feat(:restart)!: ":restart" (no bang) saves/restores session #40321

Problem:
`:restart` does not preserve window layout, etc.

Solution:
- Change `:restart` to save/restore a session automatically.
- Introduce "bang" variant `:restart!` to restart *without* session
  save/restore.
- Introduce `v:startreason`.
- `ZR` maps to `:restart!`.

runtime/lua/vim/_core/server.lua

@@ -1,3 +1,5 @@
+local fs = vim.fs
+local uv = vim.uv
 -- For "--listen" and related functionality.
 
 local M = {}
@@ -154,4 +156,63 @@ function M.rebind_after_restart(canonical_addr, expected_uis)
   end)
 end
 
+-- Called by ex_restart(). Saves the current session and calls back to
+-- ex_restart() with the necessary arguments to restore the session.
+-- TODO: https://github.com/neovim/neovim/issues/34204
+--- @param eap vim._core.ExCmdArgs
+--- @param extra { quit_cmd: string }
+function M.ex_session_restart(eap, extra)
+  -- Commands to run after restart
+  local after_cmd = eap.args
+  assert(not after_cmd:find(']==]'))
+
+  -- Use custom quit command if provided
+  local quit_cmd = 'qall'
+  if extra.quit_cmd ~= '' then
+    quit_cmd = extra.quit_cmd
+  end
+
+  -- Preserve the value of v:this_session
+  local this_session = vim.v.this_session
+  assert(not this_session:find(']==]'))
+
+  -- Get temp file to write session to
+  local temp_dir = fs.abspath(fs.dirname(fs.dirname(vim.fn.tempname())))
+  assert(not temp_dir:find(']==]'))
+  local fd, session = uv.fs_mkstemp(fs.joinpath(temp_dir, 'restart_session_XXXXXX'))
+  if not fd then
+    error('Failed to get temporary filename for restart session')
+  end
+  uv.fs_close(fd)
+
+  -- Write session
+  local session_arg = vim.fn.fnameescape(session)
+  vim.cmd('%argdelete')
+  vim.cmd.mksession { session_arg, bang = true }
+
+  -- Lua commands to restore the session and remove the session file
+  local after_list = {}
+  table.insert(after_list, ('vim.cmd("source %s")'):format(session_arg))
+  table.insert(after_list, ('pcall(vim.fs.rm, [==[%s]==])'):format(session))
+  table.insert(after_list, ('vim.v.this_session = [==[%s]==]'):format(this_session))
+  -- User provided command
+  if after_cmd ~= '' then
+    table.insert(after_list, ('vim.cmd([==[%s]==])'):format(after_cmd))
+  end
+  -- Concatenate everything together
+  local after = 'lua ' .. table.concat(after_list, ';')
+
+  -- Restart Neovim and run our Lua commands
+  local success, msg = pcall(function()
+    -- "+:::" special argument tells the C handler that this is actually a non-bang restart
+    -- That way, v:startreason can be set correctly
+    vim.cmd.restart { '+:::', quit_cmd, after, bang = true }
+  end)
+
+  if not success then
+    fs.rm(session, { force = true })
+    error(msg)
+  end
+end
+
 return M

runtime/lua/vim/_meta/vvars.gen.lua

@@ -669,7 +669,8 @@ vim.v.stacktrace = ...
 
 --- The reason Nvim started. Possible values:
 ---   - "normal"    normal startup.
----   - "restart"   started by `:restart` or `ZR`.
+---   - "restart"   started by `:restart`.
+---   - "restart!"  started by `:restart!` or `ZR`.
 ---
 --- Read-only.
 --- @type string