[AST] Represent requirement signature as a flat set of requirements.

The outside representation already went to a flat set of requirements;
make the internal representation match so we aren't tempted to use the
requirement signature as inputs to a generic signature.

Author: DougGregor

include/swift/AST/Decl.h

@@ -3501,7 +3501,7 @@ class ProtocolDecl : public NominalTypeDecl {
 
   /// The generic signature representing exactly the new requirements introduced
   /// by this protocol.
-  GenericSignature *RequirementSignature = nullptr;
+  const Requirement *RequirementSignature = nullptr;
 
   /// True if the protocol has requirements that cannot be satisfied (e.g.
   /// because they could not be imported from Objective-C).
@@ -3511,6 +3511,9 @@ class ProtocolDecl : public NominalTypeDecl {
   /// value, plus one. Otherwise, it will be 0.
   unsigned KnownProtocol : 6;
 
+  /// The number of requirements in the requirement signature.
+  unsigned NumRequirementsInSignature : 16;
+
   bool requiresClassSlow();
 
   bool existentialConformsToSelfSlow();
@@ -3687,8 +3690,6 @@ class ProtocolDecl : public NominalTypeDecl {
 
   /// Create the implicit generic parameter list for a protocol or
   /// extension thereof.
-  ///
-  /// FIXME: protocol extensions will introduce a where clause here as well.
   GenericParamList *createGenericParams(DeclContext *dc);
 
   /// Create the generic parameters of this protocol if the haven't been
@@ -3708,9 +3709,9 @@ class ProtocolDecl : public NominalTypeDecl {
   /// protocols of the inherited protocols) are not mentioned. The conformance
   /// requirements listed here become entries in the witness table.
   ArrayRef<Requirement> getRequirementSignature() const {
-    assert(RequirementSignature &&
+    assert(isRequirementSignatureComputed() &&
            "getting requirement signature before computing it");
-    return RequirementSignature->getRequirements();
+    return llvm::makeArrayRef(RequirementSignature, NumRequirementsInSignature);
   }
 
   /// Has the requirement signature been computed yet?
@@ -3720,9 +3721,7 @@ class ProtocolDecl : public NominalTypeDecl {
 
   void computeRequirementSignature();
 
-  void setRequirementSignature(GenericSignature *sig) {
-    RequirementSignature = sig;
-  }
+  void setRequirementSignature(ArrayRef<Requirement> requirements);
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) {

lib/AST/Decl.cpp

@@ -2861,10 +2861,11 @@ ProtocolDecl::ProtocolDecl(DeclContext *DC, SourceLoc ProtocolLoc,
   ProtocolDeclBits.RequiresClass = false;
   ProtocolDeclBits.ExistentialConformsToSelfValid = false;
   ProtocolDeclBits.ExistentialConformsToSelf = false;
-  KnownProtocol = 0;
   ProtocolDeclBits.Circularity
     = static_cast<unsigned>(CircularityCheck::Unchecked);
   HasMissingRequirements = false;
+  KnownProtocol = 0;
+  NumRequirementsInSignature = 0;
 }
 
 llvm::TinyPtrVector<ProtocolDecl *>
@@ -3299,8 +3300,23 @@ void ProtocolDecl::computeRequirementSignature() {
          GenericSignatureBuilder::RequirementSource
            ::forRequirementSignature(selfPA, this),
          nullptr);
-  
-  RequirementSignature = builder.computeGenericSignature(SourceLoc());
+
+  // Compute and record the signature.
+  auto requirementSig = builder.computeGenericSignature(SourceLoc());
+  RequirementSignature = requirementSig->getRequirements().data();
+  assert(RequirementSignature != nullptr);
+  NumRequirementsInSignature = requirementSig->getRequirements().size();
+}
+
+void ProtocolDecl::setRequirementSignature(ArrayRef<Requirement> requirements) {
+  assert(!RequirementSignature && "already computed requirement signature");
+  if (requirements.empty()) {
+    RequirementSignature = reinterpret_cast<Requirement *>(this + 1);
+    NumRequirementsInSignature = 0;
+  } else {
+    RequirementSignature = getASTContext().AllocateCopy(requirements).data();
+    NumRequirementsInSignature = requirements.size();
+  }
 }
 
 /// Returns the default witness for a requirement, or nullptr if there is

lib/Serialization/Deserialization.cpp

@@ -3066,9 +3066,7 @@ ModuleFile::getDeclChecked(DeclID DID, Optional<DeclContext *> ForcedContext) {
       proto->setImplicit();
     proto->computeType();
 
-    auto signature = GenericSignature::get(
-                               { proto->getProtocolSelfType() }, requirements);
-    proto->setRequirementSignature(signature);
+    proto->setRequirementSignature(requirements);
 
     proto->setMemberLoader(this, DeclTypeCursor.GetCurrentBitNo());
     proto->setCircularityCheck(CircularityCheck::Checked);