AST: Introduce AvailabilityInference::availableRangeAndAttr().

Also, clean up some documentation.

Author: tshortli

include/swift/AST/Availability.h

@@ -350,15 +350,20 @@ class AvailabilityInference {
 
   static AvailabilityRange inferForType(Type t);
 
-  /// Returns the context where a declaration is available
-  /// We assume a declaration without an annotation is always available.
+  /// Returns the range of platform versions in which the decl is available.
   static AvailabilityRange availableRange(const Decl *D);
 
+  /// Returns the range of platform versions in which the decl is available and
+  /// the attribute which determined this range (which may be `nullptr` if the
+  /// declaration is always available.
+  static std::pair<AvailabilityRange, const AvailableAttr *>
+  availableRangeAndAttr(const Decl *D);
+
   /// Returns true is the declaration is `@_spi_available`.
   static bool isAvailableAsSPI(const Decl *D);
 
-  /// Returns the availability context for a declaration with the given
-  /// @available attribute.
+  /// Returns the range of platform versions in which a declaration with the
+  /// given `@available` attribute is available.
   ///
   /// NOTE: The attribute must be active on the current platform.
   static AvailabilityRange availableRange(const AvailableAttr *attr,

lib/AST/Availability.cpp

@@ -580,12 +580,18 @@ static const AvailableAttr *attrForAvailableRange(const Decl *D) {
   return nullptr;
 }
 
-AvailabilityRange AvailabilityInference::availableRange(const Decl *D) {
-  if (auto attr = attrForAvailableRange(D))
-    return availableRange(attr, D->getASTContext());
+std::pair<AvailabilityRange, const AvailableAttr *>
+AvailabilityInference::availableRangeAndAttr(const Decl *D) {
+  if (auto attr = attrForAvailableRange(D)) {
+    return {availableRange(attr, D->getASTContext()), attr};
+  }
 
   // Treat unannotated declarations as always available.
-  return AvailabilityRange::alwaysAvailable();
+  return {AvailabilityRange::alwaysAvailable(), nullptr};
+}
+
+AvailabilityRange AvailabilityInference::availableRange(const Decl *D) {
+  return availableRangeAndAttr(D).first;
 }
 
 bool AvailabilityInference::isAvailableAsSPI(const Decl *D) {