Add support for command completion on commands, use it for :new

That means commands can be completed using other commands and their
completers. Yes that does makes sense.

Closes #296

rc/client.kak

@@ -23,6 +23,7 @@ decl str termcmd %sh{
 
 def -docstring 'create a new kak client for current session' \
     -shell-params \
+    -command-completion \
     new %{ %sh{
             if [ -z "${kak_opt_termcmd}" ]; then
                echo "echo -color Error 'termcmd option is not set'"

src/command_manager.cc

@@ -589,6 +589,40 @@ Completions CommandManager::complete(const Context& context,
     return Completions{};
 }
 
+Completions CommandManager::complete(const Context& context,
+                                     CompletionFlags flags,
+                                     CommandParameters params,
+                                     size_t token_to_complete,
+                                     ByteCount pos_in_token)
+{
+    StringView prefix = params[token_to_complete].substr(0, pos_in_token);
+
+    if (token_to_complete == 0)
+    {
+        CandidateList candidates;
+        for (auto& command : m_commands)
+        {
+            if (command.second.flags & CommandFlags::Hidden)
+                continue;
+            if (prefix_match(command.first, prefix))
+                candidates.push_back(command.first);
+        }
+        std::sort(candidates.begin(), candidates.end());
+        return {0, pos_in_token, std::move(candidates)};
+    }
+    else
+    {
+        const String& command_name = params[0];
+
+        auto command_it = find_command(context, command_name);
+        if (command_it != m_commands.end() and command_it->second.completer)
+            return command_it->second.completer(
+                context, flags, params.subrange(1, params.size()-1),
+                token_to_complete-1, pos_in_token);
+    }
+    return Completions{};
+}
+
 Completions PerArgumentCommandCompleter::operator()(const Context& context,
                                                     CompletionFlags flags,
                                                     CommandParameters params,

src/command_manager.hh

@@ -67,6 +67,10 @@ public:
     Completions complete(const Context& context, CompletionFlags flags,
                          StringView command_line, ByteCount cursor_pos);
 
+    Completions complete(const Context& context, CompletionFlags flags,
+                         CommandParameters params,
+                         size_t token_to_complete, ByteCount pos_in_token);
+
     CommandInfo command_info(const Context& context,
                              StringView command_line) const;
 

src/commands.cc

@@ -798,6 +798,16 @@ void define_command(const ParametersParser& parser, Context& context)
             return Completions{ 0_byte, pos_in_token, split(output, '\n', 0) };
         };
     }
+    else if (parser.get_switch("command-completion"))
+    {
+        completer = [](const Context& context, CompletionFlags flags,
+                       CommandParameters params,
+                       size_t token_to_complete, ByteCount pos_in_token)
+        {
+            return CommandManager::instance().complete(
+                context, flags, params, token_to_complete, pos_in_token);
+        };
+    }
 
     auto docstring = parser.get_switch("docstring").value_or(StringView{});
 
@@ -816,6 +826,7 @@ const CommandDesc define_command_cmd = {
           { "file-completion",   { false, "complete parameters using filename completion" } },
           { "client-completion", { false, "complete parameters using client name completion" } },
           { "buffer-completion", { false, "complete parameters using buffer name completion" } },
+          { "command-completion", { false, "complete parameters using kakoune command completion" } },
           { "shell-completion",  { true,  "complete the parameters using the given shell-script" } } },
         ParameterDesc::Flags::None,
         2, 2

src/normal.cc

@@ -345,7 +345,10 @@ void command(Context& context, NormalParams)
 
     context.input_handler().prompt(
         ":", "", get_face("Prompt"),
-        std::bind(&CommandManager::complete, &CommandManager::instance(), _1, _2, _3, _4),
+        [](const Context& context, CompletionFlags flags,
+           StringView cmd_line, ByteCount pos) {
+               return CommandManager::instance().complete(context, flags, cmd_line, pos);
+        },
         [](StringView cmdline, PromptEvent event, Context& context) {
             if (context.has_ui())
             {