changes involve updating configuration, validation logic, and the diagnostic output structures to accommodate this new functionality.

Highlights

• New Lint Mode: Introduced a new -lint=suggest option to buildifier.
• JSON Output Enhancement: When -lint=suggest is combined with --format=json, the output now includes a new replacement field within warnings, providing byte-offset based start/end positions and suggested content for automated fixes.
• Compatibility Enforcement: The --lint=suggest mode is strictly enforced to be compatible only with --format=json.
• Data Structure Updates: New data structures (replacement struct and Replacement field in warning struct) were added to buildifier/utils/diagnostics.go to support the new replacement information in the JSON output.

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[gemini-code-assist]

Code Review

This pull request adds a new -lint=suggest flag to include replacement suggestions in the JSON output. The implementation is straightforward and well-contained. I've identified two minor issues: an outdated error message and a missing omitempty tag on a new JSON field, for which I've provided suggestions. Overall, this is a valuable addition.