[LLDB]Provide clearer error message for invalid commands.

Sometimes users (esp. gdb-longtime users) accidentally use GDB syntax, such as `breakpoint foo`, and they would get an error message from LLDB saying simply `Invalid command "breakpoint foo"`, which is not very helpful.
This change provides additional suggestions to help correcting the mistake.

Author: oontvoo

lldb/include/lldb/Interpreter/CommandObjectMultiword.h

@@ -70,6 +70,8 @@ class CommandObjectMultiword : public CommandObject {
     return m_subcommand_dict;
   }
 
+  std::string GetTopSubcommands(int count);
+
   CommandObject::CommandMap m_subcommand_dict;
   bool m_can_be_removed;
 };

lldb/source/Commands/CommandObjectMultiword.cpp

@@ -194,28 +194,50 @@ void CommandObjectMultiword::Execute(const char *args_string,
 
   std::string error_msg;
   const size_t num_subcmd_matches = matches.GetSize();
-  if (num_subcmd_matches > 0)
+  if (num_subcmd_matches > 0) {
     error_msg.assign("ambiguous command ");
-  else
-    error_msg.assign("invalid command ");
-
-  error_msg.append("'");
-  error_msg.append(std::string(GetCommandName()));
-  error_msg.append(" ");
-  error_msg.append(std::string(sub_command));
-  error_msg.append("'.");
+    error_msg.append("'");
+    error_msg.append(std::string(GetCommandName()));
+    error_msg.append(" ");
+    error_msg.append(std::string(sub_command));
+    error_msg.append("'.");
 
-  if (num_subcmd_matches > 0) {
     error_msg.append(" Possible completions:");
     for (const std::string &match : matches) {
       error_msg.append("\n\t");
       error_msg.append(match);
     }
+  } else {
+    // Rather than simply complaining about the invalid (sub) command,
+    // try to offer some alternatives.
+    // This is especially useful for cases where the user types something
+    // seamingly trivial, such as `breakpoint foo`.
+    error_msg.assign(
+        llvm::Twine("'" + sub_command + "' is not a valid subcommand of \"" +
+                    GetCommandName() + "\". Valid subcommands are " +
+                    GetTopSubcommands(/*count=*/5) + ". Use \"help " +
+                    GetCommandName() + "\" to find out more.")
+            .str());
   }
   error_msg.append("\n");
   result.AppendRawError(error_msg.c_str());
 }
 
+std::string CommandObjectMultiword::GetTopSubcommands(int count) {
+  if (m_subcommand_dict.empty())
+    return "<NONE>";
+  std::string buffer = "{";
+  CommandMap::iterator pos;
+  for (pos = m_subcommand_dict.begin();
+       pos != m_subcommand_dict.end() && count > 0; ++pos, --count) {
+    buffer.append("'");
+    buffer.append(pos->first);
+    buffer.append("',");
+  }
+  buffer.append("...}");
+  return buffer;
+}
+
 void CommandObjectMultiword::GenerateHelpText(Stream &output_stream) {
   // First time through here, generate the help text for the object and push it
   // to the return result object as well

lldb/test/Shell/Commands/command-breakpoint-col.test

@@ -3,8 +3,10 @@
 # RUN: %clang_host -g -O0 %S/Inputs/main.c -o %t.out
 # RUN: %lldb -b -o 'help breakpoint set' -o 'breakpoint set -f main.c -l 2 -u 21' %t.out | FileCheck %s --check-prefix HELP --check-prefix CHECK
 # RUN: %lldb -b -o 'help _regexp-break' -o 'b main.c:2:21' %t.out | FileCheck %s --check-prefix HELP-REGEX --check-prefix CHECK
+# RUN: not %lldb -b -o 'breakpoint foo' %t.out -o exit 2>&1 | FileCheck %s --check-prefix ERROR-MSG
 # HELP: -u <column> ( --column <column> )
 # HELP: Specifies the column number on which to set this breakpoint.
 # HELP-REGEX: _regexp-break <filename>:<linenum>:<colnum>
 # HELP-REGEX: main.c:12:21{{.*}}Break at line 12 and column 21 of main.c
 # CHECK: at main.c:2:21
+# ERROR-MSG: 'foo' is not a valid subcommand of "breakpoint". Valid subcommands are {'clear','command','delete','disable','enable',...}. Use "help breakpoint" to find out more.