AST: Implement SE-0102, introducing new semantics for Never alongside @NoReturn

No migrator support yet, and the code for @NoReturn is still in
place.

Author: slavapestov

include/swift/AST/Builtins.def

@@ -431,10 +431,10 @@ BUILTIN_MISC_OPERATION(ZeroInitializer, "zeroInitializer", "n", Special)
 /// once has type (Builtin.RawPointer, () -> ())
 BUILTIN_MISC_OPERATION(Once, "once", "", Special)
 
-/// unreachable has type @noreturn () -> ()
+/// unreachable has type () -> Never
 BUILTIN_MISC_OPERATION(Unreachable, "unreachable", "", Special)
 
-/// conditionallyUnreachable has type @noreturn () -> ()
+/// conditionallyUnreachable has type () -> Never
 BUILTIN_MISC_OPERATION(CondUnreachable, "conditionallyUnreachable", "", Special)
 
 /// DestroyArray has type (T.Type, Builtin.RawPointer, Builtin.Word) -> ()

include/swift/AST/DiagnosticsSIL.def

@@ -179,8 +179,6 @@ ERROR(assignment_to_immutable_value,none,
 ERROR(missing_return,none,
       "missing return in a %select{function|closure}1 expected to return %0",
       (Type, unsigned))
-ERROR(return_from_noreturn,none, 
-      "return from a 'noreturn' function", ())
 ERROR(non_exhaustive_switch,none, 
       "switch must be exhaustive, consider adding a default clause", ())
 ERROR(guard_body_must_not_fallthrough,none,
@@ -190,7 +188,7 @@ WARNING(unreachable_code,none, "will never be executed", ())
 NOTE(unreachable_code_branch,none,
      "condition always evaluates to %select{false|true}0", (bool))
 NOTE(call_to_noreturn_note,none,
-     "a call to a noreturn function", ())
+     "a call to a never-returning function", ())
 WARNING(unreachable_code_after_stmt,none,
         "code after '%select{return|break|continue|throw}0' will never "
         "be executed", (unsigned))

lib/AST/Builtins.cpp

@@ -713,8 +713,7 @@ static ValueDecl *getVoidErrorOperation(ASTContext &Context, Identifier Id) {
 static ValueDecl *getUnexpectedErrorOperation(ASTContext &Context,
                                               Identifier Id) {
   return getBuiltinFunction(Id, {Context.getExceptionType()},
-                            TupleType::getEmpty(Context),
-                            AnyFunctionType::ExtInfo().withIsNoReturn());
+                            Context.getNeverType());
 }
 
 static ValueDecl *getCmpXChgOperation(ASTContext &Context, Identifier Id,
@@ -974,10 +973,8 @@ static ValueDecl *getIntToFPWithOverflowOperation(ASTContext &Context,
 
 static ValueDecl *getUnreachableOperation(ASTContext &Context,
                                           Identifier Id) {
-  // @noreturn () -> ()
-  auto VoidTy = Context.TheEmptyTupleType;
-  return getBuiltinFunction(Id, {}, VoidTy,
-                            AnyFunctionType::ExtInfo().withIsNoReturn(true));
+  // () -> Never
+  return getBuiltinFunction(Id, {}, Context.getNeverType());
 }
 
 static ValueDecl *getOnceOperation(ASTContext &Context,
@@ -1192,7 +1189,7 @@ getSwiftFunctionTypeForIntrinsic(unsigned iid, ArrayRef<Type> TypeArgs,
   Info = FunctionType::ExtInfo();
   if (attrs.hasAttribute(llvm::AttributeSet::FunctionIndex,
                          llvm::Attribute::NoReturn))
-    Info = Info.withIsNoReturn(true);
+    ResultTy = Context.getNeverType();
 
   return true;
 }

lib/ClangImporter/ImportDecl.cpp

@@ -3428,10 +3428,6 @@ namespace {
 
     void finishFuncDecl(const clang::FunctionDecl *decl,
                         AbstractFunctionDecl *result) {
-      if (decl->isNoReturn())
-        result->getAttrs().add(new (Impl.SwiftContext)
-                                   NoReturnAttr(/*IsImplicit=*/false));
-
       // Keep track of inline function bodies so that we can generate
       // IR from them using Clang's IR generator.
       if ((decl->isInlined() || decl->hasAttr<clang::AlwaysInlineAttr>() ||

lib/ClangImporter/ImportType.cpp

@@ -1518,12 +1518,12 @@ importFunctionType(DeclContext *dc,
   if (!parameterList)
     return Type();
 
-  FunctionType::ExtInfo extInfo;
-  extInfo = extInfo.withIsNoReturn(isNoReturn);
+  if (isNoReturn)
+    swiftResultTy = SwiftContext.getNeverType();
 
