[Concurrency] Implement type checking for 'async let' declarations.

include/swift/AST/Attr.def

@@ -588,6 +588,12 @@ SIMPLE_DECL_ATTR(_specializeExtension, SpecializeExtension,
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
   105)
 
+CONTEXTUAL_SIMPLE_DECL_ATTR(async, Async,
+  DeclModifier | OnVar | OnFunc | ConcurrencyOnly |
+  ABIBreakingToAdd | ABIBreakingToRemove |
+  APIBreakingToAdd | APIBreakingToRemove,
+  106)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef CONTEXTUAL_DECL_ATTR_ALIAS

include/swift/AST/Decl.h

@@ -1784,6 +1784,9 @@ class PatternBindingDecl final : public Decl,
   /// Is the pattern binding entry for this variable  currently being computed?
   bool isComputingPatternBindingEntry(const VarDecl *vd) const;
 
+  /// Is this an "async let" declaration?
+  bool isAsyncLet() const;
+
   /// Gets the text of the initializer expression for the pattern entry at the
   /// given index, stripping out inactive branches of any #ifs inside the
   /// expression.
@@ -4727,6 +4730,9 @@ class VarDecl : public AbstractStorageDecl {
   /// getSpecifier() == Specifier::Default.
   bool isLet() const { return getIntroducer() == Introducer::Let; }
 
+  /// Is this an "async let" property?
+  bool isAsyncLet() const;
+
   Introducer getIntroducer() const {
     return Introducer(Bits.VarDecl.Introducer);
   }

include/swift/AST/DiagnosticsParse.def

@@ -324,6 +324,8 @@ ERROR(func_decl_expected_arrow,none,
 ERROR(operator_static_in_protocol,none,
       "operator '%0' declared in protocol must be 'static'",
       (StringRef))
+ERROR(async_func_modifier,none,
+      "'async' must be written after the parameter list of a function", ())
 
 // Enum
 ERROR(expected_lbrace_enum,PointsToFirstBadToken,

include/swift/AST/DiagnosticsSema.def

@@ -4067,6 +4067,8 @@ ERROR(throwing_interpolation_without_try,none,
       "interpolation can throw but is not marked with 'try'", ())
 ERROR(throwing_call_without_try,none,
       "call can throw but is not marked with 'try'", ())
+ERROR(throwing_async_let_without_try,none,
+      "reading 'async let' can throw but is not marked with 'try'", ())
 NOTE(note_forgot_try,none,
       "did you mean to use 'try'?", ())
 NOTE(note_error_to_optional,none,
@@ -4087,6 +4089,9 @@ ERROR(async_call_without_await,none,
       "call is 'async' but is not marked with 'await'", ())
 ERROR(async_call_without_await_in_autoclosure,none,
       "call is 'async' in an autoclosure argument that is not marked with 'await'", ())
+ERROR(async_call_without_await_in_async_let,none,
+      "call is 'async' in an 'async let' initializer that is not marked "
+      "with 'await'", ())
 WARNING(no_async_in_await,none,
         "no calls to 'async' functions occur within 'await' expression", ())
 ERROR(async_call_in_illegal_context,none,
@@ -4098,9 +4103,9 @@ ERROR(await_in_illegal_context,none,
       "%select{<<ERROR>>|a default argument|a property initializer|a global variable initializer|an enum case raw value|a catch pattern|a catch guard expression|a defer body}0",
       (unsigned))
 ERROR(async_in_nonasync_function,none,
-      "%select{'async'|'await'}0 in %select{a function|an autoclosure}1 that "
-      "does not support concurrency",
-      (bool, bool))
+      "%select{'async'|'await'|'async let'}0 in "
+      "%select{a function|an autoclosure}1 that does not support concurrency",
+      (unsigned, bool))
 NOTE(note_add_async_to_function,none,
      "add 'async' to function %0 to make it asynchronous", (DeclName))
 NOTE(note_add_asynchandler_to_function,none,
@@ -4118,6 +4123,21 @@ NOTE(protocol_witness_async_conflict,none,
 ERROR(async_autoclosure_nonasync_function,none,
       "'async' autoclosure parameter in a non-'async' function", ())
 
