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`.

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)