   // Form the function type.
   auto argTy = parameterList->getType(SwiftContext);
-  return FunctionType::get(argTy, swiftResultTy, extInfo);
+  return FunctionType::get(argTy, swiftResultTy);
 }
 
 ParameterList *ClangImporter::Implementation::importFunctionParameterList(
@@ -2430,9 +2430,13 @@ Type ClangImporter::Implementation::importMethodType(
 
   // Form the parameter list.
   *bodyParams = ParameterList::create(SwiftContext, swiftParams);
-  
+
+  if (isNoReturn) {
+    origSwiftResultTy = SwiftContext.getNeverType();
+    swiftResultTy = SwiftContext.getNeverType();
+  }
+
   FunctionType::ExtInfo extInfo;
-  extInfo = extInfo.withIsNoReturn(isNoReturn);
 
   if (errorInfo) {
     foreignErrorInfo = getForeignErrorInfo(*errorInfo, errorParamType,

lib/IDE/CodeCompletion.cpp

@@ -4063,7 +4063,7 @@ class CompletionOverrideLookup : public swift::VisibleDeclConsumer {
         Options.setArchetypeSelfTransform(transformType, VD->getDeclContext());
       Options.PrintDefaultParameterPlaceholder = false;
       Options.PrintImplicitAttrs = false;
-      Options.ExclusiveAttrList.push_back(DAK_NoReturn);
+      Options.SkipAttributes = true;
       Options.PrintOverrideKeyword = false;
       Options.PrintPropertyAccessors = false;
       VD->print(Printer, Options);
@@ -4141,7 +4141,7 @@ class CompletionOverrideLookup : public swift::VisibleDeclConsumer {
       llvm::raw_svector_ostream OS(DeclStr);
       PrintOptions Options;
       Options.PrintImplicitAttrs = false;
-      Options.ExclusiveAttrList.push_back(DAK_NoReturn);
+      Options.SkipAttributes = true;
       Options.PrintDefaultParameterPlaceholder = false;
       CD->print(OS, Options);
     }

lib/SIL/SILVerifier.cpp

@@ -797,7 +797,7 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
     // Check that if the apply is of a noreturn callee, make sure that an
     // unreachable is the next instruction.
     if (AI->getModule().getStage() == SILStage::Raw ||
-        !AI->getCallee()->getType().getAs<SILFunctionType>()->isNoReturn())
+        !AI->isCalleeNoReturn())
       return;
     require(isa<UnreachableInst>(std::next(SILBasicBlock::iterator(AI))),
             "No return apply without an unreachable as a next instruction.");

lib/SILOptimizer/Analysis/AliasAnalysis.cpp

@@ -641,9 +641,9 @@ AliasResult AliasAnalysis::aliasInner(SILValue V1, SILValue V2,
 }
 
 bool AliasAnalysis::canApplyDecrementRefCount(FullApplySite FAS, SILValue Ptr) {
-  // Treat applications of @noreturn functions as decrementing ref counts. This
+  // Treat applications of no-return functions as decrementing ref counts. This
   // causes the apply to become a sink barrier for ref count increments.
-  if (FAS.getCallee()->getType().getAs<SILFunctionType>()->isNoReturn())
+  if (FAS.isCalleeNoReturn())
     return true;
 
   /// If the pointer cannot escape to the function we are done.

lib/SILOptimizer/Analysis/CFG.cpp

@@ -81,7 +81,7 @@ findAllNonFailureExitBBs(SILFunction *F,
     // non-failure exit BB. Add it to our list and continue.
     auto PrevIter = std::prev(SILBasicBlock::iterator(TI));
     if (auto *AI = dyn_cast<ApplyInst>(&*PrevIter)) {
-      if (AI->getSubstCalleeType()->isNoReturn() &&
+      if (AI->isCalleeNoReturn() &&
           !isTrapNoReturnFunction(AI)) {
         BBs.push_back(&BB);
         continue;

lib/SILOptimizer/IPO/ClosureSpecializer.cpp

@@ -673,7 +673,7 @@ void ClosureSpecCloner::populateCloned() {
       auto NoReturnApply = FullApplySite::isa(&*PrevIter);
 
       // We insert the release value right before the no return apply so that if
-      // the partial apply is passed into the @noreturn function as an @owned
+      // the partial apply is passed into the no-return function as an @owned
       // value, we will retain the partial apply before we release it and
       // potentially eliminate it.
       Builder.setInsertionPoint(NoReturnApply.getInstruction());

lib/SILOptimizer/IPO/EagerSpecializer.cpp

@@ -301,7 +301,7 @@ void EagerDispatch::emitDispatchTo(SILFunction *NewFunc) {
     Result = Builder.createTuple(Loc, VoidTy, { });
 