+ERROR(async_not_let,none,
+      "'async' can only be used with 'let' declarations", ())
+ERROR(async_let_not_local,none,
+      "'async let' can only be used on local declarations", ())
+ERROR(async_let_not_initialized,none,
+      "'async let' binding requires an initializer expression", ())
+ERROR(async_let_no_variables,none,
+      "'async let' requires at least one named variable", ())
+ERROR(async_let_without_await,none,
+      "reference to async let %0 is not marked with 'await'", (DeclName))
+ERROR(async_let_in_illegal_context,none,
+      "async let %0 cannot be referenced in "
+      "%select{<<ERROR>>|a default argument|a property initializer|a global variable initializer|an enum case raw value|a catch pattern|a catch guard expression|a defer body}1",
+      (DeclName, unsigned))
+
 ERROR(asynchandler_non_func,none,
       "'@asyncHandler' can only be applied to functions",
       ())

include/swift/AST/Expr.h

@@ -3903,7 +3903,10 @@ class AutoClosureExpr : public AbstractClosureExpr {
 
     // An autoclosure with type (Self) -> (Args...) -> Result. Formed from type
     // checking a partial application.
-    DoubleCurryThunk = 2
+    DoubleCurryThunk = 2,
+
+    // An autoclosure with type () -> Result that was formed for an async let.
+    AsyncLet = 3,
   };
 
   AutoClosureExpr(Expr *Body, Type ResultTy, unsigned Discriminator,

include/swift/Sema/ConstraintSystem.h

@@ -4373,7 +4373,8 @@ class ConstraintSystem {
   /// Build implicit autoclosure expression wrapping a given expression.
   /// Given expression represents computed result of the closure.
   Expr *buildAutoClosureExpr(Expr *expr, FunctionType *closureType,
-                             bool isDefaultWrappedValue = false);
+                             bool isDefaultWrappedValue = false,
+                             bool isAsyncLetWrapper = false);
 
   /// Builds a type-erased return expression that can be used in dynamic
   /// replacement.

lib/AST/Decl.cpp

@@ -1545,6 +1545,13 @@ StaticSpellingKind PatternBindingDecl::getCorrectStaticSpelling() const {
   return getCorrectStaticSpellingForDecl(this);
 }
 
+bool PatternBindingDecl::isAsyncLet() const {
+  if (auto var = getAnchoringVarDecl(0))
+    return var->isAsyncLet();
+
+  return false;
+}
+
 
 bool PatternBindingDecl::hasStorage() const {
   // Walk the pattern, to check to see if any of the VarDecls included in it
@@ -5740,6 +5747,10 @@ bool VarDecl::isMemberwiseInitialized(bool preferDeclaredProperties) const {
   return true;
 }
 
+bool VarDecl::isAsyncLet() const {
+  return getAttrs().hasAttribute<AsyncAttr>();
+}
+
 void ParamDecl::setSpecifier(Specifier specifier) {
   // FIXME: Revisit this; in particular shouldn't __owned parameters be
   // ::Let also?

lib/AST/Expr.cpp

@@ -2031,6 +2031,7 @@ Expr *AutoClosureExpr::getUnwrappedCurryThunkExpr() const {
 
   switch (getThunkKind()) {
   case AutoClosureExpr::Kind::None:
+  case AutoClosureExpr::Kind::AsyncLet:
     break;
 
   case AutoClosureExpr::Kind::SingleCurryThunk: {

lib/Parse/ParseDecl.cpp

@@ -6558,6 +6558,19 @@ ParserResult<FuncDecl> Parser::parseDeclFunc(SourceLoc StaticLoc,
 
   diagnoseWhereClauseInGenericParamList(GenericParams);
 
