I think we could have an option to keep function expressions if they're passed to a .mockImplementation() call.

For those affected by the issue: Instead of turning off the rule completely, you can use named functions for the mock implementations. Named function expressions are ignored by complexity.useArrowFunction.

Example:

myMock.mockImplementation(function someArbitraryName() {
  return 'value';
});

But yes, it would be nice, if Biome could handle anonymous functions too in this case.

As another workaround, you can add type annotation for the this parameter to suppress the warning, if you are mocking a class method:

myMock.mockImplementation(function (this: MyClass) {
  return 'value';
});