Add "#if hasAttribute(X)" support query attributes.

Author: DougGregor

include/swift/AST/Attr.h

@@ -2626,6 +2626,10 @@ inline SourceLoc extractNearestSourceLoc(const DeclAttribute *attr) {
   return attr->getLocation();
 }
 
+/// Determine whether the given attribute is available, looking up the
+/// attribute by name.
+bool hasAttribute(const LangOptions &langOpts, llvm::StringRef attributeName);
+
 } // end namespace swift
 
 #endif

lib/AST/Attr.cpp

@@ -2285,3 +2285,23 @@ void swift::simple_display(llvm::raw_ostream &out, const DeclAttribute *attr) {
   if (attr)
     attr->print(out);
 }
+
+bool swift::hasAttribute(
+    const LangOptions &langOpts, llvm::StringRef attributeName) {
+  DeclAttrKind kind = DeclAttribute::getAttrKindFromString(attributeName);
+  if (kind == DAK_Count)
+    return false;
+
+  if (DeclAttribute::isUserInaccessible(kind))
+    return false;
+  if (DeclAttribute::isDeclModifier(kind))
+    return false;
+  if (DeclAttribute::shouldBeRejectedByParser(kind))
+    return false;
+  if (DeclAttribute::isSilOnly(kind))
+    return false;
+  if (DeclAttribute::isConcurrencyOnly(kind))
+    return false;
+
+  return true;
+}

lib/Parse/ParseIfConfig.cpp

@@ -346,6 +346,16 @@ class ValidateIfConfigCondition :
       return E;
     }
 
+    if (*KindName == "hasAttribute") {
+      if (!getDeclRefStr(Arg, DeclRefKind::Ordinary)) {
+        D.diagnose(E->getLoc(), diag::unsupported_platform_condition_argument,
+                   "attribute name");
+        return nullptr;
+      }
+
+      return E;
+    }
+
     // ( 'os' | 'arch' | '_endian' | '_runtime' ) '(' identifier ')''
     auto Kind = getPlatformConditionKind(*KindName);
     if (!Kind.hasValue()) {
@@ -574,6 +584,9 @@ class EvaluateIfConfigCondition :
     } else if (KindName == "hasFeature") {
       auto featureName = getDeclRefStr(Arg);
       return Ctx.LangOpts.hasFeature(featureName);
+    } else if (KindName == "hasAttribute") {
+      auto attributeName = getDeclRefStr(Arg);
+      return hasAttribute(Ctx.LangOpts, attributeName);
     }
 
     auto Val = getDeclRefStr(Arg);

test/attr/has_attribute.swift

@@ -0,0 +1,25 @@
+// RUN: %target-typecheck-verify-swift
+
+#if !hasAttribute(dynamicCallable)
+BOOM
+#endif
+
+#if hasAttribute(fortran)
+BOOM
+#endif
+
+#if hasAttribute(cobol)
+this is unparsed junk // expected-error{{consecutive statements on a line must be separated by}}
+#endif
+
+#if hasAttribute(optional)
+ModifiersAreNotAttributes
+#endif
+
+#if hasAttribute(__raw_doc_comment)
+UserInaccessibleAreNotAttributes
+#endif
+
+#if hasAttribute(17)
+// expected-error@-1{{unexpected platform condition argument: expected attribute name}}
+#endif