SILGen: Handle nested functions in shouldSkipDecl().

Functions that are nested within other functions are not exportable to clients
since they cannot be called directly by clients. However, they need still to be
emitted during SILGen when -experimental-skip-non-exportable-decls is specified
so walk the decl context hierarchy when determining whether to skip emitting a
function.

Resolves rdar://117440503

Author: tshortli

lib/SILGen/SILGen.cpp

@@ -801,11 +801,28 @@ bool SILGenModule::shouldSkipDecl(Decl *D) {
   if (!D->isAvailableDuringLowering())
     return true;
 
-  if (getASTContext().SILOpts.SkipNonExportableDecls &&
-      !D->isExposedToClients())
+  if (!getASTContext().SILOpts.SkipNonExportableDecls)
+    return false;
+
+  if (auto *afd = dyn_cast<AbstractFunctionDecl>(D)) {
+    do {
+      if (afd->isExposedToClients())
+        return false;
+
+      // If this function is nested within another function that is exposed to
+      // clients then it should be emitted.
+      auto dc = afd->getDeclContext()->getAsDecl();
+      afd = dc ? dyn_cast<AbstractFunctionDecl>(dc) : nullptr;
+    } while (afd);
+
+    // We didn't find a parent function that is exposed.
     return true;
+  }
 
-  return false;
+  if (D->isExposedToClients())
+    return false;
+
+  return true;
 }
 
 void SILGenModule::visit(Decl *D) {

test/SILGen/skip-non-exportable-decls.swift

@@ -12,9 +12,39 @@ private func privateFunc() {}
 // CHECK-SKIP-NOT: s4Test12internalFuncyyF
 internal func internalFunc() {}
 
+// CHECK-NO-SKIP: sil hidden{{.*}} @$s4Test022internalFuncWithNestedC0yyF : $@convention(thin) () -> () {
+// CHECK-SKIP-NOT: s4Test022internalFuncWithNestedC0yyF
+internal func internalFuncWithNestedFunc() {
+  func nested() {}
+  nested()
+}
+// CHECK-NO-SKIP: sil private{{.*}} @$s4Test022internalFuncWithNestedC0yyF6nestedL_yyF : $@convention(thin) () -> () {
+// CHECK-SKIP-NOT: s4Test022internalFuncWithNestedC0yyF6nestedL_yyF
+
 // CHECK: sil{{.*}} @$s4Test10publicFuncyyF : $@convention(thin) () -> () {
 public func publicFunc() {}
 
+// CHECK: sil{{.*}} @$s4Test25publicFuncWithNestedFuncsyyF : $@convention(thin) () -> () {
+public func publicFuncWithNestedFuncs() {
+  defer { publicFunc() }
+  func nested() {}
+  nested()
+}
+// CHECK: sil private{{.*}} @$s4Test25publicFuncWithNestedFuncsyyF6$deferL_yyF : $@convention(thin) () -> () {
+// CHECK: sil private{{.*}} @$s4Test25publicFuncWithNestedFuncsyyF6nestedL_yyF : $@convention(thin) () -> () {
+
+// CHECK: sil [serialized]{{.*}} @$s4Test13inlinableFuncyyF : $@convention(thin) () -> () {
+@inlinable internal func inlinableFunc() {}
+
+// CHECK: sil [serialized]{{.*}} @$s4Test023inlinableFuncWithNestedC0yyF : $@convention(thin) () -> () {
+@inlinable internal func inlinableFuncWithNestedFunc() {
+  defer { publicFunc() }
+  func nested() {}
+  nested()
+}
+// CHECK: sil shared [serialized]{{.*}} @$s4Test023inlinableFuncWithNestedC0yyF6$deferL_yyF : $@convention(thin) () -> () {
+// CHECK: sil shared [serialized]{{.*}} @$s4Test023inlinableFuncWithNestedC0yyF6nestedL_yyF : $@convention(thin) () -> () {
+
 private class PrivateClass {
   // CHECK-NO-SKIP: sil private{{.*}} @$s4Test12PrivateClass33_E3F0E1C7B46D05C8067CB98677DE566CLLCfd : $@convention(method) (@guaranteed PrivateClass) -> @owned Builtin.NativeObject {
   // CHECK-SKIP-NOT: s4Test12PrivateClass33_E3F0E1C7B46D05C8067CB98677DE566CLLCfd