[Macros] Use private discriminators for expression macros

Expression macros ascribed to non-private contexts need private
discriminators so they don't conflict with other uses of the same macro
in other source files.

Thank you, Richard, for noticing this omission!

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 { return PrivateDiscriminator; }
+  Identifier getPrivateDiscriminator() const;
   Optional<ExternalSourceLocs::RawLocs>
   getExternalRawLocsForDecl(const Decl *D) const override;
 

lib/AST/ASTMangler.cpp

@@ -3994,10 +3994,40 @@ void ASTMangler::appendMacroExpansionOperator(
   }
 }
 
+static StringRef getPrivateDiscriminatorIfNecessary(
+      const MacroExpansionExpr *expansion) {
+  auto dc = MacroDiscriminatorContext::getInnermostMacroContext(
+      expansion->getDeclContext());
+  auto decl = dc->getAsDecl();
+  if (decl && !decl->isOutermostPrivateOrFilePrivateScope())
+    return StringRef();
+
+  // Mangle non-local private declarations with a textual discriminator
+  // based on their enclosing file.
+  auto topLevelSubcontext = dc->getModuleScopeContext();
+  SourceFile *sf = dyn_cast<SourceFile>(topLevelSubcontext);
+  if (!sf)
+    return StringRef();
+
+  Identifier discriminator = sf->getPrivateDiscriminator();
+  assert(!discriminator.empty());
+  assert(!isNonAscii(discriminator.str()) &&
+         "discriminator contains non-ASCII characters");
+  (void)&isNonAscii;
+  assert(!clang::isDigit(discriminator.str().front()) &&
+         "not a valid identifier");
+  return discriminator.str();
+}
+
 std::string ASTMangler::mangleMacroExpansion(
     const MacroExpansionExpr *expansion) {
   beginMangling();
   appendMacroExpansionContext(expansion->getLoc(), expansion->getDeclContext());
+  auto privateDiscriminator = getPrivateDiscriminatorIfNecessary(expansion);
+  if (!privateDiscriminator.empty()) {
+    appendIdentifier(privateDiscriminator);
+    appendOperator("Ll");
+  }
   appendMacroExpansionOperator(
       expansion->getMacroName().getBaseName().userFacingName(),
       MacroRole::Expression,

lib/AST/Module.cpp

@@ -3852,11 +3852,7 @@ ASTScope &SourceFile::getScope() {
   return *Scope.get();
 }
 
-Identifier
-SourceFile::getDiscriminatorForPrivateDecl(const Decl *D) const {
-  assert(D->getDeclContext()->getModuleScopeContext() == this ||
-         D->getDeclContext()->getModuleScopeContext() == getSynthesizedFile());
-
+Identifier SourceFile::getPrivateDiscriminator() const {
   if (!PrivateDiscriminator.empty())
     return PrivateDiscriminator;
 
@@ -3894,6 +3890,13 @@ SourceFile::getDiscriminatorForPrivateDecl(const Decl *D) const {
   return PrivateDiscriminator;
 }
 
+Identifier
+SourceFile::getDiscriminatorForPrivateDecl(const Decl *D) const {
+  assert(D->getDeclContext()->getModuleScopeContext() == this ||
+         D->getDeclContext()->getModuleScopeContext() == getSynthesizedFile());
+  return getPrivateDiscriminator();
+}
+
 SynthesizedFileUnit *FileUnit::getSynthesizedFile() const {
   return cast_or_null<SynthesizedFileUnit>(SynthesizedFileAndKind.getPointer());
 }

test/Macros/top_level_freestanding.swift

@@ -71,5 +71,5 @@ func testArbitraryAtGlobal() {
 
 @freestanding(expression) macro stringify<T>(_ value: T) -> (T, String) = #externalMacro(module: "MacroDefinition", type: "StringifyMacro")
 
-// DIAG_BUFFERS: @__swiftmacro_9MacroUser9stringifyfMf1_{{.*}}warning: 'deprecated()' is deprecated
-// DIAG_BUFFERS: @__swiftmacro_9MacroUser9stringifyfMf2_{{.*}}warning: 'deprecated()' is deprecated
+// DIAG_BUFFERS: @__swiftmacro_9MacroUser33_{{.*}}9stringifyfMf1_{{.*}}warning: 'deprecated()' is deprecated
+// DIAG_BUFFERS: @__swiftmacro_9MacroUser33_{{.*}}9stringifyfMf2_{{.*}}warning: 'deprecated()' is deprecated