How to fold away C++ system headers in quickfix window?

Question

If I make a mistake in a class in some C++ code, I'll typically get a ton of messages from the compiler about how hard it's tried to find something suitable in system header files. This floods the quickfix window with a lot of stuff I barely understand or which I don't need to be told about because it was a trivial typo; so I just want to start on the first message pointing to my own code and to skip over everything about system headers (except sometimes).

Is there a way to fold all the references to system headers, and to skip them with :cnext, but to have them still available if I choose to unfold them?

I suppose that ideally :cnext would respect the state of each fold, jumping right over a fold if it's closed, but advancing through a fold if I've chosen to open it.

I found one reference on the matter but it seems more concerned with grouping files (which is simplified by make's directory-change announcements) and not so much with the directories in which the headers live.

Sample output (abridged) for a couple of simple errors follows:

g++ -Wall -Werror -std=c++17 -O3 test.cc ; true
test.cc:9:4: error: #warning "example before" [-Werror=cpp]
    9 |   #warning "example before"
      |    ^~~~~~~
test.cc:12:4: error: #warning "example after" [-Werror=cpp]
   12 |   #warning "example after"
      |    ^~~~~~~
test.cc: In function ‘int main()’:
test.cc:10:31: error: no matching function for call to ‘std::vector<Test>::vector(int, int)’
   10 |   std::vector<Test> test(10, 0);
      |                               ^
In file included from /usr/include/c++/10/vector:67,
                 from test.cc:1:
/usr/include/c++/10/bits/stl_vector.h:653:2: note: candidate: ‘template<class _InputIterator, class> std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with _InputIterator = _InputIterator; <template-parameter-2-2> = <template-parameter-1-2>; _Tp = Test; _Alloc = std::allocator<Test>]’
  653 |  vector(_InputIterator __first, _InputIterator __last,
      |  ^~~~~~
/usr/include/c++/10/bits/stl_vector.h:653:2: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/10/bits/stl_algobase.h:65,
                 from /usr/include/c++/10/vector:60,
                 from test.cc:1:
/usr/include/c++/10/bits/stl_iterator_base_types.h: In substitution of ‘template<class _InIter> using _RequireInputIter = std::__enable_if_t<std::is_convertible<typename std::iterator_traits< <template-parameter-1-1> >::iterator_category, std::input_iterator_tag>::value> [with _InIter = int]’:
/usr/include/c++/10/bits/stl_vector.h:652:9:   required from here
/usr/include/c++/10/bits/stl_iterator_base_types.h:249:11: error: no type named ‘iterator_category’ in ‘struct std::iterator_traits<int>’
  249 |     using _RequireInputIter =
      |           ^~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/10/vector:67,
                 from test.cc:1:
/usr/include/c++/10/bits/stl_vector.h:625:7: note: candidate: ‘std::vector<_Tp, _Alloc>::vector(std::initializer_list<_Tp>, const allocator_type&) [with _Tp = Test; _Alloc = std::allocator<Test>; std::vector<_Tp, _Alloc>::allocator_type = std::allocator<Test>]’
  625 |       vector(initializer_list<value_type> __l,
      |       ^~~~~~
[...]
/usr/include/c++/10/bits/stl_vector.h:487:7: note:   candidate expects 0 arguments, 2 provided
test.cc:11:32: error: no matching function for call to ‘std::vector<Test>::vector(int, int)’
   11 |   std::vector<Test> test2(10, 0);
      |                                ^
In file included from /usr/include/c++/10/vector:67,
                 from test.cc:1:
/usr/include/c++/10/bits/stl_vector.h:653:2: note: candidate: ‘template<class _InputIterator, class> std::vector<_Tp, _Alloc>::vector(_InputIterator, _InputIterator, const allocator_type&) [with _InputIterator = _InputIterator; <template-parameter-2-2> = <template-parameter-1-2>; _Tp = Test; _Alloc = std::allocator<Test>]’
  653 |  vector(_InputIterator __first, _InputIterator __last,
      |  ^~~~~~

clang++ -Wall -Werror -std=c++17 -O3 test.cc
test.cc:9:4: error: "example before" [-Werror,-W#warnings]
  #warning "example before"
   ^
test.cc:10:21: error: no matching constructor for initialization of 'std::vector<Test>'
  std::vector<Test> test(10, 0);
                    ^    ~~~~~
/usr/bin/../lib/gcc/aarch64-linux-gnu/10/../../../../include/c++/10/bits/stl_vector.h:510:7: note: candidate constructor not viable: no known conversion from 'int' to 'const std::vector<Test, std::allocator<Test>>::allocator_type' (aka 'const std::allocator<Test>') for 2nd argument
      vector(size_type __n, const allocator_type& __a = allocator_type())
      ^
/usr/bin/../lib/gcc/aarch64-linux-gnu/10/../../../../include/c++/10/bits/stl_vector.h:522:7: note: candidate constructor not viable: no known conversion from 'int' to 'const std::vector<Test, std::allocator<Test>>::value_type' (aka 'const Test') for 2nd argument
      vector(size_type __n, const value_type& __value,
      ^
[...]
/usr/bin/../lib/gcc/aarch64-linux-gnu/10/../../../../include/c++/10/bits/stl_vector.h:589:7: note: candidate constructor not viable: requires 3 arguments, but 2 were provided
      vector(vector&& __rv, const allocator_type& __m, false_type)
      ^
/usr/bin/../lib/gcc/aarch64-linux-gnu/10/../../../../include/c++/10/bits/stl_vector.h:487:7: note: candidate constructor not viable: requires 0 arguments, but 2 were provided
      vector() = default;
      ^
test.cc:12:4: error: "example after" [-Werror,-W#warnings]
  #warning "example after"
   ^
4 errors generated.

Answer 1

For the first example if you define the following function (e.g. .vimrc)

function! FoldError(lnum)
  let l:line = getline(a:lnum)
  if matchstr(l:line, ' ^|| In file included') != ''
    return '>1'
  endif
  if matchstr(l:line, '^||') != ''
    return '='
  endif
  if matchstr(l:line, '^/usr/bin') != ''
    return '>2'
  endif
  return '>1'
endfunction

And run the following commands in the QuickFix window:

set foldmethod=expr
set foldexpr=FoldError(v:lnum)

After running zx to refresh the folds you'll get something like:

It doesn't solve your :cnext problem but give you a more readable view on your errors in the QuickFix window.

Answer 2

cnext does not respect folds and you can't make it to.

The procedure of creating folds is rather a standard one:

1. Create ftplugin file for your filetype, i.e. ~/after/ftplugin/qf.vim

2. Set local options foldmethod and foldexpr, i.e. setl fdm=expr fde=MyLineToFoldLevel(v:lnum)

3. Define your custom function to calc desired fold level (zero or one, unless you want nested folds) by line number.

How exactly one implements the step no. 3 is out of the scope of this question.

Answer 3

Combining both answers, I put the following into ~/.vim/after/ftplugin/qf.vim:

function! FoldError(lnum)
  let l:line = getline(a:lnum)
  if matchstr(l:line, '^/') != ''
    return 1
  endif
  if matchstr(l:line, '^|') != ''
    return -1
  endif
  return 0
endfunction

setl fde=FoldError(v:lnum)
setl fdm=expr

Since the output from the compilers has been processed into a different format, it seems that the first character is all that's needed to identify the type of line that it is. Either it begins with a |, meaning it's some kind of commentary, otherwise it's a filename. If it begins with a / then it's outside of the tree I'm working on.

---

Also, filetype plugin on to make sure that the above script is executed at the appropriate time.