feat(lua): completion for vim.fn, vim.v, vim.o #30472

Problem: Lua accessors for
- global, local, and special variables (`vim.{g,t,w,b,v}.*`), and
- options (`vim.{o,bo,wo,opt,opt_local,opt_global}.*`),

do not have command-line completion, unlike their vimscript counterparts
(e.g., `g:`, `b:`, `:set`, `:setlocal`, `:call <fn>`, etc.).

Completion for vimscript functions (`vim.fn.*`) is incomplete and does
not list all the available functions.

Solution: Implement completion for vimscript function, variable and
option accessors in `vim._expand_pat` through:

- `getcompletion()` for variable and vimscript function accessors, and
- `nvim_get_all_options_info()` for option accessors.

Note/Remark:

- Short names for options are yet to be implemented.

- Completions for accessors with handles (e.g. `vim.b[0]`, `vim.wo[0]`)
  are also yet to be implemented, and are left as future work, which
  involves some refactoring of options.

- For performance reasons, we may want to introduce caching for
  completing options, but this is not considered at this time since the
  number of the available options is not very big (only ~350) and Lua
  completion for option accessors appears to be pretty fast.

- Can we have a more "general" framework for customizing completions?
  In the future, we may want to improve the implementation by moving the
  core logic for generating completion candidates to each accessor (or
  its metatable) or through some central interface, rather than writing
  all the accessor-specific completion implementations in a single
  function: `vim._expand_pat`.

3 files changed, 234 insertions, 4 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index d8e113190a..d2e66f4596 100644
--- a/runtime/doc/news.txt
+++ b/runtime/doc/news.txt
@@ -78,6 +78,9 @@ LUA
 
 • API functions now consistently return an empty dictionary as
   |vim.empty_dict()|. Earlier, a |lua-special-tbl| was sometimes used.
+• Command-line completions for: `vim.g`, `vim.t`, `vim.w`, `vim.b`, `vim.v`,
+  `vim.o`, `vim.wo`, `vim.bo`, `vim.opt`, `vim.opt_local`, `vim.opt_global`,
+  and `vim.fn`.
 
 OPTIONS
 
diff --git a/runtime/lua/vim/_editor.lua b/runtime/lua/vim/_editor.lua
index bdca97cfb8..2e829578a7 100644
--- a/runtime/lua/vim/_editor.lua
+++ b/runtime/lua/vim/_editor.lua
@@ -787,7 +787,7 @@ function vim._expand_pat(pat, env)
       if mt and type(mt.__index) == 'table' then
         field = rawget(mt.__index, key)
       elseif final_env == vim and (vim._submodules[key] or vim._extra[key]) then
-        field = vim[key]
+        field = vim[key] --- @type any
       end
     end
     final_env = field
@@ -798,14 +798,24 @@ function vim._expand_pat(pat, env)
   end
 
   local keys = {} --- @type table<string,true>
+
   --- @param obj table<any,any>
   local function insert_keys(obj)
     for k, _ in pairs(obj) do
-      if type(k) == 'string' and string.sub(k, 1, string.len(match_part)) == match_part then
+      if
+        type(k) == 'string'
+        and string.sub(k, 1, string.len(match_part)) == match_part
+        and k:match('^[_%w]+$') ~= nil -- filter out invalid identifiers for field, e.g. 'foo#bar'
+      then
         keys[k] = true
       end
     end
   end
+  ---@param acc table<string,any>
+  local function _fold_to_map(acc, k, v)
+    acc[k] = (v or true)
+    return acc
+  end
 
   if type(final_env) == 'table' then
     insert_keys(final_env)
@@ -814,11 +824,61 @@ function vim._expand_pat(pat, env)
   if mt and type(mt.__index) == 'table' then
     insert_keys(mt.__index)
   end
+
   if final_env == vim then
     insert_keys(vim._submodules)
     insert_keys(vim._extra)
   end
 
