Handle completion of % and # completion correctly.

The current behavior if you try to complete :e %:p:h or something similar
replaces only the h with the path, which isn't the desired behavior, because
then nvim tries to open the wrong filename. Detect this case and produce edits
that will replace the entire `%` chain with modifiers.

lua/cmp_cmdline/init.lua

@@ -78,6 +78,8 @@ local definitions = {
     kind = cmp.lsp.CompletionItemKind.Variable,
     isIncomplete = true,
     exec = function(option, arglead, cmdline, force)
+      -- Any edits we produce are relative to the whole command line.
+      local cmdline_length = #cmdline
       -- Ignore range only cmdline. (e.g.: 4, '<,'>)
       if not force and ONLY_RANGE_REGEX:match_str(cmdline) then
         return {}
@@ -115,18 +117,21 @@ local definitions = {
       -- In this case, the `vim.fn.getcompletion` will return only `get_query` for `vim.treesitter.get_|`.
       -- We should detect `vim.treesitter.` and `get_query` separately.
       -- TODO: The `\h\w*` was choosed by huristic. We should consider more suitable detection.
-      local fixed_input
-      do
-        local suffix_pos = vim.regex([[\h\w*$]]):match_str(arglead)
-        fixed_input = string.sub(arglead, 1, suffix_pos or #arglead)
-      end
+      local fixed_input = arglead
 
       -- The `vim.fn.getcompletion` does not return `*no*cursorline` option.
       -- cmp-cmdline corrects `no` prefix for option name.
       local is_option_name_completion = OPTION_NAME_COMPLETION_REGEX:match_str(cmdline) ~= nil
 
       local items = {}
       local escaped = cmdline:gsub([[\\]], [[\\\\]]);
+      local is_magic_file = false
+      local input_start = string.sub(fixed_input, 1, 1)
+      if input_start == '%' then
+        is_magic_file = true
+      elseif input_start == '#' then
+        is_magic_file = true
+      end
       for _, word_or_item in ipairs(vim.fn.getcompletion(escaped, 'cmdline')) do
         local word = type(word_or_item) == 'string' and word_or_item or word_or_item.word
         local item = { label = word }
@@ -139,9 +144,25 @@ local definitions = {
         end
       end
       for _, item in ipairs(items) do
-        if not string.find(item.label, fixed_input, 1, true) then
+        if not is_magic_file and not string.find(item.label, fixed_input, 1, true) then
           item.label = fixed_input .. item.label
         end
+        if is_magic_file then
+          local replace_range = {
+            start = {
+              character = cmdline_length - #fixed_input - 1
+            },
+            ['end'] = {
+              character = cmdline_length - 1
+            }
+          }
+          item.textEdit = {
+            replace = replace_range,
+            range = replace_range,
+            newText = item.label
+          }
+          item.label = fixed_input .. ' → ' .. item.label
+        end
       end
       return items
     end
@@ -196,6 +217,20 @@ source.complete = function(self, params, callback)
   for _, item in ipairs(items) do
     item.kind = kind
     labels[item.label] = true
+    if item['textEdit'] ~= nil then
+      local new_range = {
+        start = {
+          character = item.textEdit.range.start.character,
+          line = params.context.cursor.line
+        },
+        ['end'] = {
+          character = item.textEdit.range['end'].character,
+          line = params.context.cursor.line
+        }
+      }
+      item.textEdit.replace = new_range
+      item.textEdit.range = new_range
+    end
   end
 
   -- `vim.fn.getcompletion` does not handle fuzzy matches. So, we must return all items, including items that were matched in the previous input.