Add new example: Migrate a Swift Codebase to the new if-let-Syntax

Examples/MigrateToNewIfLetSyntax.swift

@@ -0,0 +1,59 @@
+import Foundation
+import SwiftSyntax
+import SwiftParser
+
+/// MigrateToNewIfLetSyntax will visit each `if` statement in the syntax tree
+/// replacing all "old style" optional bindings by the new shorter syntax available
+/// since Swift 5.7.
+///
+/// For example, it will turn:
+/// ```
+/// if let foo = foo {
+///   ...
+/// }
+/// ```
+/// into:
+/// ```
+/// if let foo {
+///   ...
+/// }
+class MigrateToNewIfLetSyntax: SyntaxRewriter {
+  // Visit all `if` statements.
+  override func visit(_ node: IfStmtSyntax) -> StmtSyntax {
+    // Visit all conditions in the node.
+    let newConditions = node.conditions.enumerated().map { (index, condition) in
+      var conditionCopy = condition
+      // Check if the condition is an optional binding ...
+      if var binding = condition.condition.as(OptionalBindingConditionSyntax.self),
+         // ... and has an initializer ...
+         let initializer = binding.initializer,
+         // ... and both sides of the assignment are the same identifiers.
+         binding.pattern.withoutTrivia().description == initializer.value.withoutTrivia().description {
+        // Remove the initializer ... 
+        binding.initializer = nil
+        // ... and remove whitespace before the comma (in `if` statements with multiple conditions).
+        if index != node.conditions.count - 1 {
+          binding.pattern = binding.pattern.withoutTrailingTrivia()
+        }
+        conditionCopy.condition = Syntax(binding)
+      }
+      return conditionCopy
+    }
+    return StmtSyntax(node.withConditions(ConditionElementListSyntax(newConditions)))
+  }
+
+  /// Utility function to migrate all swift files in a folder and its subfolders
+  static func migrate(folderPath: String) {
+    for case let fileURL as String in FileManager.default.enumerator(atPath: folderPath)! {
+      if fileURL.hasSuffix("swift") {
+        print("Rewriting", fileURL)
+        let fullPath = folderPath + "/" + fileURL
+        let tree = try! Parser.parse(source: String(data: FileManager.default.contents(atPath: fullPath)!, encoding: .utf8 )!)
+        let newTree = MigrateToNewIfLetSyntax().visit(tree)
+        try! newTree.description.write(toFile: fullPath, atomically: true, encoding: .utf8)
+      }
+    }
+  }
+}
+
+MigrateToNewIfLetSyntax.migrate(folderPath: "/path/to/folder")