Runtime: Repurpose TargetGenericContextDescriptorHeader::NumExtraArguments for flags

For now, we have a single flag, which indicates that our generic
context has generic parameter packs.

Author: slavapestov

include/swift/ABI/GenericContext.h

@@ -30,6 +30,38 @@ struct TargetProtocolConformanceDescriptor;
 template <typename Runtime>
 struct TargetGenericContext;
 
+class GenericContextDescriptorFlags {
+  uint16_t Value;
+
+public:
+  constexpr GenericContextDescriptorFlags() : Value(0) {}
+
+  explicit constexpr GenericContextDescriptorFlags(uint16_t value)
+    : Value(value) {}
+
+  constexpr GenericContextDescriptorFlags(bool hasTypePacks)
+    : GenericContextDescriptorFlags(
+        GenericContextDescriptorFlags((uint16_t)0)
+          .withHasTypePacks(hasTypePacks)) {}
+
+  /// Whether this generic context has at least one type parameter
+  /// pack, in which case the generic context will have a trailing
+  /// GenericParamPackShapeHeader.
+  constexpr bool hasTypePacks() const {
+    return (Value & 0x1) != 0;
+  }
+
+  constexpr GenericContextDescriptorFlags
+  withHasTypePacks(bool hasTypePacks) const {
+    return GenericContextDescriptorFlags((uint16_t)(
+      (Value & ~0x1) | (hasTypePacks ? 0x1 : 0)));
+  }
+
+  constexpr uint16_t getIntValue() const {
+    return Value;
+  }
+};
+
 template <typename Runtime>
 struct TargetGenericContextDescriptorHeader {
   /// The number of (source-written) generic parameters, and thus
@@ -39,8 +71,8 @@ struct TargetGenericContextDescriptorHeader {
   ///
   /// A GenericParamDescriptor corresponds to a type metadata pointer
   /// in the arguments layout when isKeyArgument() is true.
-  /// isKeyArgument() will be false if the parameter has been unified
-  /// unified with a different parameter or an associated type.
+  /// isKeyArgument() will be false if the parameter has been made
+  /// equivalent to a different parameter or a concrete type.
   uint16_t NumParams;
 
   /// The number of GenericRequirementDescriptors in this generic
@@ -66,18 +98,22 @@ struct TargetGenericContextDescriptorHeader {
   /// hasKeyArgument()).
   uint16_t NumKeyArguments;
 
-  /// In principle, the size of the "extra" area of the argument
+  /// Originally this was the size of the "extra" area of the argument
   /// layout, in words.  The idea was that extra arguments would
   /// include generic parameters and conformances that are not part
   /// of the identity of the context; however, it's unclear why we
-  /// would ever want such a thing.  As a result, this section is
-  /// unused, and this field is always zero.  It can be repurposed
-  /// as long as it remains zero in code which must be compatible
-  /// with existing Swift runtimes.
-  uint16_t NumExtraArguments;
+  /// would ever want such a thing.  As a result, in pre-5.8 runtimes
+  /// this field is always zero.  New flags can only be added as long
+  /// as they remains zero in code which must be compatible with
+  /// older Swift runtimes.
+  GenericContextDescriptorFlags Flags;
 
   uint32_t getNumArguments() const {
-    return NumKeyArguments + NumExtraArguments;
+    // Note: this used to be NumKeyArguments + NumExtraArguments,
+    // and flags was named NumExtraArguments, which is why Flags
+    // must remain zero when backward deploying to Swift 5.7 or
+    // earlier.
+    return NumKeyArguments;
   }
 
   /// Return the total size of the argument layout, in words.
@@ -163,7 +199,7 @@ class TargetGenericRequirementDescriptor {
     return offsetof(typename std::remove_reference<decltype(*this)>::type, Type);
   }
 
-  /// Retreive the offset to the Type field
+  /// Retreive the offset to the Param field
   constexpr inline auto
   getParamOffset() const -> typename Runtime::StoredSize {
     return offsetof(typename std::remove_reference<decltype(*this)>::type, Param);