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

3 files changed, 30 insertions, 1 deletions

diff --git a/runtime/doc/news.txt b/runtime/doc/news.txt
index f7f86237f6..9c3ae3706c 100644
--- a/runtime/doc/news.txt
+++ b/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.
diff --git a/runtime/ftplugin/lua.lua b/runtime/ftplugin/lua.lua
index e0f7e95cf6..ef8f126fab 100644
--- a/runtime/ftplugin/lua.lua
+++ b/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<'
diff --git a/runtime/lua/vim/_ftplugin/lua.lua b/runtime/lua/vim/_ftplugin/lua.lua
new file mode 100644
index 0000000000..588433409b
--- /dev/null
+++ b/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