[Source Tooling] Refactoring action to convert if statement to switch

[Regno]

Implement action to convert if statements from:

var e = E.case1
if e == .case1 {}
else if e == .case2 {}
else {}

to

  var e = E.case1
  switch e {
  case .case1: break
  case .case2: break
  default: break
  }

Resolves SR-5740.

@nkcsgexi

[Regno]

@nathawes Could you review this PR please?

[nathawes]

Looking good so far! I thought of a few edge cases that might need handling and some tests to add while looking over it. I mentioned some potential enhancements to make it applicable in more situations in case you're interested in working on them, but those can certainly be left for a future PR too.

[nathawes]

I think it's ok to restrict it to just enums for now, but technically you could allow this refactoring to work for any type that implements the ~= operator, rather than just enums. e.g. the below is fine:

let x = 10
                                    // switch x {
if x == 5 {                         // case 5: ...
} else if x == 4 {                  // case 4: ...
} else if 1...3 ~= x {              // case 1...3: ...
} else {                            // default: ...
}                                   // }

Allowing any type that conforms to the Equatable protocol (which provides a default implementation for ~=) is probably easier to check for and would probably cover most cases:

struct MyType: Equatable { let v: String }
let y = MyType(v: "hello")
                                    // switch y {
if y == MyType(v: "bye") {          // case MyType(v: "bye"): ...
} else if x == MyType(v: "tchau") { // case MyType(v: "tchau"): ...
} else {                            // default: ...
}                                   // }

I think you can do the check using ModuleDecl::conformsToProtocol() to check if FirstArgumentType conforms to the Equatable protocol which you can get via Ctx->getProtocol(KnownProtocolKind::Equatable). I think you need to call it on the module the defines FirstArgumentType. Alternatively, IDETypechecking.h has an isConvertibleTo function that might to the right thing without needing the ModuleDecl.

[Regno]

I added such test cases too.

But I don't check that type conforms to the Equatable protocol. I just suppose that user will try to convert a valid if statement.

[Regno]

I can't reopen this PR after force push to branch. So I create new one: #29596