+  // If there was an 'async' modifier, put it in the right place for a function.
+  bool isAsync = asyncLoc.isValid();
+  if (auto asyncAttr = Attributes.getAttribute<AsyncAttr>()) {
+    SourceLoc insertLoc = Lexer::getLocForEndOfToken(
+        SourceMgr, BodyParams->getRParenLoc());
+
+    diagnose(asyncAttr->getLocation(), diag::async_func_modifier)
+      .fixItRemove(asyncAttr->getRange())
+      .fixItInsert(insertLoc, " async");
+    asyncAttr->setInvalid();
+    isAsync = true;
+  }
+
   // Create the decl for the func and add it to the parent scope.
   auto *FD = FuncDecl::create(Context, StaticLoc, StaticSpelling,
                               FuncLoc, FullName, NameLoc,

lib/SIL/IR/SILDeclRef.cpp

@@ -447,8 +447,15 @@ bool SILDeclRef::isTransparent() const {
 
   if (hasAutoClosureExpr()) {
     auto *ace = getAutoClosureExpr();
-    if (ace->getThunkKind() == AutoClosureExpr::Kind::None)
+    switch (ace->getThunkKind()) {
+    case AutoClosureExpr::Kind::None:
       return true;
+
+    case AutoClosureExpr::Kind::AsyncLet:
+    case AutoClosureExpr::Kind::DoubleCurryThunk:
+    case AutoClosureExpr::Kind::SingleCurryThunk:
+      break;
+    }
   }
 
   if (hasDecl()) {

lib/Sema/CSApply.cpp

@@ -7881,6 +7881,51 @@ bool ConstraintSystem::applySolutionFixes(const Solution &solution) {
   return diagnosedAnyErrors;
 }
 
+/// For the initializer of an `async let`, wrap it in an autoclosure and then
+/// a call to that autoclosure, so that the code for the child task is captured
+/// entirely within the autoclosure. This captures the semantics of the
+/// operation but not the timing, e.g., the call itself will actually occur
+/// when one of the variables in the async let is referenced.
+static Expr *wrapAsyncLetInitializer(
+      ConstraintSystem &cs, Expr *initializer, DeclContext *dc) {
+  // Form the autoclosure type. It is always 'async', and will be 'throws'.
+  Type initializerType = initializer->getType();
+  bool throws = TypeChecker::canThrow(initializer);
+  auto extInfo = ASTExtInfoBuilder()
+    .withAsync()
+    .withThrows(throws)
+    .build();
+
+  // Form the autoclosure expression. The actual closure here encapsulates the
+  // child task.
+  auto closureType = FunctionType::get({ }, initializerType, extInfo);
+  ASTContext &ctx = dc->getASTContext();
+  Expr *autoclosureExpr = cs.buildAutoClosureExpr(
+      initializer, closureType, /*isDefaultWrappedValue=*/false,
+      /*isAsyncLetWrapper=*/true);
+
+  // Call the autoclosure so that the AST types line up. SILGen will ignore the
+  // actual calls and translate them into a different mechanism.
+  auto autoclosureCall = CallExpr::createImplicit(
+      ctx, autoclosureExpr, { }, { });
+  autoclosureCall->setType(initializerType);
+  autoclosureCall->setThrows(throws);
+
+  // For a throwing expression, wrap the call in a 'try'.
+  Expr *resultInit = autoclosureCall;
+  if (throws) {
+    resultInit = new (ctx) TryExpr(
+        SourceLoc(), resultInit, initializerType, /*implicit=*/true);
+  }
+
+  // Wrap the call in an 'await'.
+  resultInit = new (ctx) AwaitExpr(
+      SourceLoc(), resultInit, initializerType, /*implicit=*/true);
+
+  cs.cacheExprTypes(resultInit);
+  return resultInit;
+}
+
 /// Apply the given solution to the initialization target.
 ///
 /// \returns the resulting initialization expression.
@@ -7955,6 +8000,16 @@ static Optional<SolutionApplicationTarget> applySolutionToInitialization(
     return None;
   }
 
+  // For an async let, wrap the initializer appropriately to make it a child
+  // task.
+  if (auto patternBinding = target.getInitializationPatternBindingDecl()) {
+    if (patternBinding->isAsyncLet()) {
+      resultTarget.setExpr(
+          wrapAsyncLetInitializer(
+            cs, resultTarget.getAsExpr(), resultTarget.getDeclContext()));
+    }
+  }
+
   return resultTarget;
 }
 

lib/Sema/ConstraintSystem.cpp

@@ -4662,7 +4662,8 @@ ConstraintSystem::isConversionEphemeral(ConversionRestrictionKind conversion,
 
