Enforce consistent usage of -experimental-hermetic-seat-at-link flag (#39986)

We've recently added the -experimental-hermetic-seal-at-link compiler flag,
which turns on aggressive dead-stripping optimizations and assumes that library
code can be optimized against client code because all users of the library
code/types are present at link/LTO time. This means that any module that's
built with -experimental-hermetic-seal-at-link requires all clients of this
module to also use -experimental-hermetic-seal-at-link. This PR enforces that
by storing a bit in the serialized module, and checking the bit when importing
modules.

Author: kubamracek

include/swift/AST/Decl.h

@@ -594,7 +594,7 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
     HasAnyUnavailableValues : 1
   );
 
-  SWIFT_INLINE_BITFIELD(ModuleDecl, TypeDecl, 1+1+1+1+1+1+1+1+1+1+1+1,
+  SWIFT_INLINE_BITFIELD(ModuleDecl, TypeDecl, 1+1+1+1+1+1+1+1+1+1+1+1+1,
     /// If the module is compiled as static library.
     StaticLibrary : 1,
 
@@ -631,6 +631,9 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
     /// Whether this module has incremental dependency information available.
     HasIncrementalInfo : 1,
 
+    /// Whether this module was built with -experimental-hermetic-seal-at-link.
+    HasHermeticSealAtLink : 1,
+
     /// Whether this module has been compiled with comprehensive checking for
     /// concurrency, e.g., Sendable checking.
     IsConcurrencyChecked : 1

include/swift/AST/DiagnosticsSema.def

@@ -832,6 +832,10 @@ WARNING(serialization_malformed_sourceinfo,none,
       "'%0' is either malformed or generated by a different Swift version. "
       "Note that it uses an unstable format and may leak internal project "
       "details, it should not be distributed alongside modules", (StringRef))
+ERROR(need_hermetic_seal_to_import_module,none,
+      "module %0 was built with -experimental-hermetic-seal-at-link, but "
+      "current compilation does not have -experimental-hermetic-seal-at-link",
+      (Identifier))
 
 ERROR(reserved_member_name,none,
       "type member must not be named %0, since it would conflict with the"

include/swift/AST/Module.h

@@ -517,6 +517,15 @@ class ModuleDecl
     Bits.ModuleDecl.HasIncrementalInfo = enabled;
   }
 
+  /// Returns true if this module was built with
+  /// -experimental-hermetic-seal-at-link.
+  bool hasHermeticSealAtLink() const {
+    return Bits.ModuleDecl.HasHermeticSealAtLink;
+  }
+  void setHasHermeticSealAtLink(bool enabled = true) {
+    Bits.ModuleDecl.HasHermeticSealAtLink = enabled;
+  }
+
   /// \returns true if this module is a system module; note that the StdLib is
   /// considered a system module.
   bool isSystemModule() const {

include/swift/Basic/LangOptions.h

@@ -439,6 +439,11 @@ namespace swift {
     /// Load swiftmodule files in memory as volatile and avoid mmap.
     bool EnableVolatileModules = false;
 
+    /// Enable experimental 'hermetic seal at link' feature. Turns on
+    /// dead-stripping optimizations assuming that all users of library code
+    /// are present at LTO time.
+    bool HermeticSealAtLink = false;
+
     /// Allow deserializing implementation only dependencies. This should only
     /// be set true by lldb and other tooling, so that deserilization
     /// recovery issues won't bring down the debugger.

include/swift/Frontend/FrontendOptions.h

@@ -341,6 +341,11 @@ class FrontendOptions {
   /// True if the "-static" option is set.
   bool Static = false;
 
+  /// True if building with -experimental-hermetic-seal-at-link. Turns on
+  /// dead-stripping optimizations assuming that all users of library code
+  /// are present at LTO time.
+  bool HermeticSealAtLink = false;
+
   /// The different modes for validating TBD against the LLVM IR.
   enum class TBDValidationMode {
     Default,        ///< Do the default validation for the current platform.

include/swift/Serialization/SerializationOptions.h

@@ -142,6 +142,7 @@ namespace swift {
     bool IsSIB = false;
     bool DisableCrossModuleIncrementalInfo = false;
     bool StaticLibrary = false;
+    bool HermeticSealAtLink = false;
     bool IsOSSA = false;
   };
 

include/swift/Serialization/Validation.h

@@ -109,7 +109,8 @@ class ExtendedValidationInfo {
   struct {
     unsigned ArePrivateImportsEnabled : 1;
     unsigned IsSIB : 1;
-    unsigned IsStaticLibrary: 1;
+    unsigned IsStaticLibrary : 1;
+    unsigned HasHermeticSealAtLink : 1;
     unsigned IsTestable : 1;
     unsigned ResilienceStrategy : 2;
     unsigned IsImplicitDynamicEnabled : 1;
@@ -148,6 +149,10 @@ class ExtendedValidationInfo {
   void setIsStaticLibrary(bool val) {
     Bits.IsStaticLibrary = val;
   }
+  bool hasHermeticSealAtLink() const { return Bits.HasHermeticSealAtLink; }
+  void setHasHermeticSealAtLink(bool val) {
+    Bits.HasHermeticSealAtLink = val;
+  }
   bool isTestable() const { return Bits.IsTestable; }
   void setIsTestable(bool val) {
     Bits.IsTestable = val;

lib/AST/Module.cpp

@@ -488,6 +488,7 @@ ModuleDecl::ModuleDecl(Identifier name, ASTContext &ctx,
   Bits.ModuleDecl.IsNonSwiftModule = 0;
   Bits.ModuleDecl.IsMainModule = 0;
   Bits.ModuleDecl.HasIncrementalInfo = 0;
+  Bits.ModuleDecl.HasHermeticSealAtLink = 0;
   Bits.ModuleDecl.IsConcurrencyChecked = 0;
 }
 

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -290,6 +290,8 @@ bool ArgsToFrontendOptionsConverter::convert(
 
   Opts.Static = Args.hasArg(OPT_static);
 
+  Opts.HermeticSealAtLink = Args.hasArg(OPT_experimental_hermetic_seal_at_link);
+
   return false;
 }
 

lib/Frontend/CompilerInvocation.cpp

@@ -736,6 +736,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.EnableVolatileModules |= Args.hasArg(OPT_enable_volatile_modules);
 
+  Opts.HermeticSealAtLink |= Args.hasArg(OPT_experimental_hermetic_seal_at_link);
+
   Opts.UseDarwinPreStableABIBit =
     (Target.isMacOSX() && Target.isMacOSXVersionLT(10, 14, 4)) ||
     (Target.isiOS() && Target.isOSVersionLT(12, 2)) ||