E490: no fold found with tree-sitter

[adrian5]

Problem

Tree-sitter computed folds don't work on files opened through commands. This breaks session restore functionality.

Related:

• foldexpr=v:lua.vim.treesitter.foldexpr() doesn't fold #25948
• nvim-treesitter #1337
• nvim-telescope #699
• auto-session #109
• possession.nvim #19

Steps to reproduce

minimal.lua:

for name, url in pairs {
  ts = "https://github.com/nvim-treesitter/nvim-treesitter/",
} do
  local install_path = vim.fn.fnamemodify('nvim_issue/' .. name, ':p')
  if vim.fn.isdirectory(install_path) == 0 then
    vim.fn.system { 'git', 'clone', '--depth=1', url, install_path }
  end
  vim.opt.runtimepath:append(install_path)
end

require("nvim-treesitter.configs").setup({
  ensure_installed = { "c" }
})

vim.opt.foldmethod = "expr"
vim.wo.foldexpr = "v:lua.vim.treesitter.foldexpr()"
vim.opt.foldlevelstart = 99

vim.cmd("e hello-world.c")
vim.cmd("setf c")

helloworld.c:

#include <stdio.h>

int main() {
    printf("Hello, world!");
    return 0;
}

Steps:

• nvim --clean -u minimal.lua
• Press za on foldable section
• message: E490: no fold found

Entering insert mode or pressing zx seems to restore fold functionality. Not working without these actions breaks a complete session restore.

Expected behavior

Folds should always work, including those restored via sessions.

Neovim version (nvim -v)

NVIM v0.10.0-dev-3103+gca735c755

Vim (not Nvim) behaves the same?

not applicable

Operating system/version

Arch Linux x86_64

Terminal name/version

Alacritty 0.13.2-2

$TERM environment variable

alacritty

Installation

GitHub nightly tarball

[clason]

You need to set these options in a FileType autocommand or ftplugin; these are not "global options".

[adrian5]

And should :mksession work with options set the way you suggest? From the help file it looks like it might only track global options.

If it does not restore local options, does that mean I cannot restore folds (sessionoptions) in conjunction with nvim-treesitter?

[clason]

The folds from treesitter behave exactly like other folds.

[adrian5]

As sparse as ever with your answers. I would have liked to figure this out with vanilla nvim, but will return to using nvim-ufo then, which somehow handles these use cases.

[clason]

That's all the time I have for, unfortunately. You can try your luck with @lewis6991.

[adrian5]

Fair enough, just my frustration speaking as I've gotten stuck on this far too long, apologies!

[wookayin]

Try evaluating vim.treesitter.foldexpr() manually, e.g. :lua= vim.treesitter.foldexpr(3) and see what it prints. Check setlocal foldexpr and setlocal foldmethod too.

Also ensure the TS parser exists and works for the buffer: :lua= vim.treesitter.get_parser(0):lang().

[tomtomjhj]

And should :mksession work with options set the way you suggest?

Yes that works. The problem in the repro script is that filetype is not set when foldexpr is first evaluated, so the foldexpr doesn't recognize the language. Setting up an FileType autocmd to set foldexpr will reevaluate foldexpr with proper filetype.

So there are two options for improvements

• document that treesitter foldexpr should be set in FileType autocmd or ftplugin; or
• let treesitter fold module setup FileType autocmd to reevaluate foldexpr, so that it works even when the user doesn't do that