Only synthesize a private discriminator when needed

Author: DougGregor

include/swift/AST/SourceFile.h

@@ -496,7 +496,7 @@ class SourceFile final : public FileUnit {
   collectLinkLibraries(ModuleDecl::LinkLibraryCallback callback) const override;
 
   Identifier getDiscriminatorForPrivateDecl(const Decl *D) const override;
-  Identifier getPrivateDiscriminator() const;
+  Identifier getPrivateDiscriminator(bool createIfMissing = false) const;
   Optional<ExternalSourceLocs::RawLocs>
   getExternalRawLocsForDecl(const Decl *D) const override;
 

lib/AST/ASTMangler.cpp

@@ -4009,7 +4009,8 @@ static StringRef getPrivateDiscriminatorIfNecessary(
   if (!sf)
     return StringRef();
 
-  Identifier discriminator = sf->getPrivateDiscriminator();
+  Identifier discriminator =
+      sf->getPrivateDiscriminator(/*createIfMissing=*/true);
   assert(!discriminator.empty());
   assert(!isNonAscii(discriminator.str()) &&
          "discriminator contains non-ASCII characters");

lib/AST/Module.cpp

@@ -3852,8 +3852,8 @@ ASTScope &SourceFile::getScope() {
   return *Scope.get();
 }
 
-Identifier SourceFile::getPrivateDiscriminator() const {
-  if (!PrivateDiscriminator.empty())
+Identifier SourceFile::getPrivateDiscriminator(bool createIfMissing) const {
+  if (!PrivateDiscriminator.empty() || !createIfMissing)
     return PrivateDiscriminator;
 
   StringRef name = getFilename();
@@ -3894,7 +3894,7 @@ Identifier
 SourceFile::getDiscriminatorForPrivateDecl(const Decl *D) const {
   assert(D->getDeclContext()->getModuleScopeContext() == this ||
          D->getDeclContext()->getModuleScopeContext() == getSynthesizedFile());
-  return getPrivateDiscriminator();
+  return getPrivateDiscriminator(/*createIfMissing=*/true);
 }
 
 SynthesizedFileUnit *FileUnit::getSynthesizedFile() const {