[ClangImporter] Predefine __swift__ in imported (Obj)C code.

This macro expands to a numeric value representing the current Swift
language version; for Swift 3.1.2, it would be 30102.

See discussion in https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20160822/002754.html.

rdar://problem/26921435

Author: jrose-apple

include/swift/Basic/Version.h

@@ -65,13 +65,15 @@ class Version {
 
   /// Return a string to be used as an internal preprocessor define.
   ///
-  /// Assuming the project version is at most X.Y.Z.a.b, the integral constant
-  /// representing the version is:
+  /// The components of the version are multiplied element-wise by
+  /// \p componentWeights, then added together (a dot product operation).
+  /// If either array is longer than the other, the missing elements are
+  /// treated as zero.
   ///
-  /// X*1000*1000*1000 + Z*1000*1000 + a*1000 + b
-  ///
-  /// The second version component is not used.
-  std::string preprocessorDefinition() const;
+  /// The resulting string will have the form "-DMACRO_NAME=XYYZZ".
+  /// The combined value must fit in a uint64_t.
+  std::string preprocessorDefinition(StringRef macroName,
+                                     ArrayRef<uint64_t> componentWeights) const;
 
   /// Return the ith version component.
   unsigned operator[](size_t i) const {

lib/Basic/Version.cpp

@@ -269,25 +269,20 @@ raw_ostream &operator<<(raw_ostream &os, const Version &version) {
   return os;
 }
 
-std::string Version::preprocessorDefinition() const {
-  SmallString<64> define("-D__SWIFT_COMPILER_VERSION=");
-  llvm::raw_svector_ostream OS(define);
+std::string
+Version::preprocessorDefinition(StringRef macroName,
+                                ArrayRef<uint64_t> componentWeights) const {
   uint64_t versionConstant = 0;
 
-  auto NumComponents = Components.size();
-
-  if (NumComponents > 0)
-    versionConstant += Components[0] * 1000 * 1000 * 1000;
-  // Component 2 is not used.
-  if (NumComponents > 2)
-    versionConstant += Components[2] * 1000 * 1000;
-  if (NumComponents > 3)
-    versionConstant += Components[3] * 1000;
-  if (NumComponents > 4)
-    versionConstant += Components[4];
+  for (size_t i = 0, e = std::min(componentWeights.size(), Components.size());
+       i < e; ++i) {
+    versionConstant += componentWeights[i] * Components[i];
+  }
 
-  OS << versionConstant;
-  return OS.str().str();
+  std::string define("-D");
+  llvm::raw_string_ostream(define) << macroName << '=' << versionConstant;
+  // This isn't using stream.str() so that we get move semantics.
+  return define;
 }
 
 bool operator>=(const class Version &lhs,

lib/ClangImporter/ClangImporter.cpp

@@ -315,6 +315,8 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
   const llvm::Triple &triple = ctx.LangOpts.Target;
   SearchPathOptions &searchPathOpts = ctx.SearchPathOpts;
 
+  auto languageVersion = swift::version::Version::getCurrentLanguageVersion();
+
   // Construct the invocation arguments for the current target.
   // Add target-independent options first.
   invocationArgStrs.insert(
@@ -332,9 +334,11 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
           // Don't emit LLVM IR.
           "-fsyntax-only",
 
-          // enable block support
+          // Enable block support.
           "-fblocks",
 
+          languageVersion.preprocessorDefinition("__swift__", {10000, 100, 1}),
+
           "-fretain-comments-from-system-headers",
 
           SHIMS_INCLUDE_FLAG, searchPathOpts.RuntimeResourcePath,
@@ -396,7 +400,8 @@ getNormalInvocationArguments(std::vector<std::string> &invocationArgStrs,
     auto V = version::Version::getCurrentCompilerVersion();
     if (!V.empty()) {
       invocationArgStrs.insert(invocationArgStrs.end(), {
-        V.preprocessorDefinition(),
+        V.preprocessorDefinition("__SWIFT_COMPILER_VERSION",
+                                 {1000000000, /*ignored*/0, 1000000, 1000, 1}),
       });
     }
   } else {

test/ClangModules/Inputs/custom-modules/PredefinedMacros.h

@@ -0,0 +1,9 @@
+#if !defined(__swift__)
+# error "__swift__ not defined"
+#elif __swift__ < 30000
+# error "Why are you using such an old version of Swift?"
+#elif __swift__ >= 810000
+# error "Is Swift 81 out already? If so, please update this test."
+#else
+void swift3ReadyToGo();
+#endif

test/ClangModules/Inputs/custom-modules/module.map

@@ -103,6 +103,11 @@ module ObjCSubscripts {
   export *
 }
 
+module PredefinedMacros {
+  header "PredefinedMacros.h"
+  export *
+}
+
 module ProtoWithInitializer {
   header "ProtoWithInitializer.h"
   export *

test/ClangModules/predefined_macros.swift

@@ -0,0 +1,5 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -parse -verify -I %S/Inputs/custom-modules/ %s
+
+import PredefinedMacros
+
+swift3ReadyToGo()