fix(lua): vim.diff is nil in uv.new_work() thread #34909

Problem:
The "gitsigns" plugin runs `vim.diff` in a thread (`uv.new_work`), but
`vim.diff` is nil in that context:

    Lua callback:
    …/gitsigns.nvim/lua/gitsigns/diff_int.lua:30: bad argument #1 to 'decode' (string expected, got nil)
    stack traceback:
      [C]: in function 'decode'
      …/gitsigns.nvim/lua/gitsigns/diff_int.lua:30: in function <…/gitsigns.nvim/lua/gitsigns/diff_int.lua:29>
    Luv thread:
    …/gitsigns.nvim/lua/gitsigns/diff_int.lua:63: attempt to call field 'diff' (a nil value)

Solution:
Revert the `stdlib.c` change (set `vim.diff` instead of `vim._diff`).

runtime/lua/vim/_editor.lua

@@ -1317,9 +1317,6 @@ require('vim._options')
 --- Remove at Nvim 1.0
 ---@deprecated
 vim.loop = vim.uv
---- Renamed to `vim.text.diff`, remove at Nvim 1.0
----@deprecated
-vim.diff = vim._diff ---@type fun(a: string, b: string, opts?: vim.text.diff.Opts): string|integer[][]?
 
 -- Deprecated. Remove at Nvim 2.0
 vim.highlight = vim._defer_deprecated_module('vim.highlight', 'vim.hl')

runtime/lua/vim/_meta/misc.lua

@@ -14,3 +14,11 @@
 --- @param ... any
 --- @return any
 function vim.call(func, ...) end
+
+--- Renamed to `vim.text.diff`, remove at Nvim 1.0
+---@deprecated
+---@param a string First string to compare
+---@param b string Second string to compare
+---@param opts? vim.text.diff.Opts
+---@return string|integer[][]? # See {opts.result_type}. `nil` if {opts.on_hunk} is given.
+function vim.diff(a, b, opts) end

runtime/lua/vim/text.lua

@@ -73,7 +73,8 @@ local M = {}
 ---@param opts? vim.text.diff.Opts
 ---@return string|integer[][]? # See {opts.result_type}. `nil` if {opts.on_hunk} is given.
 function M.diff(...)
-  return vim._diff(...)
+  ---@diagnostic disable-next-line: deprecated
+  return vim.diff(...)
 end
 
 local alphabet = '0123456789ABCDEF'

src/nvim/lua/stdlib.c

@@ -763,7 +763,8 @@ void nlua_state_add_stdlib(lua_State *const lstate, bool is_thread)
 
   // vim.text.diff
   lua_pushcfunction(lstate, &nlua_xdl_diff);
-  lua_setfield(lstate, -2, "_diff");
+  // TODO(justinmk): set vim.text.diff here, or rename this to "_diff". goddamnit.
+  lua_setfield(lstate, -2, "diff");
 
   // vim.json
   lua_cjson_new(lstate);

test/functional/lua/thread_spec.lua

@@ -217,7 +217,7 @@ describe('thread', function()
     it('diff', function()
       exec_lua [[
         local entry = function(async)
-          async:send(vim._diff('Hello\n', 'Helli\n'))
+          async:send(vim.diff('Hello\n', 'Helli\n'))
         end
         local on_async = function(ret)
           vim.rpcnotify(1, 'result', ret)
@@ -372,7 +372,7 @@ describe('threadpool', function()
     it('work', function()
       exec_lua [[
         local work_fn = function()
-          return vim._diff('Hello\n', 'Helli\n')
+          return vim.diff('Hello\n', 'Helli\n')
         end
         local after_work_fn = function(ret)
           vim.rpcnotify(1, 'result', ret)