vim-patch:8.2.0865 syntax: Add command to control how foldlevel is computed

Problem:    Syntax foldlevel is taken from the start of the line.
Solution:   Add ":syn foldlevel" to be able to use the minimal foldlevel in the line.
https://github.com/vim/vim/commit/e35a52aee718c881bdfa69a47a1068df6ab6c60a

With `foldmethod=syntax` the foldlevel of a line is computed based
on syntax items on the line.  Previously we always used the level
of the syntax item containing the start of the line.  This works
well in cases such as:

    if (...) {
      ...
    }
    else if (...) {
      ...
    }
    else {
      ...
    }

which folds like this:

    +---  3 lines: if (...) {---------------------------
    +---  3 lines: else if (...) {----------------------
    +---  3 lines: else {-------------------------------

However, the code:

    if (...) {
      ...
    } else if (...) {
      ...
    } else {
      ...
    }

folds like this:

    +---  7 lines: if (...) {---------------------------

We can make the latter case fold like this:

    +---  2 lines: if (...) {---------------------------
    +---  2 lines: } else if (...) {--------------------
    +---  3 lines: } else {-----------------------------

by choosing on each line the lowest fold level that is followed
by a higher fold level.

Add a syntax command

    :syntax foldlevel [start | minimum]

to choose between these two methods of computing the foldlevel of
a line.

1 files changed, 84 insertions, 1 deletions

diff --git a/src/nvim/testdir/test_syntax.vim b/src/nvim/testdir/test_syntax.vim
index 6cada1503f..85ee42420e 100644
--- a/src/nvim/testdir/test_syntax.vim
+++ b/src/nvim/testdir/test_syntax.vim
@@ -153,7 +153,7 @@ endfunc
 
 func Test_syntax_completion()
   call feedkeys(":syn \<C-A>\<C-B>\"\<CR>", 'tx')
-  call assert_equal('"syn case clear cluster conceal enable include iskeyword keyword list manual match off on region reset spell sync', @:)
+  call assert_equal('"syn case clear cluster conceal enable foldlevel include iskeyword keyword list manual match off on region reset spell sync', @:)
 
   call feedkeys(":syn case \<C-A>\<C-B>\"\<CR>", 'tx')
   call assert_equal('"syn case ignore match', @:)
@@ -579,3 +579,86 @@ func Test_syntax_hangs()
   set redrawtime&
   bwipe!
 endfunc
+
+func Test_syntax_foldlevel()
+  new
+  call setline(1, [
+   \ 'void f(int a)',
+   \ '{',
+   \ '    if (a == 1) {',
+   \ '        a = 0;',
+   \ '    } else if (a == 2) {',
+   \ '        a = 1;',
+   \ '    } else {',
+   \ '        a = 2;',
+   \ '    }',
+   \ '    if (a > 0) {',
+   \ '        if (a == 1) {',
+   \ '            a = 0;',
+   \ '        } /* missing newline */ } /* end of outer if */ else {',
+   \ '        a = 1;',
+   \ '    }',
+   \ '    if (a == 1)',
+   \ '    {',
+   \ '        a = 0;',
+   \ '    }',
+   \ '    else if (a == 2)',
+   \ '    {',
+   \ '        a = 1;',
+   \ '    }',
+   \ '    else',
+   \ '    {',
+   \ '        a = 2;',
+   \ '    }',
+   \ '}',
+   \ ])
+  setfiletype c
+  syntax on
+  set foldmethod=syntax
+
+  call assert_fails('syn foldlevel start start', 'E390')
+  call assert_fails('syn foldlevel not_an_option', 'E390')
+
+  set foldlevel=1
+
+  syn foldlevel start
+  redir @c
+  syn foldlevel
+  redir END
+  call assert_equal("\nsyntax foldlevel start", @c)
+  syn sync fromstart
+  let a = map(range(3,9), 'foldclosed(v:val)')
+  call assert_equal([3,3,3,3,3,3,3], a) " attached cascade folds together
+  let a = map(range(10,15), 'foldclosed(v:val)')
+  call assert_equal([10,10,10,10,10,10], a) " over-attached 'else' hidden
+  let a = map(range(16,27), 'foldclosed(v:val)')
+  let unattached_results = [-1,17,17,17,-1,21,21,21,-1,25,25,25]
+  call assert_equal(unattached_results, a) " unattached cascade folds separately
+
+  syn foldlevel minimum
+  redir @c
+  syn foldlevel
+  redir END
+  call assert_equal("\nsyntax foldlevel minimum", @c)
+  syn sync fromstart
+  let a = map(range(3,9), 'foldclosed(v:val)')
+  call assert_equal([3,3,5,5,7,7,7], a) " attached cascade folds separately
+  let a = map(range(10,15), 'foldclosed(v:val)')
+  call assert_equal([10,10,10,13,13,13], a) " over-attached 'else' visible
+  let a = map(range(16,27), 'foldclosed(v:val)')
+  call assert_equal(unattached_results, a) " unattached cascade folds separately
+
+  set foldlevel=2
+
+  syn foldlevel start
+  syn sync fromstart
+  let a = map(range(11,14), 'foldclosed(v:val)')
+  call assert_equal([11,11,11,-1], a) " over-attached 'else' hidden
+
+  syn foldlevel minimum
+  syn sync fromstart
+  let a = map(range(11,14), 'foldclosed(v:val)')
+  call assert_equal([11,11,-1,-1], a) " over-attached 'else' visible
+
+  quit!
+endfunc