#_objectFileFormat: Use @_spi and throw an error to avoid relying on the new additions

Author: kubamracek

Sources/SwiftIfConfig/BuildConfiguration.swift

@@ -37,6 +37,17 @@ public enum CanImportVersion {
   case underlyingVersion(VersionTuple)
 }
 
+enum BuildConfigurationError: Error, CustomStringConvertible {
+  case experimentalFeature(name: String)
+
+  var description: String {
+    switch self {
+    case .experimentalFeature(let name):
+      return "'name' is an experimental feature"
+    }
+  }
+}
+
 /// Captures information about the build configuration that can be
 /// queried in a `#if` expression, including OS, compiler version,
 /// enabled language features, and available modules.
@@ -227,6 +238,7 @@ public protocol BuildConfiguration {
   /// - Parameters:
   ///   - name: The name of the object file format.
   /// - Returns: Whether the target object file format matches the given name.
+  @_spi(ExperimentalLanguageFeatures)
   func isActiveTargetObjectFileFormat(name: String) throws -> Bool
 
   /// The bit width of a data pointer for the target architecture.
@@ -293,9 +305,11 @@ public protocol BuildConfiguration {
 }
 
 /// Default implementation of BuildConfiguration, to avoid a revlock with the
-/// swift repo.
+/// swift repo, and breaking clients with the new addition to the protocol.
 extension BuildConfiguration {
-  func isActiveTargetObjectFileFormat(name: String) throws -> Bool {
-    return false
+  /// FIXME: This should be @_spi(ExperimentalLanguageFeatures) but cannot due
+  /// to rdar://147943518, https://github.com/swiftlang/swift/issues/80313
+  public func isActiveTargetObjectFileFormat(name: String) throws -> Bool {
+    throw BuildConfigurationError.experimentalFeature(name: "_objectFileFormat")
   }
 }