Add rule to insert a blank line following a switch case with a multi-line body

[calda]

Summary

This PR proposes a new rule to insert a blank line following a switch case with a multi-line body:

Insert a blank line following a switch case with a multi-line body. Spacing within an individual switch statement should be consistent. If any case has a multi-line body then all cases should include a trailing blank line. The last switch case doesn't need a blank line, since it is already followed by a closing brace.

Autocorrect is implemented in the blankLineAfterMultilineSwitchCase and consistentSwitchStatementSpacing SwiftFormat rules, which were added in nicklockwood/SwiftFormat#1621.

Reasoning

Like with declarations in a file, inserting a blank line between scopes makes them easier to visually differentiate.

Switch statements with multi-line cases can become visually busy and cramped without blank lines between the cases. In this example, all of the cases start to bleed together, making it more difficult to read the code at a glance:

func handle(_ action: SpaceshipAction) {
  switch action {
  case .engageWarpDrive:
    navigationComputer.destination = targetedDestination
    warpDrive.spinUp()
    warpDrive.activate()
  case .enableArtificialGravity:
    artificialGravityEngine.enable(strength: .oneG)
  case .scanPlanet(let planet):
    scanner.target = planet
    scanner.scanAtmosphere()
    scanner.scanBiosphere()
    scanner.scanForArtificialLife()
  case .handleIncomingEnergyBlast:
    energyShields.engage()
  }
}

Blank lines between the individual cases make the switch statement much easier to read:

func handle(_ action: SpaceshipAction) {
  switch action {
  case .engageWarpDrive:
    navigationComputer.destination = targetedDestination
    warpDrive.spinUp()
    warpDrive.activate()

  case .enableArtificialGravity:
    artificialGravityEngine.enable(strength: .oneG)

  case .scanPlanet(let planet):
    scanner.target = planet
    scanner.scanAtmosphere()
    scanner.scanBiosphere()
    scanner.scanForArtificialLife()

  case .handleIncomingEnergyBlast:
    energyShields.engage()
  }
}

Blank lines are not required in cases where all of the switch cases are a single line, but are still permitted if it helps with readability:

// RIGHT. Since none of the cases are multi-line, blank lines are not required.
func handle(_ action: SpaceshipAction) {
  switch action {
  case .engageWarpDrive:
      warpDrive.engage()
  case .enableArtificialGravity:
      artificialGravityEngine.enable(strength: .oneG)
  case .scanPlanet(let planet):
      scanner.scan(planet)
  case .handleIncomingEnergyBlast:
      energyShields.engage()
  }
}

// ALSO RIGHT. Blank lines are still permitted after single-line switch cases if it helps with readability. 
func handle(_ action: SpaceshipAction) {
  switch action {
  case .engageWarpDrive:
      warpDrive.engage()

  case .enableArtificialGravity:
      artificialGravityEngine.enable(strength: .oneG)

  case .scanPlanet(let planet):
      scanner.scan(planet)

  case .handleIncomingEnergyBlast:
      energyShields.engage()
  }
}

Please react with 👍/👎 if you agree or disagree with this proposal.

[bachand]

Nice work @calda . LGTM with some comments.

[bachand]

👍

[bachand]

👍

[bachand]

👍

[bachand]

👍

[calda]

There wasn't much public engagement on this PR, but three engineers expressed support for this rule internally on Slack.

[bachand]

Thanks @calda !

[bachand]

Nice!