IRGen: Hollow out NecessaryBindings

This removes the "optimization" where a function type, metatype or
tuple type was split up into structural components, because it seems
that in general we need this structural type metadata again.

Similarly, this no longer tries to split up dependent concrete
conformances and instead passes the witness table in the context.

This makes the context larger potentially, but it avoids calls to
metadata access functions and swift_getWitnessTable() every time the
closure is invoked.

Author: slavapestov

include/swift/IRGen/GenericRequirement.h

@@ -76,7 +76,6 @@ class GenericRequirement {
   }
 
   static GenericRequirement forShape(CanType type) {
-    assert(!isa<PackType>(type));
     assert(type->isParameterPack() || isa<PackArchetypeType>(type));
     return GenericRequirement(Kind::Shape, type, nullptr);
   }
@@ -92,7 +91,6 @@ class GenericRequirement {
   }
 
   static GenericRequirement forMetadata(CanType type) {
-    assert(!isa<PackType>(type));
     auto kind = ((type->isParameterPack() ||
                   isa<PackArchetypeType>(type))
                  ? Kind::MetadataPack : Kind::Metadata);
@@ -110,7 +108,6 @@ class GenericRequirement {
   }
 
   static GenericRequirement forWitnessTable(CanType type, ProtocolDecl *proto) {
-    assert(!isa<PackType>(type));
     auto kind = ((type->isParameterPack() ||
                   isa<PackArchetypeType>(type))
                  ? Kind::WitnessTablePack

lib/IRGen/GenArchetype.cpp

@@ -441,8 +441,8 @@ withOpaqueTypeGenericArgs(IRGenFunction &IGF,
         opaqueDecl->getGenericSignature().getCanonicalSignature(),
         [&](GenericRequirement reqt) {
           auto arg = emitGenericRequirementFromSubstitutions(
-              IGF, reqt, archetype->getSubstitutions(),
-              MetadataState::Abstract);
+              IGF, reqt, MetadataState::Abstract,
+              archetype->getSubstitutions());
           args.push_back(arg);
           types.push_back(args.back()->getType());
         });

lib/IRGen/GenKeyPath.cpp

@@ -89,12 +89,11 @@ bindPolymorphicArgumentsFromComponentIndices(IRGenFunction &IGF,
     args =
         IGF.Builder.CreateInBoundsGEP(IGF.IGM.Int8Ty, args, genericArgsOffset);
   }
+
   bindFromGenericRequirementsBuffer(
       IGF, requirements,
       Address(args, IGF.IGM.Int8Ty, IGF.IGM.getPointerAlignment()),
-      MetadataState::Complete, [&](CanType t) {
-        return genericEnv->mapTypeIntoContext(t)->getCanonicalType();
-      });
+      MetadataState::Complete, genericEnv->getForwardingSubstitutionMap());
 }
 
 static llvm::Function *
@@ -295,9 +294,7 @@ getLayoutFunctionForComputedComponent(IRGenModule &IGM,
       bindFromGenericRequirementsBuffer(
           IGF, requirements,
           Address(args, IGM.Int8Ty, IGF.IGM.getPointerAlignment()),
-          MetadataState::Complete, [&](CanType t) {
-            return genericEnv->mapTypeIntoContext(t)->getCanonicalType();
-          });
+          MetadataState::Complete, genericEnv->getForwardingSubstitutionMap());
     }
 
     // Run through the captured index types to determine the size and alignment
@@ -583,9 +580,7 @@ getInitializerForComputedComponent(IRGenModule &IGM,
       bindFromGenericRequirementsBuffer(
           IGF, requirements,
           Address(src, IGM.Int8Ty, IGF.IGM.getPointerAlignment()),
-          MetadataState::Complete, [&](CanType t) {
-            return genericEnv->mapTypeIntoContext(t)->getCanonicalType();
-          });
+          MetadataState::Complete, genericEnv->getForwardingSubstitutionMap());
 
     } else {
       offset = llvm::ConstantInt::get(IGM.SizeTy, 0);

lib/IRGen/GenMeta.cpp

@@ -2318,11 +2318,10 @@ namespace {
             bindFromGenericRequirementsBuffer(
                 IGF, requirements,
                 Address(bindingsBufPtr, IGM.Int8Ty, IGM.getPointerAlignment()),
-                MetadataState::Complete, [&](CanType t) {
-                  return genericEnv ? genericEnv->mapTypeIntoContext(t)
-                                          ->getCanonicalType()
-                                    : t;
-                });
+                MetadataState::Complete,
+                (genericEnv
+                 ? genericEnv->getForwardingSubstitutionMap()
+                 : SubstitutionMap()));
           }
 
           SmallVector<llvm::BasicBlock *, 4> conditionalTypes;