[Macros] Add a flag -dump-macro-expansions as a debugging aid.

include/swift/Basic/LangOptions.h

@@ -528,6 +528,9 @@ namespace swift {
     /// Enables dumping type witness systems from associated type inference.
     bool DumpTypeWitnessSystems = false;
 
+    /// Enables dumping macro expansions.
+    bool DumpMacroExpansions = false;
+
     /// The model of concurrency to be used.
     ConcurrencyModel ActiveConcurrencyModel = ConcurrencyModel::Standard;
 

include/swift/Option/FrontendOptions.td

@@ -343,6 +343,9 @@ def dump_requirement_machine : Flag<["-"], "dump-requirement-machine">,
 def debug_requirement_machine : Joined<["-"], "debug-requirement-machine=">,
   HelpText<"Fine-grained debug output from the generics implementation">;
 
+def dump_macro_expansions : Flag<["-"], "dump-macro-expansions">,
+  HelpText<"Dumps the results of each macro expansion">;
+
 def analyze_requirement_machine : Flag<["-"], "analyze-requirement-machine">,
   Flags<[FrontendOption, HelpHidden, DoesNotAffectIncrementalBuild]>,
   HelpText<"Print out requirement machine statistics at the end of the compilation job">;

lib/Frontend/CompilerInvocation.cpp

@@ -1042,6 +1042,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.AnalyzeRequirementMachine = Args.hasArg(
       OPT_analyze_requirement_machine);
 
+  Opts.DumpMacroExpansions = Args.hasArg(
+      OPT_dump_macro_expansions);
+
   if (const Arg *A = Args.getLastArg(OPT_debug_requirement_machine))
     Opts.DebugRequirementMachine = A->getValue();
 

lib/Sema/TypeCheckMacros.cpp

@@ -433,6 +433,14 @@ Expr *swift::expandMacroExpr(
     }
   }
 
+  // Dump macro expansions to standard output, if requested.
+  if (ctx.LangOpts.DumpMacroExpansions) {
+    llvm::errs() << bufferName << " as " << expandedType.getString()
+      << "\n------------------------------\n"
+      << evaluatedSource
+      << "\n------------------------------\n";
+  }
+
   // Create a new source buffer with the contents of the expanded macro.
   auto macroBuffer =
       llvm::MemoryBuffer::getMemBuffer(evaluatedSource, bufferName);