[clang-repl] add %help, documentation, and tests for %commands

clang/docs/ClangRepl.rst

@@ -197,6 +197,20 @@ Comments:
    clang-repl> // Comments in Clang-Repl
    clang-repl> /* Comments in Clang-Repl */
 
+Help:
+=====
+
+.. code-block:: text
+
+   clang-repl>%help
+
+Undo:
+=====
+
+.. code-block:: text
+
+   clang-repl>%undo
+
 
 Closure or Termination:
 =======================

clang/tools/clang-repl/ClangRepl.cpp

@@ -28,6 +28,7 @@
 #include "llvm/Support/ManagedStatic.h" // llvm_shutdown
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/TargetSelect.h"
+#include "llvm/Support/raw_ostream.h"
 #include "llvm/TargetParser/Host.h"
 #include <optional>
 
@@ -185,7 +186,7 @@ struct ReplListCompleter {
   clang::Interpreter &MainInterp;
   ReplListCompleter(clang::IncrementalCompilerBuilder &CB,
                     clang::Interpreter &Interp)
-      : CB(CB), MainInterp(Interp){};
+      : CB(CB), MainInterp(Interp) {};
 
   std::vector<llvm::LineEditor::Completion> operator()(llvm::StringRef Buffer,
                                                        size_t Pos) const;
@@ -347,7 +348,15 @@ int main(int argc, const char **argv) {
     }
   }
 
+  // if we add more % commands, there should be better architecture than this
+  const char *help_output = "%help\tlist clang-repl %commands\n"
+                            "%undo\tundo the previous input\n"
+                            "%quit\texit clang-repl\n";
+  const char *help_prompt = "type %help to list clang-repl commands\n";
+
+  llvm::raw_ostream &OS = llvm::outs();
   if (OptInputs.empty()) {
+    OS << help_prompt;
     llvm::LineEditor LE("clang-repl");
     std::string Input;
     LE.setListCompleter(ReplListCompleter(CB, *Interp));
@@ -370,6 +379,10 @@ int main(int argc, const char **argv) {
       if (Input == R"(%undo)") {
         if (auto Err = Interp->Undo())
           llvm::logAllUnhandledErrors(std::move(Err), llvm::errs(), "error: ");
+      } else if (Input == R"(%help)") {
+        OS << help_output << '\n';
+      } else if (Input[0] == '%') { // make sure this is evaluated last
+        OS << "Invalid % command: \"" << Input << "\". " << help_prompt;
       } else if (Input.rfind("%lib ", 0) == 0) {
         if (auto Err = Interp->LoadDynamicLibrary(Input.data() + 5))
           llvm::logAllUnhandledErrors(std::move(Err), llvm::errs(), "error: ");