feat: add completion to pre-filtering (#626)

Works by pressing `<C-l>` in insert mode. Supported are all builtins that have prefiltering. Means:
- lsp_workspace_symbols
- lsp_workspace_diagnostics
- lsp_document_symbols
- lsp_document_diagnostics

lua/telescope/actions/init.lua

@@ -420,6 +420,49 @@ actions.smart_send_to_qflist = function(prompt_bufnr)
   end
 end
 
+actions.complete_tag = function(prompt_bufnr)
+  local current_picker = action_state.get_current_picker(prompt_bufnr)
+  local tags = current_picker.sorter.tags
+  local delimiter = current_picker.sorter._delimiter
+
+  if not tags then
+    print('No tag pre-filtering set for this picker')
+    return
+  end
+
+  -- format tags to match filter_function
+  local prefilter_tags = {}
+  for tag, _ in pairs(tags) do
+    table.insert(prefilter_tags, string.format('%s%s%s ', delimiter, tag:lower(), delimiter))
+  end
+
+  local line = action_state.get_current_line()
+  local filtered_tags = {}
+  -- retrigger completion with already selected tag anew
+  -- trim and add space since we can match [[:pattern: ]]  with or without space at the end
+  if vim.tbl_contains(prefilter_tags, vim.trim(line) .. " ") then
+    filtered_tags = prefilter_tags
+  else
+    -- match tag by substring
+    for _, tag in pairs(prefilter_tags) do
+      local start, _ = tag:find(line)
+      if start then
+        table.insert(filtered_tags, tag)
+      end
+    end
+  end
+
+  if vim.tbl_isempty(filtered_tags) then
+    print('No matches found')
+    return
+  end
+
+  -- incremental completion by substituting string starting from col - #line byte offset
+  local col = vim.api.nvim_win_get_cursor(0)[2] + 1
+  vim.fn.complete(col - #line, filtered_tags)
+
+end
+
 --- Open the quickfix list
 actions.open_qflist = function(_)
   vim.cmd [[copen]]

lua/telescope/mappings.lua

@@ -28,6 +28,7 @@ mappings.default_mappings = config.values.default_mappings or {
       ["<S-Tab>"] = actions.toggle_selection + actions.move_selection_better,
       ["<C-q>"] = actions.send_to_qflist + actions.open_qflist,
       ["<M-q>"] = actions.send_selected_to_qflist + actions.open_qflist,
+      ["<C-l>"] = actions.complete_tag
     },
 
     n = {

lua/telescope/sorters.lua

@@ -39,6 +39,7 @@ function Sorter:new(opts)
 
   return setmetatable({
     state = {},
+    tags = opts.tags,
     filter_function = opts.filter_function,
     scoring_function = opts.scoring_function,
     highlighter = opts.highlighter,
@@ -87,6 +88,7 @@ function Sorter:score(prompt, entry)
 
   local filter_score
   if self.filter_function ~= nil then
+    if self.tags then self.tags:insert(entry) end
     filter_score, prompt = self:filter_function(prompt, entry)
   end
 
@@ -476,7 +478,7 @@ end
 
 local substr_matcher = function(_, prompt, line, _)
   local display = line:lower()
-  local search_terms = util.max_split(prompt, "%s")
+  local search_terms = util.max_split(prompt:lower(), "%s")
   local matched = 0
   local total_search_terms = 0
   for _, word in pairs(search_terms) do
@@ -492,10 +494,9 @@ end
 local filter_function = function(opts)
   local scoring_function = vim.F.if_nil(opts.filter_function, substr_matcher)
   local tag = vim.F.if_nil(opts.tag, "ordinal")
-  local delimiter = vim.F.if_nil(opts.delimiter, ":")
 
   return function(_, prompt, entry)
-    local filter = "^(" .. delimiter .. "(%S+)" .. "[" .. delimiter .. "%s]" .. ")"
+    local filter = "^(" .. opts.delimiter .. "(%S+)" .. "[" .. opts.delimiter .. "%s]" .. ")"
     local matched = prompt:match(filter)
 
     if matched == nil then
@@ -503,13 +504,29 @@ local filter_function = function(opts)
     end
     -- clear prompt of tag
     prompt = prompt:sub(#matched + 1, -1)
-    local query = vim.trim(matched:gsub(delimiter, ""))
+    local query = vim.trim(matched:gsub(opts.delimiter, ""))
     return scoring_function(_, query, entry[tag], _), prompt
   end
 end
 
+local function create_tag_set(tag)
+  tag = vim.F.if_nil(tag, 'ordinal')
+  local set = {}
+  return setmetatable(set, {
+    __index = {
+      insert = function(set_, entry)
+        local value = entry[tag]
+        if not set_[value] then set_[value] = true end
+      end
+    }
+  })
+end
+
 sorters.prefilter = function(opts)
   local sorter = opts.sorter
+  opts.delimiter = util.get_default(opts.delimiter, ':')
+  sorter._delimiter = opts.delimiter
+  sorter.tags = create_tag_set(opts.tag)
   sorter.filter_function = filter_function(opts)
   sorter._was_discarded = function() return false end
   return sorter