feat: add error message when compile bad mainfile

refer: #2465

Author: lervag

autoload/vimtex/compiler.vim

@@ -187,6 +187,14 @@ endfunction
 
 " }}}1
 function! vimtex#compiler#start() abort " {{{1
+  if b:vimtex.main_parser ==# 'fallback current file'
+    call vimtex#log#error(
+          \ 'Compilation error due to failed mainfile detection!',
+          \ 'Please ensure that VimTeX can locate the proper main .tex file.',
+          \ 'Read ":help vimtex-multi-file" for more info.'
+          \)
+    return
+  endif
   if b:vimtex.compiler.is_running()
     call vimtex#log#warning(
           \ 'Compiler is already running for `' . b:vimtex.base . "'")

autoload/vimtex/state.vim

@@ -14,15 +14,15 @@ endfunction
 
 " }}}1
 function! vimtex#state#init() abort " {{{1
-  let [l:main, l:main_type] = s:get_main()
+  let [l:main, l:main_parser] = s:get_main()
   let l:id = s:get_main_id(l:main)
 
   if l:id >= 0
     let b:vimtex_id = l:id
     let b:vimtex = s:vimtex_states[l:id]
   else
     let b:vimtex_id = s:vimtex_next_id
-    let b:vimtex = vimtex#state#class#new(l:main, l:main_type, 0)
+    let b:vimtex = vimtex#state#class#new(l:main, l:main_parser, 0)
     let s:vimtex_next_id += 1
     let s:vimtex_states[b:vimtex_id] = b:vimtex
   endif
@@ -268,7 +268,7 @@ function! s:get_main() abort " {{{1
       return [expand('%:p'), 'bib file']
     endif
   else
-    return [expand('%:p'), 'current file']
+    return [expand('%:p'), 'fallback current file']
   endif
 endfunction