Add lint warning for retroactive conformances

[harlanhaskins]

This provides a lint rule whenever an @retroactive appears in a source file.

I've marked the rule as opt-out, but I'm also wondering if we have a syntax for opting out of a rule in general.

[harlanhaskins]

@swift-ci please test

[allevato]

I've marked the rule as opt-out, but I'm also wondering if we have a syntax for opting out of a rule in general.

You can override the isOptIn property from Rule if you want to make the rule disabled by default, and when you run generate-swift-format it will use flip the default in the generated configuration. Is that what you had in mind?

[harlanhaskins]

Oh, I meant like a comment-like syntax for clients to silence the rule within a file, like

// swift-format ignore: AvoidRetroactiveConformances

did we ever get anything like that?

[allevato]

Yeah, // swift-format-ignore: <rulle name> should work for any visitor-based rule. https://github.com/swiftlang/swift-format/blob/main/Documentation/IgnoringSource.md#ignoring-source-transforming-rules

[allevato]

Can you please update the CMake files to include the new AvoidRetroactiveConformances.swift file? (Just the Sources, The Tests don't need it.)

[allevato]

Thanks!