fix(filetype): call on_detect before setting buffer filetype

The on_detect functions returned by filetype.lua set buffer local
variables which are often used by filetype plugins. For example, the
on_detect function for shell buffers sets variables such as b:is_bash or
b:is_sh, which are used by the sh ftplugin.

When called after setting the buffer's filetype, these variables cannot
be used by the ftplugin (because they are not yet defined). Instead,
call on_detect before setting the buffer filetype so that any buffer
variables set by on_detect can be used in the ftplugin.

1 files changed, 6 insertions, 3 deletions

diff --git a/runtime/filetype.lua b/runtime/filetype.lua
index cf5fe39656..4a4f37a1c4 100644
--- a/runtime/filetype.lua
+++ b/runtime/filetype.lua
@@ -18,12 +18,15 @@ vim.api.nvim_create_autocmd({ 'BufRead', 'BufNewFile', 'StdinReadPost' }, {
         end)
       end
     else
-      vim.api.nvim_buf_call(args.buf, function()
-        vim.api.nvim_cmd({ cmd = 'setf', args = { ft } }, {})
-      end)
+      -- on_detect is called before setting the filetype so that it can set any buffer local
+      -- variables that may be used the filetype's ftplugin
       if on_detect then
         on_detect(args.buf)
       end
+
+      vim.api.nvim_buf_call(args.buf, function()
+        vim.api.nvim_cmd({ cmd = 'setf', args = { ft } }, {})
+      end)
     end
   end,
 })