   // Function marked as @NoReturn must be followed by 'unreachable'.
-  if (NewFunc->getLoweredFunctionType()->isNoReturn())
+  if (NewFunc->isNoReturnFunction())
     Builder.createUnreachable(Loc);
   else {
     auto GenResultTy = GenericFunc->mapTypeIntoContext(

lib/SILOptimizer/Mandatory/DataflowDiagnostics.cpp

@@ -46,11 +46,9 @@ static void diagnoseMissingReturn(const UnreachableInst *UI,
     llvm_unreachable("unhandled case in MissingReturn");
   }
 
-  bool isNoReturn = F->getLoweredFunctionType()->isNoReturn();
-
   // No action required if the function returns 'Void' or that the
   // function is marked 'noreturn'.
-  if (ResTy->isVoid() || isNoReturn)
+  if (ResTy->isVoid() || F->isNoReturnFunction())
     return;
 
   SILLocation L = UI->getLoc();
@@ -100,24 +98,6 @@ static void diagnoseUnreachable(const SILInstruction *I,
   }
 }
 
-static void diagnoseReturn(const SILInstruction *I, ASTContext &Context) {
-  auto *TI = dyn_cast<TermInst>(I);
-  if (!TI || !(isa<BranchInst>(TI) || isa<ReturnInst>(TI)))
-    return;
-
-  const SILBasicBlock *BB = TI->getParent();
-  const SILFunction *F = BB->getParent();
-
-  // Warn if we reach a return inside a noreturn function.
-  if (F->getLoweredFunctionType()->isNoReturn()) {
-    SILLocation L = TI->getLoc();
-    if (L.is<ReturnLocation>())
-      diagnose(Context, L.getSourceLoc(), diag::return_from_noreturn);
-    if (L.is<ImplicitReturnLocation>())
-      diagnose(Context, L.getSourceLoc(), diag::return_from_noreturn);
-  }
-}
-
 /// \brief Issue diagnostics whenever we see Builtin.static_report(1, ...).
 static void diagnoseStaticReports(const SILInstruction *I,
                                   SILModule &M) {
@@ -151,7 +131,6 @@ class EmitDFDiagnostics : public SILFunctionTransform {
     for (auto &BB : *getFunction())
       for (auto &I : BB) {
         diagnoseUnreachable(&I, M.getASTContext());
-        diagnoseReturn(&I, M.getASTContext());
         diagnoseStaticReports(&I, M);
       }
   }

lib/SILOptimizer/Mandatory/DiagnoseUnreachable.cpp

@@ -413,7 +413,7 @@ static void setOutsideBlockUsesToUndef(SILInstruction *I) {
 
 static SILInstruction *getAsCallToNoReturn(SILInstruction *I) {
   if (auto *AI = dyn_cast<ApplyInst>(I))
-    if (AI->getOrigCalleeType()->isNoReturn())
+    if (AI->isCalleeNoReturn())
       return AI;
 
   if (auto *BI = dyn_cast<BuiltinInst>(I)) {
@@ -448,7 +448,7 @@ static SILInstruction *getPrecedingCallToNoReturn(SILBasicBlock &BB) {
     // The predecessor must be the normal edge from a try_apply
     // that invokes a noreturn function.
     if (auto TAI = dyn_cast<TryApplyInst>((*i)->getTerminator())) {
-      if (TAI->getOrigCalleeType()->isNoReturn() &&
+      if (TAI->isCalleeNoReturn() &&
           TAI->isNormalSuccessorRef(i.getSuccessorRef())) {
         if (!first) first = TAI;
         continue;

lib/SILOptimizer/Transforms/FunctionSignatureOpts.cpp

@@ -499,7 +499,7 @@ void FunctionSignatureTransform::createFunctionSignatureOptimizedFunction() {
   }
 
   // Set up the return results.
-  if (NewF->getLoweredFunctionType()->isNoReturn()) {
+  if (NewF->isNoReturnFunction()) {
     Builder.createUnreachable(Loc);
   } else {
     Builder.createReturn(Loc, ReturnValue);

stdlib/public/core/Assert.swift

@@ -123,11 +123,11 @@ public func assertionFailure(
 /// * In -Ounchecked builds, the optimizer may assume that this
 ///   function will never be called. Failure to satisfy that assumption
 ///   is a serious programming error.
-@_transparent @noreturn
+@_transparent
 public func preconditionFailure(
   _ message: @autoclosure () -> String = String(),
   file: StaticString = #file, line: UInt = #line
-) {
+) -> Never {
   // Only check in debug and release mode.  In release mode just trap.
   if _isDebugAssertConfiguration() {
     _assertionFailed("fatal error", message(), file, line,
@@ -139,11 +139,11 @@ public func preconditionFailure(
 }
 
