[Diagnostics] Add -enable-experimental-descriptive-diagnostics frontend flag

This flag will feature-gate work on producing more descriptive diagnostic messages.
It will remain a hidden frontend option until these improvements are ready to ship.

Author: owenv

include/swift/AST/DiagnosticEngine.h

@@ -670,6 +670,9 @@ namespace swift {
     /// Print diagnostic names after their messages
     bool printDiagnosticNames = false;
 
+    /// Use descriptive diagnostic style when available.
+    bool useDescriptiveDiagnostics = false;
+
     friend class InFlightDiagnostic;
     friend class DiagnosticTransaction;
     friend class CompoundDiagnosticTransaction;
@@ -713,6 +716,13 @@ namespace swift {
       return printDiagnosticNames;
     }
 
+    void setUseDescriptiveDiagnostics(bool val) {
+       useDescriptiveDiagnostics = val;
+    }
+    bool getUseDescriptiveDiagnostics() const {
+      return useDescriptiveDiagnostics;
+    }
+
     void ignoreDiagnostic(DiagID id) {
       state.setDiagnosticBehavior(id, DiagnosticState::Behavior::Ignore);
     }

include/swift/Basic/DiagnosticOptions.h

@@ -55,6 +55,10 @@ class DiagnosticOptions {
   // When printing diagnostics, include the diagnostic name at the end
   bool PrintDiagnosticNames = false;
 
+  /// If set to true, produce more descriptive diagnostic output if available.
+  /// Descriptive diagnostic output is not intended to be machine-readable.
+  bool EnableDescriptiveDiagnostics = false;
+
   /// Return a hash code of any components from these options that should
   /// contribute to a Swift Bridging PCH hash.
   llvm::hash_code getPCHHashComponents() const {

include/swift/Option/FrontendOptions.td

@@ -109,6 +109,9 @@ def verify_syntax_tree : Flag<["-"], "verify-syntax-tree">,
 
 def show_diagnostics_after_fatal : Flag<["-"], "show-diagnostics-after-fatal">,
   HelpText<"Keep emitting subsequent diagnostics after a fatal error">;
+  
+def enable_descriptive_diagnostics : Flag<["-"], "enable-descriptive-diagnostics">,
+  HelpText<"Show descriptive diagnostic information, if available.">;
 
 def enable_swiftcall : Flag<["-"], "enable-swiftcall">,
   HelpText<"Enable the use of LLVM swiftcall support">;

lib/Frontend/CompilerInvocation.cpp

@@ -678,6 +678,8 @@ static bool ParseDiagnosticArgs(DiagnosticOptions &Opts, ArgList &Args,
   Opts.SuppressWarnings |= Args.hasArg(OPT_suppress_warnings);
   Opts.WarningsAsErrors |= Args.hasArg(OPT_warnings_as_errors);
   Opts.PrintDiagnosticNames |= Args.hasArg(OPT_debug_diagnostic_names);
+  Opts.EnableDescriptiveDiagnostics |=
+      Args.hasArg(OPT_enable_descriptive_diagnostics);
 
   assert(!(Opts.WarningsAsErrors && Opts.SuppressWarnings) &&
          "conflicting arguments; should have been caught by driver");

lib/Frontend/Frontend.cpp

@@ -315,6 +315,9 @@ void CompilerInstance::setUpDiagnosticOptions() {
   if (Invocation.getDiagnosticOptions().PrintDiagnosticNames) {
     Diagnostics.setPrintDiagnosticNames(true);
   }
+  if (Invocation.getDiagnosticOptions().EnableDescriptiveDiagnostics) {
+    Diagnostics.setUseDescriptiveDiagnostics(true);
+  }
 }
 
 bool CompilerInstance::setUpModuleLoaders() {