Add accessor with potentially unavailable attrs

DeclAttributes::getUnavailable() only cares about attributes which make a declaration definitely unavailable, but you sometimes need a version which will also return a potentially unavailable (i.e. “introduced:”) attribute. This adds that.

Author: beccadax

include/swift/AST/Attr.h

@@ -1446,6 +1446,11 @@ class DeclAttributes {
   bool
   isUnavailableInSwiftVersion(const version::Version &effectiveVersion) const;
 
+  /// Returns the first @available attribute that indicates
+  /// a declaration is unavailable, or the first one that indicates it's
+  /// potentially unavailable, or null otherwise.
+  const AvailableAttr *getPotentiallyUnavailable(const ASTContext &ctx) const;
+
   /// Returns the first @available attribute that indicates
   /// a declaration is unavailable, or null otherwise.
   const AvailableAttr *getUnavailable(const ASTContext &ctx) const;

lib/AST/Attr.cpp

@@ -131,6 +131,47 @@ DeclAttributes::isUnavailableInSwiftVersion(
   return false;
 }
 
+const AvailableAttr *DeclAttributes::getPotentiallyUnavailable(
+                          const ASTContext &ctx) const {
+  const AvailableAttr *potential = nullptr;
+  const AvailableAttr *conditional = nullptr;
+
+  for (auto Attr : *this)
+    if (auto AvAttr = dyn_cast<AvailableAttr>(Attr)) {
+      if (AvAttr->isInvalid())
+        continue;
+
+      if (!AvAttr->isActivePlatform(ctx) &&
+          !AvAttr->isLanguageVersionSpecific() &&
+          !AvAttr->isPackageDescriptionVersionSpecific())
+        continue;
+
+      // Definitely not available.
+      if (AvAttr->isUnconditionallyDeprecated())
+        return AvAttr;
+
+      switch (AvAttr->getVersionAvailability(ctx)) {
+        case AvailableVersionComparison::Available:
+          // Doesn't limit the introduced version.
+          break;
+
+        case AvailableVersionComparison::PotentiallyUnavailable:
+          // We'll return this if we don't see something that proves it's
+          // not available in this version.
+          potential = AvAttr;
+          break;
+
+        case AvailableVersionComparison::Unavailable:
+        case AvailableVersionComparison::Obsoleted:
+          conditional = AvAttr;
+      }
+    }
+
+  if (conditional)
+    return conditional;
+  return potential;
+}
+
 const AvailableAttr *DeclAttributes::getUnavailable(
                           const ASTContext &ctx) const {
   const AvailableAttr *conditional = nullptr;