+  -- Completion for dict accessors (special vim variables and vim.fn)
+  if mt and vim.tbl_contains({ vim.g, vim.t, vim.w, vim.b, vim.v, vim.fn }, final_env) then
+    local prefix, type = unpack(
+      vim.fn == final_env and { '', 'function' }
+        or vim.g == final_env and { 'g:', 'var' }
+        or vim.t == final_env and { 't:', 'var' }
+        or vim.w == final_env and { 'w:', 'var' }
+        or vim.b == final_env and { 'b:', 'var' }
+        or vim.v == final_env and { 'v:', 'var' }
+        or { nil, nil }
+    )
+    assert(prefix, "Can't resolve final_env")
+    local vars = vim.fn.getcompletion(prefix .. match_part, type) --- @type string[]
+    insert_keys(vim
+      .iter(vars)
+      :map(function(s) ---@param s string
+        s = s:gsub('[()]+$', '') -- strip '(' and ')' for function completions
+        return s:sub(#prefix + 1) -- strip the prefix, e.g., 'g:foo' => 'foo'
+      end)
+      :fold({}, _fold_to_map))
+  end
+
+  -- Completion for option accessors (full names only)
+  if
+    mt
+    and vim.tbl_contains(
+      { vim.o, vim.go, vim.bo, vim.wo, vim.opt, vim.opt_local, vim.opt_global },
+      final_env
+    )
+  then
+    --- @type fun(option_name: string, option: vim.api.keyset.get_option_info): boolean
+    local filter = function(_, _)
+      return true
+    end
+    if vim.bo == final_env then
+      filter = function(_, option)
+        return option.scope == 'buf'
+      end
+    elseif vim.wo == final_env then
+      filter = function(_, option)
+        return option.scope == 'win'
+      end
+    end
+
+    --- @type table<string, vim.api.keyset.get_option_info>
+    local options = vim.api.nvim_get_all_options_info()
+    insert_keys(vim.iter(options):filter(filter):fold({}, _fold_to_map))
+  end
+
   keys = vim.tbl_keys(keys)
   table.sort(keys)
 
diff --git a/test/functional/lua/command_line_completion_spec.lua b/test/functional/lua/command_line_completion_spec.lua
index 2ba432133b..f8786a45bb 100644
--- a/test/functional/lua/command_line_completion_spec.lua
+++ b/test/functional/lua/command_line_completion_spec.lua
@@ -5,12 +5,14 @@ local clear = n.clear
 local eq = t.eq
 local exec_lua = n.exec_lua
 
+--- @return { [1]: string[], [2]: integer }
 local get_completions = function(input, env)
-  return exec_lua('return {vim._expand_pat(...)}', input, env)
+  return exec_lua('return { vim._expand_pat(...) }', input, env)
 end
 
+--- @return { [1]: string[], [2]: integer }
 local get_compl_parts = function(parts)
-  return exec_lua('return {vim._expand_pat_get_parts(...)}', parts)
+  return exec_lua('return { vim._expand_pat_get_parts(...) }', parts)
 end
 
 before_each(clear)
@@ -123,6 +125,171 @@ describe('nlua_expand_pat', function()
     )
   end)
 
