[Experimental Dependencies] Add -enable-experimental-dependencies and push it through.

include/swift/Basic/LangOptions.h

@@ -288,6 +288,10 @@ namespace swift {
     /// Whether to verify the parsed syntax tree and emit related diagnostics.
     bool VerifySyntaxTree = false;
 
+    /// Scaffolding to permit experimentation with finer-grained dependencies
+    /// and faster rebuilds.
+    bool EnableExperimentalDependencies = false;
+
     /// Sets the target we are building for and updates platform conditions
     /// to match.
     ///

include/swift/Driver/Compilation.h

@@ -206,6 +206,10 @@ class Compilation {
   /// limit filelists will be used.
   size_t FilelistThreshold;
 
+  /// Scaffolding to permit experimentation with finer-grained dependencies and
+  /// faster rebuilds.
+  const bool EnableExperimentalDependencies;
+
   template <typename T>
   static T *unwrap(const std::unique_ptr<T> &p) {
     return p.get();
@@ -234,7 +238,8 @@ class Compilation {
               Optional<unsigned> BatchSizeLimit = None,
               bool SaveTemps = false,
               bool ShowDriverTimeCompilation = false,
-              std::unique_ptr<UnifiedStatsReporter> Stats = nullptr);
+              std::unique_ptr<UnifiedStatsReporter> Stats = nullptr,
+              bool EnableExperimentalDependencies = false);
   ~Compilation();
 
   ToolChain const &getToolChain() const {
@@ -289,6 +294,10 @@ class Compilation {
     EnableIncrementalBuild = false;
   }
 
+  bool getEnableExperimentalDependencies() const {
+    return EnableExperimentalDependencies;
+  }
+
   bool getBatchModeEnabled() const {
     return EnableBatchMode;
   }

include/swift/Option/Options.td

@@ -129,6 +129,10 @@ def driver_always_rebuild_dependents :
   Flag<["-"], "driver-always-rebuild-dependents">, InternalDebugOpt,
   HelpText<"Always rebuild dependents of files that have been modified">;
 
+def enable_experimental_dependencies :
+Flag<["-"], "enable-experimental-dependencies">, Flags<[FrontendOption, HelpHidden]>,
+HelpText<"Experimental work-in-progress to be more selective about incremental recompilation">;
+
 def driver_mode : Joined<["--"], "driver-mode=">, Flags<[HelpHidden]>,
   HelpText<"Set the driver mode to either 'swift' or 'swiftc'">;
 

lib/Driver/Compilation.cpp

@@ -117,7 +117,8 @@ Compilation::Compilation(DiagnosticEngine &Diags,
                          Optional<unsigned> BatchSizeLimit,
                          bool SaveTemps,
                          bool ShowDriverTimeCompilation,
-                         std::unique_ptr<UnifiedStatsReporter> StatsReporter)
+                         std::unique_ptr<UnifiedStatsReporter> StatsReporter,
+                         bool EnableExperimentalDependencies)
   : Diags(Diags), TheToolChain(TC),
     TheOutputInfo(OI),
     Level(Level),
@@ -138,7 +139,9 @@ Compilation::Compilation(DiagnosticEngine &Diags,
     SaveTemps(SaveTemps),
     ShowDriverTimeCompilation(ShowDriverTimeCompilation),
     Stats(std::move(StatsReporter)),
-    FilelistThreshold(FilelistThreshold) {
+    FilelistThreshold(FilelistThreshold),
+    EnableExperimentalDependencies(EnableExperimentalDependencies) {
+
 };
 
 static bool writeFilelistIfNecessary(const Job *job, const ArgList &args,

lib/Driver/Driver.cpp

@@ -920,7 +920,9 @@ Driver::buildCompilation(const ToolChain &TC,
         ArgList->hasArg(options::OPT_driver_time_compilation);
     std::unique_ptr<UnifiedStatsReporter> StatsReporter =
         createStatsReporter(ArgList.get(), Inputs, OI, DefaultTargetTriple);
-
+    const bool EnableExperimentalDependencies =
+        ArgList->hasArg(options::OPT_enable_experimental_dependencies);
+
     C = llvm::make_unique<Compilation>(
         Diags, TC, OI, Level,
         std::move(ArgList),
@@ -939,7 +941,8 @@ Driver::buildCompilation(const ToolChain &TC,
         DriverBatchSizeLimit,
         SaveTemps,
         ShowDriverTimeCompilation,
-        std::move(StatsReporter));
+        std::move(StatsReporter),
+        EnableExperimentalDependencies);
   }
 
   // Construct the graph of Actions.

lib/Driver/ToolChains.cpp

@@ -218,6 +218,8 @@ static void addCommonFrontendArgs(const ToolChain &TC, const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_O_Group);
   inputArgs.AddLastArg(arguments, options::OPT_RemoveRuntimeAsserts);
   inputArgs.AddLastArg(arguments, options::OPT_AssumeSingleThreaded);
+  inputArgs.AddLastArg(arguments,
+                       options::OPT_enable_experimental_dependencies);
 
   // Pass on any build config options
   inputArgs.AddAllArgs(arguments, options::OPT_D);

lib/Frontend/CompilerInvocation.cpp

@@ -273,6 +273,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.BuildSyntaxTree = true;
     Opts.VerifySyntaxTree = true;
   }
+
+  if (Args.hasArg(OPT_enable_experimental_dependencies))
+    Opts.EnableExperimentalDependencies = true;
 
   Opts.DebuggerSupport |= Args.hasArg(OPT_debugger_support);
   if (Opts.DebuggerSupport)