Compute prompt completion only from characters left of the cursor

If I type

	:echo -mx

I get no completions, even when I move the cursor on the x.
If I replace the x  with a k, I get a completion "-markup".
The odd thing is that if I accept the completion while the cursor is
on the k, then the commandline will be

	:echo markupk

Evidently, the characters under cursor (x/k) influence the completion
(actually all letters right of the cursor do), but they are not
incorporated in the final result, which is weird.

I think there are two consistent behaviors:
1. Compute completions only from characters left of the cursor. We already
   do this in insert mode completion, and when completing the command name
   in a prompt.
2. Compute completions from the whole token, and when accepting a completion,
   replace the whole token.

Most readline-style completion systems out there implement 1. A
notable exception is fish's tab-completion. I think we should stick
to 1 because it's more predictable and familiar. Do that.

This allows us to get rid of a special case for completing command
names, because the new behavior subsumes it.

In fact, I think this would allow us to get rid of most "pos_in_token"
or "cursor_pos" completer parameters. I believe the only place where we
it's actually different from the end of the query is in "shell-script"
completers, where we expose "$kak_pos_in_token". I think we can still
remove that parameter and just cut the commandline at the cursor
position before passing it to a "shell-script" completer. Then we
also don't need "$kak_token_to_complete" (but we can still keep
expose them for backwards compatibility).

1 files changed, 5 insertions, 5 deletions

diff --git a/src/command_manager.cc b/src/command_manager.cc
index 30684d85..35b3d665 100644
--- a/src/command_manager.cc
+++ b/src/command_manager.cc
@@ -653,8 +653,9 @@ Completions CommandManager::complete(const Context& context,
                                      StringView command_line,
                                      ByteCount cursor_pos)
 {
-    CommandParser parser{command_line};
-    const char* cursor = command_line.begin() + (int)cursor_pos;
+    auto prefix = command_line.substr(0_byte, cursor_pos);
+    CommandParser parser{prefix};
+    const char* cursor = prefix.begin() + (int)cursor_pos;
     Vector<Token> tokens;
 
     bool is_last_token = true;
@@ -675,7 +676,7 @@ Completions CommandManager::complete(const Context& context,
     }
 
     if (is_last_token)
-        tokens.push_back({Token::Type::Raw, command_line.length(), {}});
+        tokens.push_back({Token::Type::Raw, prefix.length(), {}});
     kak_assert(not tokens.empty());
     const auto& token = tokens.back();
 
@@ -712,8 +713,7 @@ Completions CommandManager::complete(const Context& context,
     if (tokens.size() == 1 and (token.type == Token::Type::Raw or
                                 token.type == Token::Type::RawQuoted))
     {
-        StringView query = command_line.substr(start, pos_in_token);
-        return offset_pos(requote(complete_command_name(context, query), token.type), start);
+        return offset_pos(requote(complete_command_name(context, prefix), token.type), start);
     }
 
     switch (token.type)