[ASTGen] Implement bridging for using declaration

include/swift/AST/ASTBridging.h

@@ -1744,6 +1744,19 @@ BridgedImportDecl BridgedImportDecl_createParsed(
     BridgedSourceLoc cImportKeywordLoc, BridgedImportKind cImportKind,
     BridgedSourceLoc cImportKindLoc, BridgedArrayRef cImportPathElements);
 
+enum ENUM_EXTENSIBILITY_ATTR(open) BridgedUsingSpecifier {
+  BridgedUsingSpecifierMainActor,
+  BridgedUsingSpecifierNonisolated,
+};
+
+SWIFT_NAME("BridgedUsingDecl.createParsed(_:declContext:usingKeywordLoc:"
+           "specifierLoc:specifier:)")
+BridgedUsingDecl BridgedUsingDecl_createParsed(BridgedASTContext cContext,
+                                               BridgedDeclContext cDeclContext,
+                                               BridgedSourceLoc usingKeywordLoc,
+                                               BridgedSourceLoc specifierLoc,
+                                               BridgedUsingSpecifier specifier);
+
 SWIFT_NAME("BridgedSubscriptDecl.createParsed(_:declContext:staticLoc:"
            "staticSpelling:subscriptKeywordLoc:genericParamList:parameterList:"
            "arrowLoc:returnType:genericWhereClause:)")

lib/AST/Bridging/DeclBridging.cpp

@@ -637,6 +637,17 @@ BridgedImportDecl BridgedImportDecl_createParsed(
       std::move(builder).get());
 }
 
+BridgedUsingDecl BridgedUsingDecl_createParsed(BridgedASTContext cContext,
+                                               BridgedDeclContext cDeclContext,
+                                               BridgedSourceLoc usingKeywordLoc,
+                                               BridgedSourceLoc specifierLoc,
+                                               BridgedUsingSpecifier specifier) {
+  ASTContext &ctx = cContext.unbridged();
+  return UsingDecl::create(
+      ctx, usingKeywordLoc.unbridged(), specifierLoc.unbridged(),
+      static_cast<UsingSpecifier>(specifier), cDeclContext.unbridged());
+}
+
 BridgedSubscriptDecl BridgedSubscriptDecl_createParsed(
     BridgedASTContext cContext, BridgedDeclContext cDeclContext,
     BridgedSourceLoc cStaticLoc, BridgedStaticSpelling cStaticSpelling,

lib/ASTGen/Sources/ASTGen/Decls.swift

@@ -69,6 +69,8 @@ extension ASTGenVisitor {
       return self.generate(typeAliasDecl: node)?.asDecl
     case .variableDecl(let node):
       return self.generate(variableDecl: node)
+    case .usingDecl(let node):
+      return self.generate(usingDecl: node)?.asDecl
     }
   }
 
@@ -1083,6 +1085,37 @@ extension ASTGenVisitor {
   }
 }
 
+extension ASTGenVisitor {
+  func generate(usingDecl node: UsingDeclSyntax) -> BridgedUsingDecl? {
+    var specifier: BridgedUsingSpecifier? = nil
+
+    switch node.specifier {
+    case .attribute(let attr):
+      if let identifier = attr.attributeName.as(IdentifierTypeSyntax.self),
+         identifier.name.tokenKind == .identifier("MainActor") {
+        specifier = .mainActor
+      }
+    case .modifier(let modifier):
+      if case .identifier("nonisolated") = modifier.tokenKind {
+        specifier = .nonisolated
+      }
+    }
+
+    guard let specifier else {
+      self.diagnose(.invalidDefaultIsolationSpecifier(node.specifier))
+      return nil
+    }
+
+    return BridgedUsingDecl.createParsed(
+      self.ctx,
+      declContext: self.declContext,
+      usingKeywordLoc: self.generateSourceLoc(node.usingKeyword),
+      specifierLoc: self.generateSourceLoc(node.specifier),
+      specifier: specifier
+    )
+  }
+}
+
 extension ASTGenVisitor {
   func generate(memberBlockItem node: MemberBlockItemSyntax) -> BridgedDecl? {
     if let node = node.decl.as(MacroExpansionDeclSyntax.self) {

lib/ASTGen/Sources/ASTGen/Diagnostics.swift

@@ -146,6 +146,13 @@ extension ASTGenDiagnostic {
       message: "expressions are not allowed at the top level"
     )
   }
+
+  static func invalidDefaultIsolationSpecifier(_ specifier: some SyntaxProtocol) -> Self {
+    Self(
+      node: specifier,
+      message: "default isolation can only be set to '@MainActor' or 'nonisolated'"
+    )
+  }
 }
 
 /// DeclAttributes diagnostics

test/ASTGen/decls.swift

@@ -31,6 +31,10 @@ import protocol Swift.Sequence
 import func Swift.max
 import var Swift._playgroundPrintHook
 
+
+using @MainActor
+// FIXME: cannot add `using nonisolated` here because it's a re-declaration
+
 func
 test1
 (

test/ASTGen/diagnostics.swift

@@ -4,6 +4,7 @@
 
 // REQUIRES: swift_swift_parser
 // REQUIRES: swift_feature_ParserASTGen
+
 // rdar://116686158
 // UNSUPPORTED: asan
 
@@ -66,3 +67,12 @@ func misisngPatternTest(arr: [Int]) {
   for {} // expected-error {{expected pattern, 'in', and expression in 'for' statement}}
          // expected-note@-1 {{insert pattern, 'in', and expression}} {{7-7=<#pattern#> }} {{7-7=in }} {{7-7=<#expression#> }}
 }
+
+using @MainActor // expected-note {{default isolation was previously declared here}}
+using nonisolated // expected-error {{invalid redeclaration of file-level default actor isolation}}
+
+using @Test
+// expected-error@-1 {{default isolation can only be set to '@MainActor' or 'nonisolated'}}
+
+using test
+// expected-error@-1 {{default isolation can only be set to '@MainActor' or 'nonisolated'}}