[Mangler] Use '_' to represent nameless entities.

This can show up when trying to generate USRs for a document with
errors in it. This isn't a great answer because the names it generates
aren't unique (there may be more than one nameless entity with the
same type), but it at least generates valid mangled names.

When generating mangled names for purposes other than USRs, nameless
entities are now checked for by an assertion.

Author: jrose-apple

include/swift/AST/ASTMangler.h

@@ -38,6 +38,11 @@ class ASTMangler : public Mangler {
   /// If enabled, Arche- and Alias types are mangled with context.
   bool DWARFMangling;
 
+  /// If enabled, entities that ought to have names but don't get a placeholder.
+  ///
+  /// If disabled, it is an error to try to mangle such an entity.
+  bool AllowNamelessEntities = false;
+
 public:
   enum class SymbolKind {
     Default,

lib/AST/ASTMangler.cpp

@@ -327,11 +327,14 @@ std::string ASTMangler::mangleObjCRuntimeName(const NominalTypeDecl *Nominal) {
 }
 
 std::string ASTMangler::mangleTypeAsContextUSR(const NominalTypeDecl *type) {
+  llvm::SaveAndRestore<bool> allowUnnamedRAII(AllowNamelessEntities, true);
   appendContext(type);
   return finalize();
 }
 
-std::string ASTMangler::mangleDeclAsUSR(ValueDecl *Decl, StringRef USRPrefix) {
+std::string ASTMangler::mangleDeclAsUSR(const ValueDecl *Decl,
+                                        StringRef USRPrefix) {
+  llvm::SaveAndRestore<bool> allowUnnamedRAII(AllowNamelessEntities, true);
   Buffer << USRPrefix;
   bindGenericParameters(Decl->getDeclContext());
 
@@ -354,6 +357,7 @@ std::string ASTMangler::mangleAccessorEntityAsUSR(AccessorKind kind,
                                                   AddressorKind addressorKind,
                                                   const ValueDecl *decl,
                                                   StringRef USRPrefix) {
+  llvm::SaveAndRestore<bool> allowUnnamedRAII(AllowNamelessEntities, true);
   Buffer << USRPrefix;
   appendAccessorEntity(kind, addressorKind, decl, /*isStatic*/ false);
   return finalize();
@@ -444,8 +448,13 @@ void ASTMangler::appendDeclName(const ValueDecl *decl) {
         appendOperator("oi");
         break;
     }
-  } else {
+  } else if (decl->hasName()) {
     appendIdentifier(decl->getName().str());
+  } else {
+    assert(AllowNamelessEntities && "attempt to mangle unnamed decl");
+    // Fall back to an unlikely name, so that we still generate a valid
+    // mangled name.
+    appendIdentifier("_");
   }
 
   if (decl->getDeclContext()->isLocalContext()) {

test/Demangle/Inputs/manglings.txt

@@ -250,3 +250,4 @@ _T08mangling14varargsVsArrayySaySiG3arrd_SS1ntF ---> mangling.varargsVsArray(arr
 _T08mangling14varargsVsArrayySaySiG3arrd_tF ---> mangling.varargsVsArray(arr: Swift.Int...) -> ()
 _T0s13_UnicodeViewsVss22RandomAccessCollectionRzs0A8EncodingR_11SubSequence_5IndexQZAFRtzsAcERpzAE_AEQZAIRSs15UnsignedInteger8Iterator_7ElementRPzAE_AlMQZANRS13EncodedScalar_AlMQY_AORSr0_lE13CharacterViewVyxq__G ---> (extension in Swift):Swift._UnicodeViews<A, B><A, B where A: Swift.RandomAccessCollection, B: Swift.UnicodeEncoding, A.Index == A.SubSequence.Index, A.SubSequence: Swift.RandomAccessCollection, A.SubSequence == A.SubSequence.SubSequence, A.Iterator.Element: Swift.UnsignedInteger, A.Iterator.Element == A.SubSequence.Iterator.Element, A.SubSequence.Iterator.Element == B.EncodedScalar.Iterator.Element>.CharacterView
 _T010Foundation11MeasurementV12SimulatorKitSo9UnitAngleCRszlE11OrientationO2eeoiSbAcDEAGOyAF_G_AKtFZ ---> static (extension in SimulatorKit):Foundation.Measurement<A where A == __ObjC.UnitAngle>.Orientation.== infix((extension in SimulatorKit):Foundation.Measurement<__ObjC.UnitAngle>.Orientation, (extension in SimulatorKit):Foundation.Measurement<__ObjC.UnitAngle>.Orientation) -> Swift.Bool
+_T04main1_yyF ---> main._() -> ()

test/Demangle/Inputs/simplified-manglings.txt

@@ -202,3 +202,4 @@ _TFE1a ---> _TFE1a
 _TFC4testP33_83378C430F65473055F1BD53F3ADCDB71C5doFoofT_T_ ---> C.doFoo()
 _TTRXFo_oCSo13SKPhysicsBodydVSC7CGPointdVSC8CGVectordGSpV10ObjectiveC8ObjCBool___XFdCb_dS_dS0_dS1_dGSpS3____ ---> thunk for @callee_owned (@owned SKPhysicsBody, @unowned CGPoint, @unowned CGVector, @unowned UnsafeMutablePointer<ObjCBool>) -> ()
 _T0So13SKPhysicsBodyCSC7CGPointVSC8CGVectorVSpy10ObjectiveC8ObjCBoolVGIxxyyy_AbdFSpyAIGIyByyyy_TR ---> thunk for @callee_owned (@owned SKPhysicsBody, @unowned CGPoint, @unowned CGVector, @unowned UnsafeMutablePointer<ObjCBool>) -> ()
+_T04main1_yyF ---> _()