ModuleInterface: Skip attributes referencing SPI platforms.

When printing the public `.swiftinterface` for a `-library-level=api` module,
skip emitting `@available`, `@backDeployed`, and `@_originallyDefinedIn`
attributes that reference "SPI" platforms.

Resolves rdar://133990140.

Author: tshortli

include/swift/AST/PlatformKind.h

@@ -91,6 +91,10 @@ bool inheritsAvailabilityFromPlatform(PlatformKind Child, PlatformKind Parent);
 llvm::VersionTuple canonicalizePlatformVersion(
     PlatformKind platform, const llvm::VersionTuple &version);
 
+/// Returns true if \p Platform should be considered to be SPI and therefore not
+/// printed in public `.swiftinterface` files, for example.
+bool isPlatformSPI(PlatformKind Platform);
+
 } // end namespace swift
 
 #endif

lib/AST/Attr.cpp

@@ -973,6 +973,30 @@ static void printDifferentiableAttrArguments(
   printer << '(' << stream.str() << ')';
 }
 
+/// Returns the `PlatformKind` referenced by \p attr if applicable, or
+/// `std::nullopt` otherwise.
+static std::optional<PlatformKind>
+referencedPlatform(const DeclAttribute *attr) {
+  switch (attr->getKind()) {
+  case DeclAttrKind::Available:
+    return static_cast<const AvailableAttr *>(attr)->Platform;
+  case DeclAttrKind::BackDeployed:
+    return static_cast<const BackDeployedAttr *>(attr)->Platform;
+  case DeclAttrKind::OriginallyDefinedIn:
+    return static_cast<const OriginallyDefinedInAttr *>(attr)->Platform;
+  default:
+    return std::nullopt;
+  }
+}
+
+/// Returns true if \p attr contains a reference to a `PlatformKind` that should
+/// be considered SPI.
+static bool referencesSPIPlatform(const DeclAttribute *attr) {
+  if (auto platform = referencedPlatform(attr))
+    return isPlatformSPI(*platform);
+  return false;
+}
+
 void DeclAttributes::print(ASTPrinter &Printer, const PrintOptions &Options,
                            const Decl *D) const {
   if (!DeclAttrs)
@@ -995,6 +1019,8 @@ void DeclAttributes::print(ASTPrinter &Printer, const PrintOptions &Options,
   AttributeVector longAttributes;
   AttributeVector attributes;
   AttributeVector modifiers;
+  bool libraryLevelAPI =
+      D->getASTContext().LangOpts.LibraryLevel == LibraryLevel::API;
 
   for (auto DA : llvm::reverse(FlattenedAttrs)) {
     // Don't skip implicit custom attributes. Custom attributes like global
@@ -1010,6 +1036,12 @@ void DeclAttributes::print(ASTPrinter &Printer, const PrintOptions &Options,
     if (Options.excludeAttrKind(DA->getKind()))
       continue;
 
+    // In the public interfaces of -library-level=api modules, skip attributes
+    // that reference SPI platforms.
+    if (Options.printPublicInterface() && libraryLevelAPI &&
+        referencesSPIPlatform(DA))
+      continue;
+
     // If we're supposed to suppress expanded macros, check whether this is
     // a macro.
     if (Options.SuppressExpandedMacros) {

lib/AST/PlatformKind.cpp

@@ -262,3 +262,25 @@ llvm::VersionTuple swift::canonicalizePlatformVersion(
 
   return version;
 }
+
+bool swift::isPlatformSPI(PlatformKind Platform) {
+  switch (Platform) {
+  case PlatformKind::macOS:
+  case PlatformKind::macOSApplicationExtension:
+  case PlatformKind::iOS:
+  case PlatformKind::iOSApplicationExtension:
+  case PlatformKind::macCatalyst:
+  case PlatformKind::macCatalystApplicationExtension:
+  case PlatformKind::tvOS:
+  case PlatformKind::tvOSApplicationExtension:
+  case PlatformKind::watchOS:
+  case PlatformKind::watchOSApplicationExtension:
+  case PlatformKind::visionOS:
+  case PlatformKind::visionOSApplicationExtension:
+  case PlatformKind::OpenBSD:
+  case PlatformKind::Windows:
+  case PlatformKind::none:
+    return false;
+  }
+  llvm_unreachable("bad PlatformKind");
+}