[Concurrency] SILGenModule::emitEntryPoint crashes when exit hasn't been imported

If `exit` hasn't been imported, SILGenModule::emitEntryPoint crashes because it doesn't synthesize an exit function like SILGenFunction::emitCallToMain does. Make it do so.

rdar://122413238

Author: ian-twilightcoder

lib/SILGen/SILGenTopLevel.cpp

@@ -20,6 +20,23 @@
 using namespace swift;
 using namespace Lowering;
 
+static FuncDecl *synthesizeExit(ASTContext &ctx, ModuleDecl *moduleDecl) {
+  // Synthesize an exit function with this interface.
+  // @_extern(c)
+  // func exit(_: Int32) -> Never
+  ParameterList *params =
+      ParameterList::createWithoutLoc(ParamDecl::createImplicit(
+          ctx, Identifier(), Identifier(), ctx.getInt32Type(), moduleDecl));
+  FuncDecl *exitFuncDecl = FuncDecl::createImplicit(
+      ctx, StaticSpellingKind::None,
+      DeclName(ctx, DeclBaseName(ctx.getIdentifier("exit")), params), {},
+      /*async*/ false, /*throws*/ false, /*thrownType*/ Type(), {}, params,
+      ctx.getNeverType(), moduleDecl);
+  exitFuncDecl->getAttrs().add(new (ctx) ExternAttr(
+      llvm::None, llvm::None, ExternKind::C, /*implicit*/ true));
+  return exitFuncDecl;
+}
+
 void SILGenModule::emitEntryPoint(SourceFile *SF, SILFunction *TopLevel) {
 
   auto EntryRef = SILDeclRef::getMainFileEntryPoint(SF);
@@ -85,7 +102,10 @@ void SILGenModule::emitEntryPoint(SourceFile *SF, SILFunction *TopLevel) {
   SILType returnType;
   if (isAsyncTopLevel) {
     FuncDecl *exitFuncDecl = getExit();
-    assert(exitFuncDecl && "Failed to find exit function declaration");
+    if (!exitFuncDecl) {
+      // If it doesn't exist, we can conjure one up instead of crashing
+      exitFuncDecl = synthesizeExit(getASTContext(), TopLevel->getModule().getSwiftModule());
+    }
     exitFunc = getFunction(
         SILDeclRef(exitFuncDecl, SILDeclRef::Kind::Func, /*isForeign*/ true),
         NotForDefinition);
@@ -216,20 +236,7 @@ void SILGenFunction::emitCallToMain(FuncDecl *mainFunc) {
     FuncDecl *exitFuncDecl = SGM.getExit();
     if (!exitFuncDecl) {
       // If it doesn't exist, we can conjure one up instead of crashing
-      // @_extern(c)
-      // func exit(_: Int32) -> Never
-      ASTContext &ctx = getASTContext();
-      ModuleDecl *moduleDecl = mainFunc->getModuleContext();
-      ParameterList *params =
-          ParameterList::createWithoutLoc(ParamDecl::createImplicit(
-              ctx, Identifier(), Identifier(), ctx.getInt32Type(), moduleDecl));
-      exitFuncDecl = FuncDecl::createImplicit(
-          ctx, StaticSpellingKind::None,
-          DeclName(ctx, DeclBaseName(ctx.getIdentifier("exit")), params), {},
-          /*async*/ false, /*throws*/ false, /*thrownType*/ Type(), {}, params,
-          ctx.getNeverType(), moduleDecl);
-      exitFuncDecl->getAttrs().add(new (ctx) ExternAttr(
-          llvm::None, llvm::None, ExternKind::C, /*implicit*/ true));
+      exitFuncDecl = synthesizeExit(getASTContext(), mainFunc->getModuleContext());
     }
     SILFunction *exitSILFunc = SGM.getFunction(
         SILDeclRef(exitFuncDecl, SILDeclRef::Kind::Func, /*isForeign*/ true),

test/Concurrency/async_main_no_exit.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift -parse-as-library -disable-availability-checking -parse-stdlib
+// expect-no-diagnostics
+
+import _Concurrency
+
+@main struct Main {
+  static func main() async {
+  }
+}

test/Concurrency/async_top_level_no_exit.swift

@@ -0,0 +1,9 @@
+// RUN: %target-typecheck-verify-swift -disable-availability-checking -parse-stdlib
+// expect-no-diagnostics
+
+import _Concurrency
+
+func foo() async {
+}
+
+await foo()