feat(linter/plugins): add ESLint compat mode to RuleTester

[overlookmotel]

The problem

ESLint's RuleTester has a really unintutive behavior that it alters the column of diagnostics before comparing them to the test case.

This means that, bizarrely, this test fails:

const rule = {
  create(context) {
    return {
      DebuggerStatement(node) {
        context.report({
          message: "No debugger",
          loc: {
            start: {
              line: 1,
              column: 0,
            },
            end: {
              line: 1,
              column: 8,
            },
          },
        });
      },
    };
  },
};

const tester = new RuleTester();

tester.run("no-debugger", rule, {
  valid: [],
  invalid: [{
    code: "debugger;",
    errors: [{
      message: "No debugger",
      line: 1,
      column: 0, // ESLint says this is wrong - should be 1!
      endLine: 1,
      endColumn: 8, // ESLint says this is wrong - should be 9!
    }],
  }],
});

How to handle this?

This behavior is to my mind very unintuitive, and I don't think we want to follow it by default.

However, we want to allow people to be able to copy existing test cases written for ESLint's RuleTester when migrating to Oxlint. We also need to copy ESLint's behavior for running the tests of existing ESLint plugins against Oxlint, to avoid spurious test failures due to column mismatches.

Solution

Add an eslintCompat option that replicates ESLint's behavior. The option is disabled by default.

The test above now passes by default. But using eslintCompat: true, the test passes when column and endColumn are provided in ESLint style:

const tester = new RuleTester({
  eslintCompat: true,
});

tester.run("no-debugger", rule, {
  valid: [],
  invalid: [{
    code: "debugger;",
    errors: [{
      message: "No debugger",
      line: 1,
      column: 1, // Correct with `eslintCompat: true`
      endLine: 1,
      endColumn: 9, // Correct with `eslintCompat: true`
    }],
  }],
});

[overlookmotel]

• feat(linter/plugins): add ESLint compat mode to RuleTester #16538 👈 (View in Graphite)
• fix(linter/plugins): add config support skeleton to RuleTester #16537
• main

---

How to use the Graphite Merge Queue

Add either label to this PR to merge it via the merge queue:

• 0-merge - adds this PR to the back of the merge queue
• hotfix - for urgent hot fixes, skip the queue and merge this PR next

You must have a Graphite account in order to use the merge queue. Sign up using this link.

An organization admin has enabled the Graphite Merge Queue in this repository.

Please do not merge from GitHub as this will restart CI on PRs being processed by the merge queue.

This stack of pull requests is managed by Graphite. Learn more about stacking.

[Copilot]

Pull request overview

This PR adds an ESLint compatibility mode to the RuleTester to handle column offset differences between ESLint and Oxlint. ESLint uses 1-based column indexing in test assertions while Oxlint uses 0-based indexing. The new eslintCompat option (disabled by default) adds 1 to column offsets when comparing diagnostics, allowing users to migrate existing ESLint test cases without modifying column values.

Key changes:

• Added eslintCompat boolean option to Config interface and TestCase interface for both global and per-test configuration
• Modified assertInvalidTestCaseLocationIsCorrect to apply a column offset (+1 when eslintCompat: true) to column and endColumn values
• Implemented config merging logic in createConfigForTest to support per-test overrides

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[overlookmotel]

Merge activity

• Dec 5, 1:41 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Dec 5, 1:41 PM UTC: overlookmotel added this pull request to the Graphite merge queue.
• Dec 5, 1:49 PM UTC: Merged by the Graphite merge queue.