SwiftRemoteMirror: Fix crash emitting capture descriptors involving implicit self (#2638)

When emitting capture descriptors for functions with a smaller number of parameters
than SIL parameters, the compiler can crash indexing into the heap layout's element
types, because the capture index underflows to UINT_MAX.

rdar://problem/26404583

Author: bitjammer

lib/IRGen/GenFunc.cpp

@@ -1120,10 +1120,6 @@ void irgen::emitFunctionPartialApplication(IRGenFunction &IGF,
     argConventions.push_back(ParameterConvention::Direct_Unowned);
   }
 
-  // For capture descriptors, we need to know where the capture list begins
-  // in the lowered SIL type of the callee.
-  unsigned firstCaptureIndex = origType->getNumSILArguments() - params.size();
-
   // Collect the type infos for the context parameters.
   for (auto param : params) {
     SILType argType = param.getSILType();
@@ -1305,8 +1301,7 @@ void irgen::emitFunctionPartialApplication(IRGenFunction &IGF,
 
   auto descriptor = IGF.IGM.getAddrOfCaptureDescriptor(SILFn, origType,
                                                        substType, subs,
-                                                       layout,
-                                                       firstCaptureIndex);
+                                                       layout);
 
   llvm::Value *data;
   if (layout.isKnownEmpty()) {

lib/IRGen/GenReflection.cpp

@@ -525,20 +525,18 @@ class CaptureDescriptorBuilder : public ReflectionMetadataBuilder {
   CanSILFunctionType SubstCalleeType;
   ArrayRef<Substitution> Subs;
   HeapLayout &Layout;
-  unsigned FirstCaptureIndex;
 public:
   CaptureDescriptorBuilder(IRGenModule &IGM,
                            llvm::SetVector<CanType> &BuiltinTypes,
                            SILFunction &Caller,
                            CanSILFunctionType OrigCalleeType,
                            CanSILFunctionType SubstCalleeType,
                            ArrayRef<Substitution> Subs,
-                           HeapLayout &Layout,
-                           unsigned FirstCaptureIndex)
+                           HeapLayout &Layout)
     : ReflectionMetadataBuilder(IGM, BuiltinTypes),
       Caller(Caller), OrigCalleeType(OrigCalleeType),
       SubstCalleeType(SubstCalleeType), Subs(Subs),
-      Layout(Layout), FirstCaptureIndex(FirstCaptureIndex) {}
+      Layout(Layout) {}
 
   using MetadataSourceMap
     = std::vector<std::pair<CanType, const reflection::MetadataSource*>>;
@@ -617,15 +615,6 @@ class CaptureDescriptorBuilder : public ReflectionMetadataBuilder {
         // dereferencing an isa pointer or a generic argument). Record
         // the path. We assume captured values map 1-1 with function
         // parameters.
-        auto ParamIndex = ConventionSource.getParamIndex();
-        auto Index = ParamIndex - FirstCaptureIndex;
-
-        auto ParamType = SubstCalleeType->getParameters()[ParamIndex].getSILType();
-        auto CaptureType = getElementTypes()[Index];
-        assert(ParamType == CaptureType);
-        (void) ParamType;
-        (void) CaptureType;
-
         auto Root = ConventionSource.getMetadataSource(SourceBuilder);
         auto Src = Fulfillment->Path.getMetadataSource(SourceBuilder, Root);
 
@@ -772,15 +761,14 @@ IRGenModule::getAddrOfCaptureDescriptor(SILFunction &Caller,
                                         CanSILFunctionType OrigCalleeType,
                                         CanSILFunctionType SubstCalleeType,
                                         ArrayRef<Substitution> Subs,
-                                        HeapLayout &Layout,
-                                        unsigned FirstCaptureIndex) {
+                                        HeapLayout &Layout) {
   if (!IRGen.Opts.EnableReflectionMetadata)
     return llvm::Constant::getNullValue(CaptureDescriptorPtrTy);
 
   llvm::SetVector<CanType> BuiltinTypes;
   CaptureDescriptorBuilder builder(*this, BuiltinTypes, Caller,
                                    OrigCalleeType, SubstCalleeType, Subs,
-                                   Layout, FirstCaptureIndex);
+                                   Layout);
 
   auto var = builder.emit();
   if (var)

lib/IRGen/IRGenModule.h

@@ -633,8 +633,7 @@ class IRGenModule {
                                              CanSILFunctionType origCalleeType,
                                              CanSILFunctionType substCalleeType,
                                              ArrayRef<Substitution> subs,
-                                             HeapLayout &layout,
-                                             unsigned firstCaptureIndex);
+                                             HeapLayout &layout);
   std::string getBuiltinTypeMetadataSectionName();
   std::string getFieldTypeMetadataSectionName();
   std::string getAssociatedTypeMetadataSectionName();