guard nonisolated(unsafe) by experimental feature

Author: sophiapoirier

Sources/SwiftParser/ExperimentalFeatures.swift

@@ -26,4 +26,7 @@ extension Parser.ExperimentalFeatures {
 
   /// Whether to enable the parsing of 'then' statements.
   public static let thenStatements = Self(rawValue: 1 << 1)
+
+  /// Whether to enable the parsing of strict concurrency for globals.
+  public static let globalConcurrency = Self(rawValue: 1 << 2)
 }

Sources/SwiftParser/Modifiers.swift

@@ -61,7 +61,12 @@ extension Parser {
       case (.declarationModifier(.unowned), let handle)?:
         elements.append(self.parseUnownedModifier(handle))
       case (.declarationModifier(.nonisolated), let handle)?:
-        elements.append(parseNonisolatedModifier(handle))
+        if experimentalFeatures.contains(.globalConcurrency) {
+          elements.append(parseNonisolatedModifier(handle))
+        } else {
+          let (unexpectedBeforeKeyword, keyword) = self.eat(handle)
+          elements.append(RawDeclModifierSyntax(unexpectedBeforeKeyword, name: keyword, detail: nil, arena: self.arena))
+        }
       case (.declarationModifier(.final), let handle)?,
         (.declarationModifier(.required), let handle)?,
         (.declarationModifier(.optional), let handle)?,

Tests/SwiftParserTest/DeclarationTests.swift

@@ -210,7 +210,8 @@ final class DeclarationTests: ParserTestCase {
           nonisolated(unsafe) var c: Int { 0 }
           nonisolated(unsafe) let d = 0
         }
-        """
+        """,
+      experimentalFeatures: [.globalConcurrency]
     )
   }