+  describe('should complete vim.fn', function()
+    it('correctly works for simple completion', function()
+      local actual = get_completions('vim.fn.did')
+      local expected = {
+        { 'did_filetype' },
+        #'vim.fn.',
+      }
+      eq(expected, actual)
+    end)
+    it('should not suggest items with #', function()
+      exec_lua [[
+        -- ensure remote#host#... functions exist
+        vim.cmd [=[
+          runtime! autoload/remote/host.vim
+        ]=]
+        -- make a dummy call to ensure vim.fn contains an entry: remote#host#...
+        vim.fn['remote#host#IsRunning']('python3')
+      ]]
+      local actual = get_completions('vim.fn.remo')
+      local expected = {
+        { 'remove' }, -- there should be no completion "remote#host#..."
+        #'vim.fn.',
+      }
+      eq(expected, actual)
+    end)
+  end)
+
+  describe('should complete for variable accessors for', function()
+    it('vim.v', function()
+      local actual = get_completions('vim.v.t_')
+      local expected = {
+        { 't_blob', 't_bool', 't_dict', 't_float', 't_func', 't_list', 't_number', 't_string' },
+        #'vim.v.',
+      }
+      eq(expected, actual)
+    end)
+
+    it('vim.g', function()
+      exec_lua [[
+        vim.cmd [=[
+          let g:nlua_foo = 'completion'
+          let g:nlua_foo_bar = 'completion'
+          let g:nlua_foo#bar = 'nocompletion'  " should be excluded from lua completion
+        ]=]
+      ]]
+      local actual = get_completions('vim.g.nlua')
+      local expected = {
+        { 'nlua_foo', 'nlua_foo_bar' },
+        #'vim.g.',
+      }
+      eq(expected, actual)
+    end)
+
+    it('vim.b', function()
+      exec_lua [[
+        vim.b.nlua_foo_buf = 'bar'
+        vim.b.some_other_vars = 'bar'
+      ]]
+      local actual = get_completions('vim.b.nlua')
+      local expected = {
+        { 'nlua_foo_buf' },
+        #'vim.b.',
+      }
+      eq(expected, actual)
+    end)
+
+    it('vim.w', function()
+      exec_lua [[
+        vim.w.nlua_win_var = 42
+      ]]
+      local actual = get_completions('vim.w.nlua')
+      local expected = {
+        { 'nlua_win_var' },
+        #'vim.w.',
+      }
+      eq(expected, actual)
+    end)
+
+    it('vim.t', function()
+      exec_lua [[
+        vim.t.nlua_tab_var = 42
+      ]]
+      local actual = get_completions('vim.t.')
+      local expected = {
+        { 'nlua_tab_var' },
+        #'vim.t.',
+      }
+      eq(expected, actual)
+    end)
+  end)
+
+  describe('should complete for option accessors for', function()
+    -- for { vim.o, vim.go, vim.opt, vim.opt_local, vim.opt_global }
+    local test_opt = function(accessor)
+      do
+        local actual = get_completions(accessor .. '.file')
+        local expected = {
+          'fileencoding',
+          'fileencodings',
+          'fileformat',
+          'fileformats',
+          'fileignorecase',
+          'filetype',
+        }
+        eq({ expected, #accessor + 1 }, actual, accessor .. '.file')
+      end
+      do
+        local actual = get_completions(accessor .. '.winh')
+        local expected = {
+          'winheight',
+          'winhighlight',
+        }
+        eq({ expected, #accessor + 1 }, actual, accessor .. '.winh')
+      end
+    end
+
+    test_opt('vim.o')
+    test_opt('vim.go')
+    test_opt('vim.opt')
+    test_opt('vim.opt_local')
+    test_opt('vim.opt_global')
+
+    it('vim.o, suggesting all the known options', function()
+      local completions = get_completions('vim.o.')[1] ---@type string[]
+      eq(
+        exec_lua [[
+        return vim.tbl_count(vim.api.nvim_get_all_options_info())
+      ]],
+        #completions
+      )
+    end)
+
+    it('vim.bo', function()
+      do
+        local actual = get_completions('vim.bo.file')
+        local compls = {
+          -- should contain buffer options only
+          'fileencoding',
+          'fileformat',
+          'filetype',
+        }
+        eq({ compls, #'vim.bo.' }, actual)
+      end
+      do
+        local actual = get_completions('vim.bo.winh')
+        local compls = {}
+        eq({ compls, #'vim.bo.' }, actual)
+      end
+    end)
+
+    it('vim.wo', function()
+      do
+        local actual = get_completions('vim.wo.file')
+        local compls = {}
+        eq({ compls, #'vim.wo.' }, actual)
+      end
+      do
+        local actual = get_completions('vim.wo.winh')
+        -- should contain window options only
+        local compls = { 'winhighlight' }
+        eq({ compls, #'vim.wo.' }, actual)
+      end
+    end)
+  end)
+
   it('should return everything if the input is of length 0', function()
     eq({ { 'other', 'vim' }, 0 }, get_completions('', { vim = true, other = true }))
   end)