AST: Add getNextDepth() and getMaxDepth() methods to GenericSignature

Author: slavapestov

include/swift/AST/GenericSignature.h

@@ -197,6 +197,11 @@ class GenericSignature {
   /// array of the generic parameters for the innermost generic type.
   ArrayRef<GenericTypeParamType *> getInnermostGenericParams() const;
 
+  /// Returns the depth that a generic parameter at the next level of
+  /// nesting would have. This is zero for the empty signature,
+  /// and one plus the depth of the final generic parameter otherwise.
+  unsigned getNextDepth() const;
+
   /// Retrieve the requirements.
   ArrayRef<Requirement> getRequirements() const;
 
@@ -307,6 +312,9 @@ class alignas(1 << TypeAlignInBits) GenericSignatureImpl final
     return Mem;
   }
 
+  /// Returns the depth of the last generic parameter.
+  unsigned getMaxDepth() const;
+
   /// Transform the requirements into a form where implicit Copyable and
   /// Escapable conformances are omitted, and their absence is explicitly
   /// noted.

lib/AST/GenericSignature.cpp

@@ -95,6 +95,16 @@ GenericSignatureImpl::getInnermostGenericParams() const {
   return params.slice(sliceCount);
 }
 
+unsigned GenericSignatureImpl::getMaxDepth() const {
+  return getGenericParams().back()->getDepth();
+}
+
+unsigned GenericSignature::getNextDepth() const {
+  if (!getPointer())
+    return 0;
+  return getPointer()->getMaxDepth() + 1;
+}
+
 void GenericSignatureImpl::forEachParam(
     llvm::function_ref<void(GenericTypeParamType *, bool)> callback) const {
   // Figure out which generic parameters are concrete or same-typed to another