[flang] Introduce DiagnosticConsumer classes in libflangFrontend

Currently Flang uses TextDiagnostic, TextDiagnosticPrinter &
TestDiagnosticBuffer classes from Clang (more specifically, from
libclangFrontend). This patch introduces simplified equivalents of these
classes in Flang (i.e. it removes the dependency on libclangFrontend).

Flang only needs these diagnostics classes for the compiler driver
diagnostics. This is unlike in Clang in which similar diagnostic classes
are used for e.g. Lexing/Parsing/Sema diagnostics. For this reason, the
implementations introduced here are relatively basic. We can extend them
in the future if this is required.

This patch also enhances how the diagnostics are printed. In particular,
this is the diagnostic that you'd get _before_  the changes introduced here
(no text formatting):

```
$ bin/flang-new
error: no input files
```

This is the diagnostic that you get _after_ the changes introduced here
(in terminals that support it, the text is formatted - bold + red):

```
$ bin/flang-new
flang-new: error: no input files
```

Tests are updated accordingly and options related to enabling/disabling
color diagnostics are flagged as supported by Flang.

Reviewed By: sameeranjoshi, CarolineConcatto

Differential Revision: https://reviews.llvm.org/D87774

Author: banach-space

clang/include/clang/Driver/Options.td

@@ -876,7 +876,8 @@ def fclang_abi_compat_EQ : Joined<["-"], "fclang-abi-compat=">, Group<f_clang_Gr
   Flags<[CC1Option]>, MetaVarName<"<version>">, Values<"<major>.<minor>,latest">,
   HelpText<"Attempt to match the ABI of Clang <version>">;
 def fclasspath_EQ : Joined<["-"], "fclasspath=">, Group<f_Group>;
-defm color_diagnostics : OptInFFlag<"color-diagnostics", "Enable", "Disable", " colors in diagnostics", [CoreOption]>;
+defm color_diagnostics : OptInFFlag<"color-diagnostics", "Enable", "Disable", " colors in diagnostics",
+  [CoreOption, FlangOption]>;
 def fdiagnostics_color : Flag<["-"], "fdiagnostics-color">, Group<f_Group>,
   Flags<[CoreOption, DriverOption]>;
 def fdiagnostics_color_EQ : Joined<["-"], "fdiagnostics-color=">, Group<f_Group>;

flang/include/flang/Frontend/CompilerInvocation.h

@@ -11,8 +11,17 @@
 #include "flang/Frontend/FrontendOptions.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticOptions.h"
+#include "llvm/Option/ArgList.h"
 
 namespace Fortran::frontend {
+
+/// Fill out Opts based on the options given in Args.
+///
+/// When errors are encountered, return false and, if Diags is non-null,
+/// report the error(s).
+bool ParseDiagnosticArgs(clang::DiagnosticOptions &opts,
+    llvm::opt::ArgList &args, bool defaultDiagColor = true);
+
 class CompilerInvocationBase {
 public:
   /// Options controlling the diagnostic engine.$

flang/include/flang/Frontend/TextDiagnostic.h

@@ -0,0 +1,70 @@
+//===--- TextDiagnostic.h - Text Diagnostic Pretty-Printing -----*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// A utility class that provides support for textual pretty-printing of
+// diagnostics. Based on clang::TextDiagnostic (this is a trimmed version).
+//
+// TODO: If expanding, consider sharing the implementation with Clang.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FLANG_FRONTEND_TEXTDIAGNOSTIC_H
+#define LLVM_FLANG_FRONTEND_TEXTDIAGNOSTIC_H
+
+#include "clang/Basic/Diagnostic.h"
+#include "llvm/ADT/IntrusiveRefCntPtr.h"
+
+namespace Fortran::frontend {
+
+/// Class to encapsulate the logic for formatting and printing a textual
+/// diagnostic message.
+///
+/// The purpose of this class is to isolate the implementation of printing
+/// beautiful text diagnostics from any particular interfaces. Currently only
+/// simple diagnostics that lack source location information are supported (e.g.
+/// Flang driver errors).
+///
+/// In the future we can extend this class (akin to Clang) to support more
+/// complex diagnostics that would include macro backtraces, caret diagnostics,
+/// FixIt Hints and code snippets.
+///
+class TextDiagnostic {
+public:
+  TextDiagnostic();
+
+  ~TextDiagnostic();
+
+  /// Print the diagnostic level to a llvm::raw_ostream.
+  ///
+  /// This is a static helper that handles colorizing the level and formatting
+  /// it into an arbitrary output stream.
+  ///
+  /// \param os Where the message is printed
+  /// \param level The diagnostic level (e.g. error or warning)
+  /// \param showColors Enable colorizing of the message.
+  static void PrintDiagnosticLevel(llvm::raw_ostream &os,
+      clang::DiagnosticsEngine::Level level, bool showColors);
+
+  /// Pretty-print a diagnostic message to a llvm::raw_ostream.
+  ///
+  /// This is a static helper to handle the colorizing and rendering diagnostic
+  /// message to a particular ostream. In the future we can
+  /// extend it to support e.g. line wrapping. It is
+  /// publicly visible as at this stage we don't require any state data to
+  /// print a diagnostic.
+  ///
+  /// \param os Where the message is printed
+  /// \param isSupplemental true if this is a continuation note diagnostic
+  /// \param message The text actually printed
+  /// \param showColors Enable colorizing of the message.
+  static void PrintDiagnosticMessage(llvm::raw_ostream &os, bool isSupplemental,
+      llvm::StringRef message, bool showColors);
+};
+
+} // namespace Fortran::frontend
+
+#endif

