neovim/runtime/filetype.lua

if vim.g.did_load_filetypes and vim.g.did_load_filetypes ~= 0 then
  return
end

-- For now, make this opt-in with a global variable
if vim.g.do_filetype_lua ~= 1 then
  return
end

vim.api.nvim_create_augroup("filetypedetect", {clear = false})

vim.api.nvim_create_autocmd({"BufRead", "BufNewFile"}, {
  group = "filetypedetect",
  callback = function()
    vim.filetype.match(vim.fn.expand("<afile>"))
  end,
})

-- These *must* be sourced after the autocommand above is created
if not vim.g.did_load_ftdetect then
  vim.cmd [[
  augroup filetypedetect
  runtime! ftdetect/*.vim
  runtime! ftdetect/*.lua
  augroup END
  ]]
end

-- Set a marker so that the ftdetect scripts are not sourced a second time by filetype.vim
vim.g.did_load_ftdetect = 1

-- If filetype.vim is disabled, set up the autocmd to use scripts.vim
if vim.g.did_load_filetypes then
  vim.api.nvim_create_autocmd({"BufRead", "BufNewFile"}, {
    group = "filetypedetect",
    command = "if !did_filetype() && expand('<amatch>') !~ g:ft_ignore_pat | runtime! scripts.vim | endif",
  })

  vim.api.nvim_create_autocmd("StdinReadPost", {
    group = "filetypedetect",
    command = "if !did_filetype() | runtime! scripts.vim | endif",
  })
end

if not vim.g.ft_ignore_pat then
  vim.g.ft_ignore_pat = "\\.\\(Z\\|gz\\|bz2\\|zip\\|tgz\\)$"
end