 /// Unconditionally print a `message` and stop execution.
-@_transparent @noreturn
+@_transparent
 public func fatalError(
   _ message: @autoclosure () -> String = String(),
   file: StaticString = #file, line: UInt = #line
-) {
+) -> Never {
   _assertionFailed("fatal error", message(), file, line,
     flags: _fatalErrorFlags())
 }
@@ -171,11 +171,11 @@ public func _precondition(
   }
 }
 
-@_transparent @noreturn
+@_transparent
 public func _preconditionFailure(
   _ message: StaticString = StaticString(),
   file: StaticString = #file, line: UInt = #line
-) {
+) -> Never {
   _precondition(false, message, file: file, line: line)
   _conditionallyUnreachable()
 }
@@ -222,10 +222,11 @@ public func _debugPrecondition(
   }
 }
 
-@_transparent @noreturn
+@_transparent
 public func _debugPreconditionFailure(
   _ message: StaticString = StaticString(),
-  file: StaticString = #file, line: UInt = #line) {
+  file: StaticString = #file, line: UInt = #line
+) -> Never {
   if _isDebugAssertConfiguration() {
     _precondition(false, message, file: file, line: line)
   }
@@ -251,11 +252,11 @@ public func _sanityCheck(
 #endif
 }
 
-@_transparent @noreturn
+@_transparent
 public func _sanityCheckFailure(
   _ message: StaticString = StaticString(),
   file: StaticString = #file, line: UInt = #line
-) {
+) -> Never {
   _sanityCheck(false, message, file: file, line: line)
   _conditionallyUnreachable()
 }

stdlib/public/core/AssertCommon.swift

@@ -109,14 +109,14 @@ func _reportUnimplementedInitializer(
 /// This function should not be inlined because it is cold and inlining just
 /// bloats code.
 @_versioned
-@noreturn @inline(never)
+@inline(never)
 @_semantics("stdlib_binary_only")
 func _assertionFailed(
   // FIXME(ABI): add argument labels to conform to API guidelines.
   _ prefix: StaticString, _ message: StaticString,
   _ file: StaticString, _ line: UInt,
   flags: UInt32
-) {
+) -> Never {
   prefix.withUTF8Buffer {
     (prefix) -> Void in
     message.withUTF8Buffer {
@@ -141,14 +141,14 @@ func _assertionFailed(
 /// This function should not be inlined because it is cold and inlining just
 /// bloats code.
 @_versioned
-@noreturn @inline(never)
+@inline(never)
 @_semantics("stdlib_binary_only")
 func _assertionFailed(
   // FIXME(ABI): add argument labels to conform to API guidelines.
   _ prefix: StaticString, _ message: String,
   _ file: StaticString, _ line: UInt,
   flags: UInt32
-) {
+) -> Never {
   prefix.withUTF8Buffer {
     (prefix) -> Void in
     message._withUnsafeBufferPointerToUTF8 {
@@ -173,15 +173,15 @@ func _assertionFailed(
 /// This function should not be inlined because it is cold and it inlining just
 /// bloats code.
 @_versioned
-@noreturn @inline(never)
+@inline(never)
 @_semantics("stdlib_binary_only")
 @_semantics("arc.programtermination_point")
 func _fatalErrorMessage(
   // FIXME(ABI): add argument labels to conform to API guidelines.
   _ prefix: StaticString, _ message: StaticString,
   _ file: StaticString, _ line: UInt,
   flags: UInt32
-) {
+) -> Never {
 #if INTERNAL_CHECKS_ENABLED
   prefix.withUTF8Buffer {
     (prefix) in
@@ -215,14 +215,14 @@ func _fatalErrorMessage(
 
 /// Prints a fatal error message when an unimplemented initializer gets
 /// called by the Objective-C runtime.
-@_transparent @noreturn
+@_transparent
 public // COMPILER_INTRINSIC
 func _unimplementedInitializer(className: StaticString,
                                initName: StaticString = #function,
                                file: StaticString = #file,
                                line: UInt = #line,
                                column: UInt = #column
-) {
+) -> Never {
   // This function is marked @_transparent so that it is inlined into the caller
   // (the initializer stub), and, depending on the build configuration,
   // redundant parameter values (#file etc.) are eliminated, and don't leak
@@ -260,7 +260,6 @@ func _unimplementedInitializer(className: StaticString,
 }
 
 // FIXME(ABI): rename to something descriptive.
-@noreturn
 public // COMPILER_INTRINSIC
 func _undefined<T>(
   _ message: @autoclosure () -> String = String(),