flang/include/flang/Frontend/TextDiagnosticBuffer.h

@@ -0,0 +1,52 @@
+//===- TextDiagnosticBuffer.h - Buffer Text Diagnostics ---------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This is a concrete diagnostic client. The diagnostics are buffered rather
+// than printed. In order to print them, use the FlushDiagnostics method.
+// Pretty-printing is not supported.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FLANG_FRONTEND_TEXTDIAGNOSTICBUFFER_H
+#define LLVM_FLANG_FRONTEND_TEXTDIAGNOSTICBUFFER_H
+
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Basic/SourceLocation.h"
+#include <cstddef>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace Fortran::frontend {
+
+class TextDiagnosticBuffer : public clang::DiagnosticConsumer {
+public:
+  using DiagList = std::vector<std::pair<clang::SourceLocation, std::string>>;
+  using DiagnosticsLevelAndIndexPairs =
+      std::vector<std::pair<clang::DiagnosticsEngine::Level, size_t>>;
+
+private:
+  DiagList errors_, warnings_, remarks_, notes_;
+
+  /// All diagnostics in the order in which they were generated. That order
+  /// likely doesn't correspond to user input order, but at least it keeps
+  /// notes in the right places. Each pair is a diagnostic level and an index
+  /// into the corresponding DiagList above.
+  DiagnosticsLevelAndIndexPairs all_;
+
+public:
+  void HandleDiagnostic(clang::DiagnosticsEngine::Level diagLevel,
+      const clang::Diagnostic &info) override;
+
+  /// Flush the buffered diagnostics to a given diagnostic engine.
+  void FlushDiagnostics(clang::DiagnosticsEngine &diags) const;
+};
+
+} // namespace Fortran::frontend
+
+#endif // LLVM_CLANG_FRONTEND_TEXTDIAGNOSTICBUFFER_H

flang/include/flang/Frontend/TextDiagnosticPrinter.h

@@ -0,0 +1,55 @@
+//===--- TextDiagnosticPrinter.h - Text Diagnostic Client -------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This is a concrete diagnostic client. In terminals that support it, the
+// diagnostics are pretty-printed (colors + bold). The printing/flushing
+// happens in HandleDiagnostics (usually called at the point when the
+// diagnostic is generated).
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FLANG_FRONTEND_TEXTDIAGNOSTICPRINTER_H
+#define LLVM_FLANG_FRONTEND_TEXTDIAGNOSTICPRINTER_H
+
+#include "clang/Basic/Diagnostic.h"
+#include "llvm/ADT/IntrusiveRefCntPtr.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace clang {
+class DiagnosticOptions;
+class DiagnosticsEngine;
+}; // namespace clang
+
+using llvm::IntrusiveRefCntPtr;
+using llvm::raw_ostream;
+
+namespace Fortran::frontend {
+class TextDiagnostic;
+
+class TextDiagnosticPrinter : public clang::DiagnosticConsumer {
+  raw_ostream &os_;
+  llvm::IntrusiveRefCntPtr<clang::DiagnosticOptions> diagOpts_;
+
+  /// A string to prefix to error messages.
+  std::string prefix_;
+
+public:
+  TextDiagnosticPrinter(raw_ostream &os, clang::DiagnosticOptions *diags);
+  ~TextDiagnosticPrinter() override;
+
+  /// Set the diagnostic printer prefix string, which will be printed at the
+  /// start of any diagnostics. If empty, no prefix string is used.
+  void set_prefix(std::string value) { prefix_ = std::move(value); }
+
+  void HandleDiagnostic(clang::DiagnosticsEngine::Level level,
+      const clang::Diagnostic &info) override;
+};
+
+} // namespace Fortran::frontend
+
+#endif

