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Rslint

    prefer-to-contain

    Unreleased

    Configuration

    rslint.config.ts
    import { defineConfig, rstestPlugin } from '@rslint/core';

export default defineConfig([
  rstestPlugin.configs.recommended,
  {
    rules: {
      'rstest/prefer-to-contain': 'error',
    },
  },
]);

    Rule Details

    This rule requires toContain() when an equality assertion checks the boolean result of a single-argument includes() call. The dedicated matcher states the intended containment check directly and produces a more useful failure message.

    The rule recognizes toBe(), toEqual() and toStrictEqual(), including negated forms, and preserves the assertion's meaning when deciding whether the replacement needs .not. It recognizes Rstest globals, imports and aliases, namespace and require bindings, rstack/test, import.meta.rstest, expect.soft(), Playwright's ordinary value assertions, and the expect supplied by a test context.

    Polling assertions, promise modifiers, browser-only expect.element() assertions, Chai-style equality assertions with .deep, assertion chains that continue after the equality matcher, dynamic accessors and explicit NaN items are left unchanged. A regular-expression item is also excluded unless the receiver is visibly an array literal. This avoids suggesting a non-equivalent matcher for a possibly string-valued receiver because native string includes() rejects regular expressions. Rstest's array toContain() also does not match Array.prototype.includes() for NaN.

    Incorrect

    expect(flavors.includes('lime')).toBe(true);
expect(roles.includes('admin')).not.toEqual(false);
expect(tags.includes('deprecated')).toStrictEqual(false);

    Correct

    expect(flavors).toContain('lime');
expect(roles).toContain('admin');
expect(tags).not.toContain('deprecated');

    Autofix

    The autofix moves the includes() receiver and item into expect(receiver).toContain(item), removes explicit type arguments from the expect() factory and equality matcher, then adds or removes .not to preserve the boolean assertion. Quoted and template-literal matcher accessors retain their delimiters. If the moved item may have side effects, expect() has a custom message argument, or an edited expression or removed type argument list contains a comment, the rule reports the assertion without a fix.