fix(cmdline): cmdline completion of _defer_require() modules #33007

Problem:
`:lua vim.lsp.c<tab>` does not list vim.lsp.completion in the completion
list after 24cea4c7f7417c7fe99a98a0487f51dd68c4f409.

Solution:
- Always include `vim.lsp._submodule` keys in candidates.
  - Fixes `vim.lsp.c<tab>` -> `vim.lsp.completion`.
- Eager-load `vim.lsp.completion` to get its completion.
  - Fixes `vim.lsp.completion.g<tab>` -> `vim.lsp.completion.get`.

3 files changed, 84 insertions, 17 deletions

diff --git a/runtime/lua/vim/_editor.lua b/runtime/lua/vim/_editor.lua
index 6a8b23ba40..f0b8587476 100644
--- a/runtime/lua/vim/_editor.lua
+++ b/runtime/lua/vim/_editor.lua
@@ -923,6 +923,39 @@ function vim._expand_pat(pat, env)
 
   local final_env = env
 
+  --- @private
+  ---
+  --- Allows submodules to be defined on a `vim.<module>` table without eager-loading the module.
+  ---
+  --- Cmdline completion (`:lua vim.lsp.c<tab>`) accesses `vim.lsp._submodules` when no other candidates.
+  --- Cmdline completion (`:lua vim.lsp.completion.g<tab>`) will eager-load the module anyway. #33007
+  ---
+  --- @param m table
+  --- @param k string
+  --- @return any
+  local function safe_tbl_get(m, k)
+    local val = rawget(m, k)
+    if val ~= nil then
+      return val
+    end
+
+    local mt = getmetatable(m)
+    if not mt then
+      return m == vim and vim._extra[k] or nil
+    end
+
+    -- use mt.__index, _submodules as fallback
+    if type(mt.__index) == 'table' then
+      return rawget(mt.__index, k)
+    end
+
+    local sub = rawget(m, '_submodules')
+    if sub and type(sub) == 'table' and rawget(sub, k) then
+      -- Access the module to force _defer_require() to load the module.
+      return m[k]
+    end
+  end
+
   for _, part in ipairs(parts) do
     if type(final_env) ~= 'table' then
       return {}, 0
@@ -953,16 +986,7 @@ function vim._expand_pat(pat, env)
 
       key = result
     end
-    local field = rawget(final_env, key)
-    if field == nil then
-      local mt = getmetatable(final_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] --- @type any
-      end
-    end
-    final_env = field
+    final_env = safe_tbl_get(final_env, key)
 
     if not final_env then
       return {}, 0
@@ -992,17 +1016,20 @@ function vim._expand_pat(pat, env)
 
   if type(final_env) == 'table' then
     insert_keys(final_env)
+    local sub = rawget(final_env, '_submodules')
+    if type(sub) == 'table' then
+      insert_keys(sub)
+    end
+    if final_env == vim then
+      insert_keys(vim._extra)
+    end
   end
+
   local mt = getmetatable(final_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(
diff --git a/test/functional/editor/completion_spec.lua b/test/functional/editor/completion_spec.lua
index 106e0df347..3e5d0e48e8 100644
--- a/test/functional/editor/completion_spec.lua
+++ b/test/functional/editor/completion_spec.lua
@@ -928,6 +928,13 @@ describe('completion', function()
       command('set wildoptions+=fuzzy')
       eq({ 'vim' }, fn.getcompletion('vi', 'lua'))
     end)
+
+    it('completes _defer_require() modules', function()
+      -- vim.lsp.c<tab> -> vim.lsp.completion
+      ok(vim.tbl_contains(fn.getcompletion('lua vim.lsp.c', 'cmdline'), 'completion'))
+      -- vim.lsp.completion.g<tab> -> vim.lsp.completion.get
+      ok(vim.tbl_contains(fn.getcompletion('lua vim.lsp.completion.g', 'cmdline'), 'get'))
+    end)
   end)
 
   it('cmdline completion supports various string options', function()
diff --git a/test/functional/lua/command_line_completion_spec.lua b/test/functional/lua/command_line_completion_spec.lua
index 7dac7448e9..ee3d0325e1 100644
--- a/test/functional/lua/command_line_completion_spec.lua
+++ b/test/functional/lua/command_line_completion_spec.lua
@@ -24,6 +24,23 @@ describe('nlua_expand_pat', function()
 
   it('returns empty table when nothing matches', function()
     eq({ {}, 0 }, get_completions('foo', { bar = true }))
+
+    -- can access non-exist field
+    for _, m in ipairs {
+      'vim.',
+      'vim.lsp.',
+      'vim.treesitter.',
+      'vim.deepcopy.',
+      'vim.fn.',
+      'vim.api.',
+      'vim.o.',
+      'vim.b.',
+    } do
+      eq({ {}, m:len() }, get_completions(m .. 'foo'))
+      eq({ {}, 0 }, get_completions(m .. 'foo.'))
+      eq({ {}, 0 }, get_completions(m .. 'foo.bar'))
+      eq({ {}, 0 }, get_completions(m .. 'foo.bar.'))
+    end
   end)
 
   it('returns nice completions with function call prefix', function()
@@ -99,12 +116,28 @@ describe('nlua_expand_pat', function()
 
   it('with lazy submodules of "vim" global', function()
     eq({ { 'inspect', 'inspect_pos' }, 4 }, get_completions('vim.inspec'))
-
     eq({ { 'treesitter' }, 4 }, get_completions('vim.treesi'))
-
+    eq({ { 'dev' }, 15 }, get_completions('vim.treesitter.de'))
+    eq({ { 'edit_query' }, 19 }, get_completions('vim.treesitter.dev.edit_'))
     eq({ { 'set' }, 11 }, get_completions('vim.keymap.se'))
   end)
 
+  it('include keys in mt.__index and ._submodules', function()
+    eq(
+      { { 'bar1', 'bar2', 'bar3' }, 4 },
+      exec_lua(function() -- metatable cannot be serialized
+        return {
+          vim._expand_pat('foo.', {
+            foo = setmetatable(
+              { bar1 = true, _submodules = { bar2 = true } },
+              { __index = { bar3 = true } }
+            ),
+          }),
+        }
+      end)
+    )
+  end)
+
   it('excludes private fields after "."', function()
     eq(
       { { 'bar' }, 4 },