feat(runtime): Lua ftplugin 'includeexpr' #32719

Problem:
Current `'includeexpr'` in runtime/ftplugin/lua.vim doesn't work with Nvim Lua.

Solution:
Provide an improved 'includeexpr' for Lua in "ftplugin/lua.lua".

Closes: https://github.com/neovim/neovim/issues/32490

runtime/doc/news.txt

@@ -326,6 +326,8 @@ LUA
   `vim.o`, `vim.wo`, `vim.bo`, `vim.opt`, `vim.opt_local`, `vim.opt_global`,
   and `vim.fn`.
 • Documentation for |lua-bit|.
+• |gf| in Lua buffers can go to module in same repo, |runtime-search-path| and
+  |package.path|.
 • |vim.fs.rm()| can delete files and directories.
 • |vim.validate()| now has a new signature which uses less tables,
   is more performant and easier to read.

runtime/ftplugin/lua.lua

@@ -1,9 +1,10 @@
 -- use treesitter over syntax
 vim.treesitter.start()
 
+vim.bo.includeexpr = 'v:lua.require"vim._ftplugin.lua".includeexpr()'
 vim.bo.omnifunc = 'v:lua.vim.lua_omnifunc'
 vim.wo[0][0].foldexpr = 'v:lua.vim.treesitter.foldexpr()'
 
 vim.b.undo_ftplugin = (vim.b.undo_ftplugin or '')
   .. '\n call v:lua.vim.treesitter.stop()'
-  .. '\n setl omnifunc< foldexpr<'
+  .. '\n setl omnifunc< foldexpr< includeexpr<'

runtime/lua/vim/_ftplugin/lua.lua

@@ -0,0 +1,26 @@
+local M = {}
+
+--- @param module string
+---@return string
+function M.includeexpr(module)
+  ---@param fname string
+  ---@return boolean
+  local function filereadable(fname)
+    return vim.fn.filereadable(fname) == 1
+  end
+
+  local fname = module:gsub('%.', '/')
+
+  local root = vim.fs.root(vim.api.nvim_buf_get_name(0), 'lua') or vim.fn.getcwd()
+  for _, suf in ipairs { '.lua', '/init.lua' } do
+    local path = vim.fs.joinpath(root, 'lua', fname .. suf)
+    if filereadable(path) then
+      return path
+    end
+  end
+
+  local modInfo = vim.loader.find(module)[1]
+  return modInfo and modInfo.modpath or module
+end
+
+return M

test/functional/editor/ftplugin_spec.lua

@@ -0,0 +1,85 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local exec_lua = n.exec_lua
+local command = n.command
+local eq = t.eq
+
+---@param type string
+---@return string
+local function stdpath(type)
+  return exec_lua(([[return vim.fs.normalize(vim.fn.stdpath("%s"))]]):format(type))
+end
+
+---@return string
+local function vimruntime()
+  return exec_lua [[ return vim.fs.normalize(vim.env.VIMRUNTIME) ]]
+end
+
+---@param module string
+---@return string
+local function lua_includeexpr(module)
+  return exec_lua(([[return require('vim._ftplugin.lua').includeexpr("%s")]]):format(module))
+end
+
+local root = exec_lua [[ return vim.fs.normalize(vim.fn.getcwd()) ]]
+
+describe("Lua 'includeexpr'", function()
+  setup(n.clear)
+  local temp_dir = t.tmpname(false)
+  before_each(function()
+    command(([[
+      edit `=stdpath('config') .. '/lua/user-foo/init.lua'`
+      write ++p
+      edit `=stdpath('config') .. '/lua/user-foo/bar.lua'`
+      write ++p
+      edit `=stdpath('data') .. '/site/pack/packer/start/plugin-foo/lua/plugin-foo/init.lua'`
+      write ++p
+      edit `=stdpath('data') .. '/site/pack/packer/start/plugin-foo/lua/plugin-foo/bar.lua'`
+      write ++p
+
+      edit runtime/lua/foo/init.lua
+      write ++p
+      edit runtime/lua/foo/bar/init.lua
+      write ++p
+
+      edit %s/lua/runtime-foo/init.lua
+      write ++p
+      edit %s/lua/runtime-foo/bar.lua
+      write ++p
+    ]]):format(temp_dir, temp_dir))
+  end)
+
+  it('finds module in current repo', function()
+    command [[ edit runtime/lua/vim/_ftplugin/lua.lua ]]
+    eq(root .. '/runtime/lua/vim/_ftplugin/lua.lua', lua_includeexpr('vim._ftplugin.lua'))
+    eq(root .. '/runtime/lua/editorconfig.lua', lua_includeexpr('editorconfig'))
+    eq(root .. '/runtime/lua/foo/init.lua', lua_includeexpr('foo'))
+    eq(root .. '/runtime/lua/foo/bar/init.lua', lua_includeexpr('foo.bar'))
+  end)
+
+  it('find module in packpath/start', function()
+    eq(
+      stdpath('data') .. '/site/pack/packer/start/plugin-foo/lua/plugin-foo/init.lua',
+      lua_includeexpr('plugin-foo')
+    )
+    eq(
+      stdpath('data') .. '/site/pack/packer/start/plugin-foo/lua/plugin-foo/bar.lua',
+      lua_includeexpr('plugin-foo.bar')
+    )
+  end)
+
+  it('finds module in $VIMRUNTIME', function()
+    command('edit ' .. root)
+    eq(vimruntime() .. '/lua/vim/_ftplugin/lua.lua', lua_includeexpr('vim._ftplugin.lua'))
+    eq(vimruntime() .. '/lua/editorconfig.lua', lua_includeexpr('editorconfig'))
+  end)
+
+  it('find module in runtimepath', function()
+    eq(stdpath('config') .. '/lua/user-foo/init.lua', lua_includeexpr('user-foo'))
+    eq(stdpath('config') .. '/lua/user-foo/bar.lua', lua_includeexpr('user-foo.bar'))
+    command('set rtp+=' .. temp_dir)
+    eq(temp_dir .. '/lua/runtime-foo/init.lua', lua_includeexpr('runtime-foo'))
+    eq(temp_dir .. '/lua/runtime-foo/bar.lua', lua_includeexpr('runtime-foo.bar'))
+  end)
+end)