Thanks.

I'm not opposed to it in principle but I wonder how many people, even those working with Lua, build Vim with the Lua interface? It should probably be ported to one of the Vim scripts in the future, if not now.

The syntax file looks like a few fold options added to the :syn-regions would also achieve much the same.

Yes, I agree. I was just delighted to have found a starting point here, but wondered as well if Vimscript would not be the better tool for this job. Here it is

👍 but looks like approval no longer needed from me.

I am wondering if a foldexpression is really a recommended approach compared with proper syntax folding? I think the fold-expression may cause signifcant lag when used with large files, no? Wouldn't syntax folding be the prefered way here?

@chrisbra I suppose so too, I am not sure which is faster. For me this works very well, but I'm also using fastfold to avoid unnecessary recomputes (as the more direct approach failed. I am fine with any &foldmethod and invite further implementations, but at least we've something working on every Vim.

This said, generally speaking, aren't &syntax and &foldexpr always two viable solutions? When is one clearly better than the other? For example, Dr Chip's built-in Tex folding uses syntax, whereas the popular vimtex switched to &foldexpr

Well, I found it still to be slow on bigger lua files indeed and now made it only recompute on changes

okay thanks. Let me include this now then.

Actually I have to take this back. Does the following patch make sense?

diff --git a/runtime/ftplugin/lua.vim b/runtime/ftplugin/lua.vim
index 8a1e86db6..15497841f 100644
--- a/runtime/ftplugin/lua.vim
+++ b/runtime/ftplugin/lua.vim
@@ -5,7 +5,7 @@
 " Contributor:         Dorai Sitaram <ds26@gte.com>
 "                      C.D. MacEachern <craig.daniel.maceachern@gmail.com>
 "                      Tyler Miller <tmillr@proton.me>
-" Last Change:         2024 Jan 14
+" Last Change:         2024 Dec 03

 if exists("b:did_ftplugin")
   finish
@@ -55,11 +55,13 @@ if has("folding") && get(g:, "lua_folding", 0)
   let b:undo_ftplugin ..= "|setl foldexpr< foldmethod< | unlet b:lua_lasttick b:lua_foldlists"
 endif

-let &cpo = s:cpo_save
-unlet s:cpo_save

 " The rest of the file needs to be :sourced only once per Vim session
-if exists('s:loaded_lua') || &cp | finish | endif
+if exists('s:loaded_lua') || &cp
+  let &cpo = s:cpo_save
+  unlet s:cpo_save
+  finish
+endif
 let s:loaded_lua = 1

 " From https://github.com/ElPiloto/Lua-Omni-Vim-Completion/blob/7044c7010d4e6f59da60704a4a8e2118af7e973b/ftplugin/lua_omni.lua#L715C1-L738C4
@@ -106,4 +108,7 @@ function! LuaFold(lnum) abort
   return len(lua_foldlists[a:lnum-1])
 endfunction

+let &cpo = s:cpo_save
+unlet s:cpo_save
+
 " vim: nowrap sw=2 sts=2 ts=8 noet:

The reasoning is, we are still using line-continuation lines after the test for s:loaded_lua, so resetting &cpo might be a bit too early.

Then it makes perfect sense; I never thought this &cpo dance through

okay, I'll squash this in

Oh, I did this

Since there were some other details

But feel free to refine further

okay, thanks. I'll merge it now.