Rule request: add unavailable to unimplemented function

[marcelofabri]

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Rule Request

1. Why should this rule be added? Share links to existing discussion about what
   the community thinks about this.

It's common to add unimplemented methods just to fill protocol/init requirements. For example, if you don't use Interface Builder and create an UIView subclass overriding init, you'll be asked to implement init?(coder:). Xcode suggests a fix-it that adds an implementation with a fatalError .

In addition to the fatalError, one can also mark the method with @available(*, unavailable) to avoid someone from calling directly. It's just an extra safety net.

I'm not sure if we should trigger a warning for every method that only contains a fatalError inside or only the ones with the default template. Also not sure about protocols/overrides.

2. Provide several examples of what would and wouldn't trigger violations.

// would trigger
public required init?(coder aDecoder: NSCoder) {
    fatalError("init(coder:) has not been implemented")
}

// wouldn't trigger
@available(*, unavailable)
public required init?(coder aDecoder: NSCoder) {
     fatalError("init(coder:) has not been implemented")
}

3. Should the rule be configurable, if so what parameters should be configurable?

No.

4. Should the rule be opt-in or enabled by default? Why?
   See README.md for guidelines on when to mark a
   rule as opt-in.

Probably opt-in.

[quinntaylor]

Thanks for implementing this rule! Follow-up issue: #3374