Fix crashing when loading lean config

This fixes a crash that occurs when lean is configured outside of a
`config` function from packer. Technically---instead of communicating
changes to the filetype configuration through a shared configuration
global, the relevant configuration (default filetype) is shared through
one single global.
Furthermore, if this global is written by the user before the plugin is
loaded, then this global will not be managed by the plugin. This allows
a user to configure default filetype even if the plugin is not loaded!

Fixes Julian#275

.luacheckrc

@@ -1,6 +1,6 @@
 globals = {
     "_clean_buffer_count",
     "vim",
-    "lean_nvim_ft_options",
+    "lean_nvim_default_filetype",
     assert = {fields = {"message"}},
 }

ftdetect/lean.lua

@@ -8,24 +8,17 @@ local find_project_root = require('lspconfig.util').root_pattern(
   'lean-toolchain'
 )
 
-lean_nvim_ft_options = {
-  default = "lean",
-  nomodifiable = {
-    '.*/src/lean/.*',       -- Lean 4 standard library
-    '.*/lib/lean/src/.*',   -- Lean 4 legacy standard library
-    '.*/lean_packages/.*',  -- Lean 4 dependencies
-    _LEAN3_STANDARD_LIBRARY .. '.*',
-    '/_target/.*/.*.lean'   -- Lean 3 dependencies
-  }
-}
-
 local function detect(filename)
   if filename:match('^fugitive://.*') then
     filename = pcall(vim.fn.FugitiveReal, filename)
   end
 
   local abspath = vim.fn.fnamemodify(filename, ":p")
-  local filetype = lean_nvim_ft_options.default
+  local filetype = lean_nvim_default_filetype
+  if not filetype then
+    filetype = 'lean'
+  end
+
   if abspath:match(_LEAN3_STANDARD_LIBRARY) then
     filetype = 'lean3'
   else

lua/lean/ft.lua

@@ -1,15 +1,36 @@
 local ft = {}
+local _LEAN3_STANDARD_LIBRARY = '.*/[^/]*lean[%-]+3.+/lib/'
 
-lean_nvim_ft_options._DEFAULTS = vim.deepcopy(lean_nvim_ft_options)
+local options = {
+  default = "lean",
+  nomodifiable = {
+    '.*/src/lean/.*',       -- Lean 4 standard library
+    '.*/lib/lean/src/.*',   -- Lean 4 legacy standard library
+    '.*/lean_packages/.*',  -- Lean 4 dependencies
+    _LEAN3_STANDARD_LIBRARY .. '.*',
+    '/_target/.*/.*.lean'   -- Lean 3 dependencies
+  }
+}
 
+options._DEFAULTS = vim.deepcopy(options)
+
+local global_default_managed = false
 function ft.enable(opts)
-  lean_nvim_ft_options= vim.tbl_extend("force", lean_nvim_ft_options._DEFAULTS, opts)
+  if not global_default_managed and not lean_nvim_default_filetype then
+    -- we are not managing the ft global, but it has no value.
+    -- this means we have to manage it
+    global_default_managed = true
+  end
+  options= vim.tbl_extend("force", options._DEFAULTS, opts)
+  if global_default_managed then
+    lean_nvim_default_filetype = options.default
+  end
 end
 
 ---Make the given buffer `nomodifiable` if its file name matches a configured list.
 function ft.__maybe_make_nomodifiable(bufnr)
   local name = vim.api.nvim_buf_get_name(bufnr)
-  for _, pattern in ipairs(lean_nvim_ft_options.nomodifiable) do
+  for _, pattern in ipairs(options.nomodifiable) do
     if name:match(pattern) then
       vim.api.nvim_buf_set_option(bufnr, 'modifiable', false)
       return