AST: Refactor existential generalization to use buildGenericSignature() instead of GenericSignatureBuilder

The GenericSignatureBuilder is no longer used for anything by default, and we
don't want to create instances of it directly.

Instead, use the buildGenericSignature() utility method, which is backed by
the Requirement Machine.

Author: slavapestov

lib/AST/ExistentialGeneralization.cpp

@@ -15,9 +15,11 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "GenericSignatureBuilder.h"
-#include "swift/AST/Types.h"
 #include "swift/AST/CanTypeVisitor.h"
+#include "swift/AST/Decl.h"
+#include "swift/AST/GenericSignature.h"
+#include "swift/AST/Requirement.h"
+#include "swift/AST/Types.h"
 #include "llvm/ADT/DenseMap.h"
 
 using namespace swift;
@@ -34,13 +36,8 @@ class Generalizer : public CanTypeVisitor<Generalizer, Type> {
   llvm::DenseMap<std::pair<CanType, ProtocolDecl*>,
                  ProtocolConformanceRef> substConformances;
 
-  Optional<GenericSignatureBuilder> sigBuilderStorage;
-
-  GenericSignatureBuilder &sigBuilder() {
-    if (!sigBuilderStorage)
-      sigBuilderStorage.emplace(ctx);
-    return *sigBuilderStorage;
-  }
+  SmallVector<GenericTypeParamType *, 2> addedParameters;
+  SmallVector<Requirement, 2> addedRequirements;
 
 public:
   Generalizer(ASTContext &ctx) : ctx(ctx) {}
@@ -53,11 +50,13 @@ class Generalizer : public CanTypeVisitor<Generalizer, Type> {
 
   SubstitutionMap getGeneralizationSubstitutions() {
     // If we never introduced a generalization parameter, we're done.
-    if (!sigBuilderStorage) return SubstitutionMap();
+    if (addedParameters.empty() && addedRequirements.empty())
+      return SubstitutionMap();
 
     // Finish the signature.
-    auto sig = std::move(*sigBuilderStorage).computeGenericSignature();
-    sigBuilderStorage.reset();
+    auto sig = buildGenericSignature(ctx, GenericSignature(),
+                                     addedParameters,
+                                     addedRequirements);
 
     // TODO: minimize the signature by removing redundant generic
     // parameters.
@@ -217,10 +216,7 @@ class Generalizer : public CanTypeVisitor<Generalizer, Type> {
         assert(optNewReq && "generalization substitution failed");
         auto &newReq = *optNewReq;
 
-        auto source = GenericSignatureBuilder::
-          FloatingRequirementSource::forInferred(SourceLoc());
-
-        sigBuilder().addRequirement(newReq, source, nullptr);
+        addedRequirements.push_back(newReq);
 
         substConformances.insert({{newReq.getFirstType()->getCanonicalType(),
                                    newReq.getProtocolDecl()},
@@ -259,7 +255,8 @@ class Generalizer : public CanTypeVisitor<Generalizer, Type> {
                                               /*depth*/ 0,
                                               /*index*/ substTypes.size(),
                                               ctx);
-    sigBuilder().addGenericParameter(newParam);
+    addedParameters.push_back(newParam);
+
     substTypes.insert({CanType(newParam), origArg});
     return newParam;
   }