What does adding ',%\&chdir '.escape(root, ',') to errroformat actually do?

[iamFIREcracker]

I was looking at how the "make" property projection was implemented and spotted this line at the end of the supporting code:

let &l:errorformat .= ',%\&chdir '.escape(root, ',')

Does this tell vim that the output is expected to be relative to root? I would think so, but I could not find any reference to this inside :help errorformat; does anybody have an idea of how this works?

Thanks in advance.

[tpope]

That's a deliberately invalid pattern; it will never match anything. Instead, Projectionist handles it itself in QuickFixCmdPre and QuickFixCmdPost by extracting the value and using it to temporarily :cd:

vim-projectionist/autoload/projectionist.vim

Lines 898 to 913 in e5144ba

	function! s:qf_pre() abort
	let dir = substitute(matchstr(','.&l:errorformat, ',\%(%\\&\)\=\%(ch\)\=dir[ =]\zs\%(\\.\|[^,]\)*'), '\\,' ,',', 'g')
	let cwd = getcwd()
	if !empty(dir) && dir !=# cwd
	let cd = exists('*haslocaldir') && haslocaldir() ? 'lcd' : 'cd'
	execute cd fnameescape(dir)
	let s:qf_post = cd . ' ' . fnameescape(cwd)
	endif
	endfunction
	augroup projectionist_make
	autocmd!
	autocmd QuickFixCmdPre *make* call s:qf_pre()
	autocmd QuickFixCmdPost *make*
	\ if exists('s:qf_post') | execute remove(s:, 'qf_post') | endif
	augroup END