[Macros] Add simplistic MacroExpansionContext.createUniqueLocalName()

Author: DougGregor

Sources/_SwiftSyntaxMacros/MacroExpansionContext.swift

@@ -25,12 +25,23 @@ public struct MacroExpansionContext {
   /// name, so the combination of file and module name is unique.
   public let fileName: String
 
+  /// Counter used to generate names local to the macro.
+  var localNameCounter = 0
+
   /// Create a new macro evaluation context.
   public init(moduleName: String, fileName: String) {
     self.moduleName = moduleName
     self.fileName = fileName
   }
 
+  /// Generate a unique local name for use in the macro.
+  public mutating func createUniqueLocalName() -> TokenSyntax {
+    let name = "__macro_local_\(localNameCounter)"
+    let token = TokenSyntax(.identifier(name), presence: .present)
+    localNameCounter += 1
+    return token
+  }
+
   /// Create a new macro evaluation context.
   @available(*, deprecated, message: "Use init(moduleName:fileName:)")
   public init(

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -152,4 +152,15 @@ final class MacroSystemTests: XCTestCase {
       """
     )
   }
+
+  func testContextUniqueLocalNames() {
+    var context = MacroExpansionContext(
+      moduleName: "MyModule", fileName: "taylor.swift"
+    )
+
+    let t1 = context.createUniqueLocalName()
+    let t2 = context.createUniqueLocalName()
+    XCTAssertNotEqual(t1.description, t2.description)
+    XCTAssertEqual(t1.description, "__macro_local_0")
+  }
 }