flang/lib/Frontend/CMakeLists.txt

@@ -2,16 +2,16 @@ add_flang_library(flangFrontend
   CompilerInstance.cpp
   CompilerInvocation.cpp
   FrontendOptions.cpp
+  TextDiagnosticPrinter.cpp
+  TextDiagnosticBuffer.cpp
+  TextDiagnostic.cpp
 
   DEPENDS
   clangBasic
 
   LINK_LIBS
   clangBasic
   clangDriver
-  # TODO: Added to re-use clang's TextDiagnosticBuffer & TextDiagnosticPrinter.
-  # Add a custom implementation for Flang and remove this dependency.
-  clangFrontend
 
   LINK_COMPONENTS
   Option

flang/lib/Frontend/CompilerInstance.cpp

@@ -8,7 +8,7 @@
 
 #include "flang/Frontend/CompilerInstance.h"
 #include "flang/Frontend/CompilerInvocation.h"
-#include "clang/Frontend/TextDiagnosticPrinter.h"
+#include "flang/Frontend/TextDiagnosticPrinter.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace Fortran::frontend;
@@ -36,7 +36,7 @@ CompilerInstance::CreateDiagnostics(clang::DiagnosticOptions *opts,
   if (client) {
     diags->setClient(client, shouldOwnClient);
   } else {
-    diags->setClient(new clang::TextDiagnosticPrinter(llvm::errs(), opts));
+    diags->setClient(new TextDiagnosticPrinter(llvm::errs(), opts));
   }
   return diags;
 }

flang/lib/Frontend/CompilerInvocation.cpp

@@ -17,6 +17,7 @@
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/OptTable.h"
+#include "llvm/Support/Process.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace Fortran::frontend;
@@ -35,6 +36,48 @@ CompilerInvocationBase::~CompilerInvocationBase() = default;
 //===----------------------------------------------------------------------===//
 // Deserialization (from args)
 //===----------------------------------------------------------------------===//
+static bool parseShowColorsArgs(
+    const llvm::opt::ArgList &args, bool defaultColor) {
+  // Color diagnostics default to auto ("on" if terminal supports) in the driver
+  // but default to off in cc1, needing an explicit OPT_fdiagnostics_color.
+  // Support both clang's -f[no-]color-diagnostics and gcc's
+  // -f[no-]diagnostics-colors[=never|always|auto].
+  enum {
+    Colors_On,
+    Colors_Off,
+    Colors_Auto
+  } ShowColors = defaultColor ? Colors_Auto : Colors_Off;
+
+  for (auto *a : args) {
+    const llvm::opt::Option &O = a->getOption();
+    if (O.matches(clang::driver::options::OPT_fcolor_diagnostics) ||
+        O.matches(clang::driver::options::OPT_fdiagnostics_color)) {
+      ShowColors = Colors_On;
+    } else if (O.matches(clang::driver::options::OPT_fno_color_diagnostics) ||
+        O.matches(clang::driver::options::OPT_fno_diagnostics_color)) {
+      ShowColors = Colors_Off;
+    } else if (O.matches(clang::driver::options::OPT_fdiagnostics_color_EQ)) {
+      llvm::StringRef value(a->getValue());
+      if (value == "always")
+        ShowColors = Colors_On;
+      else if (value == "never")
+        ShowColors = Colors_Off;
+      else if (value == "auto")
+        ShowColors = Colors_Auto;
+    }
+  }
+
+  return ShowColors == Colors_On ||
+      (ShowColors == Colors_Auto && llvm::sys::Process::StandardErrHasColors());
+}
+
+bool Fortran::frontend::ParseDiagnosticArgs(clang::DiagnosticOptions &opts,
+    llvm::opt::ArgList &args, bool defaultDiagColor) {
+  opts.ShowColors = parseShowColorsArgs(args, defaultDiagColor);
+
+  return true;
+}
+
 static InputKind ParseFrontendArgs(FrontendOptions &opts,
     llvm::opt::ArgList &args, clang::DiagnosticsEngine &diags) {
   // Identify the action (i.e. opts.ProgramAction)