 Expr *ConstraintSystem::buildAutoClosureExpr(Expr *expr,
                                              FunctionType *closureType,
-                                             bool isDefaultWrappedValue) {
+                                             bool isDefaultWrappedValue,
+                                             bool isAsyncLetWrapper) {
   auto &Context = DC->getASTContext();
   bool isInDefaultArgumentContext = false;
   if (auto *init = dyn_cast<Initializer>(DC)) {
@@ -4684,6 +4685,9 @@ Expr *ConstraintSystem::buildAutoClosureExpr(Expr *expr,
 
   closure->setParameterList(ParameterList::createEmpty(Context));
 
+  if (isAsyncLetWrapper)
+    closure->setThunkKind(AutoClosureExpr::Kind::AsyncLet);
+
   Expr *result = closure;
 
   if (!newClosureType->isEqual(closureType)) {

lib/Sema/MiscDiagnostics.cpp

@@ -1495,8 +1495,16 @@ static void diagnoseImplicitSelfUseInClosure(const Expr *E,
                   const AbstractClosureExpr *CE) {
       // If the closure's type was inferred to be noescape, then it doesn't
       // need qualification.
-      return !AnyFunctionRef(const_cast<AbstractClosureExpr *>(CE))
-               .isKnownNoEscape();
+      if (AnyFunctionRef(const_cast<AbstractClosureExpr *>(CE))
+               .isKnownNoEscape())
+        return false;
+
+      if (auto autoclosure = dyn_cast<AutoClosureExpr>(CE)) {
+        if (autoclosure->getThunkKind() == AutoClosureExpr::Kind::AsyncLet)
+          return false;
+      }
+
+      return true;
     }
 
 

lib/Sema/TypeCheckAttr.cpp

@@ -344,6 +344,59 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
 
     (void)nominal->isGlobalActor();
   }
+
+  void visitAsyncAttr(AsyncAttr *attr) {
+    auto var = dyn_cast<VarDecl>(D);
+    if (!var)
+      return;
+
+    auto patternBinding = var->getParentPatternBinding();
+    if (!patternBinding)
+      return; // already diagnosed
+
+    // "Async" modifier can only be applied to local declarations.
+    if (!patternBinding->getDeclContext()->isLocalContext()) {
+      diagnoseAndRemoveAttr(attr, diag::async_let_not_local);
+      return;
+    }
+
+    // Check each of the pattern binding entries.
+    bool diagnosedVar = false;
+    for (unsigned index : range(patternBinding->getNumPatternEntries())) {
+      auto pattern = patternBinding->getPattern(index);
+
+      // Look for variables bound by this pattern.
+      bool foundAnyVariable = false;
+      bool isLet = true;
+      pattern->forEachVariable([&](VarDecl *var) {
+        if (!var->isLet())
+          isLet = false;
+        foundAnyVariable = true;
+      });
+
+      // Each entry must bind at least one named variable, so that there is
+      // something to "await".
+      if (!foundAnyVariable) {
+        diagnose(pattern->getLoc(), diag::async_let_no_variables);
+        attr->setInvalid();
+        return;
+      }
+
+      // Async can only be used on an "async let".
+      if (!isLet && !diagnosedVar) {
+        diagnose(patternBinding->getLoc(), diag::async_not_let)
+          .fixItReplace(patternBinding->getLoc(), "let");
+        diagnosedVar = true;
+      }
+
+      // Each pattern entry must have an initializer expression.
+      if (patternBinding->getEqualLoc(index).isInvalid()) {
+        diagnose(pattern->getLoc(), diag::async_let_not_initialized);
+        attr->setInvalid();
+        return;
+      }
+    }
+  }
 };
 } // end anonymous namespace
 

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1516,6 +1516,7 @@ namespace  {
     UNINTERESTING_ATTR(Actor)
     UNINTERESTING_ATTR(ActorIndependent)
     UNINTERESTING_ATTR(GlobalActor)
+    UNINTERESTING_ATTR(Async)
 #undef UNINTERESTING_ATTR
 
     void visitAvailableAttr(AvailableAttr *attr) {