[SwiftIfConfig] Make ConfiguredRegions and a BuildConfiguration generally interchangeable

Sources/SwiftIfConfig/ActiveClauseEvaluator.swift

@@ -0,0 +1,50 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftDiagnostics
+import SwiftSyntax
+
+/// Captures sufficient information to determine the active clause for an `#if`
+/// either by querying existing configured regions or by evaluating the
+/// clause's conditions against a build configuration.
+enum ActiveClauseEvaluator {
+  case configuredRegions(ConfiguredRegions)
+  case configuration(any BuildConfiguration)
+
+  /// Previously-known diagnostics.
+  var priorDiagnostics: [Diagnostic] {
+    switch self {
+    case .configuredRegions(let configuredRegions):
+      return configuredRegions.diagnostics
+    case .configuration:
+      return []
+    }
+  }
+
+  /// Determine which clause of an `#if` declaration is active, if any.
+  ///
+  /// If this evaluation produced any diagnostics, they will be appended to
+  /// the diagnostics parameter.
+  func activeClause(
+    for node: IfConfigDeclSyntax,
+    diagnostics: inout [Diagnostic]
+  ) -> IfConfigClauseSyntax? {
+    switch self {
+    case .configuredRegions(let configuredRegions):
+      return configuredRegions.activeClause(for: node)
+    case .configuration(let configuration):
+      let (activeClause, localDiagnostics) = node.activeClause(in: configuration)
+      diagnostics.append(contentsOf: localDiagnostics)
+      return activeClause
+    }
+  }
+}