feat(inccommand): preview 'nomodifiable' buffers #32034

Problem:
Incremental preview is not allowed on 'nomodifiable' buffers.

Solution:
- Allow preview on 'nomodifiable' buffers.
- Restore the 'modifiable' option in case the preview function changes it.

runtime/doc/news.txt

@@ -319,6 +319,8 @@ PLUGINS
 
 • EditorConfig
   • spelling_language property is now supported.
+• 'inccommand' incremental preview can run on 'nomodifiable' buffers and
+  restores their 'modifiable' state
 
 STARTUP
 

src/nvim/ex_getln.c

@@ -163,6 +163,7 @@ typedef struct {
 typedef struct {
   buf_T *buf;
   OptInt save_b_p_ul;
+  int save_b_p_ma;
   int save_b_changed;
   pos_T save_b_op_start;
   pos_T save_b_op_end;
@@ -2419,6 +2420,7 @@ static void cmdpreview_prepare(CpInfo *cpinfo)
     if (!set_has(ptr_t, &saved_bufs, buf)) {
       CpBufInfo cp_bufinfo;
       cp_bufinfo.buf = buf;
+      cp_bufinfo.save_b_p_ma = buf->b_p_ma;
       cp_bufinfo.save_b_p_ul = buf->b_p_ul;
       cp_bufinfo.save_b_changed = buf->b_changed;
       cp_bufinfo.save_b_op_start = buf->b_op_start;
@@ -2509,6 +2511,7 @@ static void cmdpreview_restore_state(CpInfo *cpinfo)
     }
 
     buf->b_p_ul = cp_bufinfo.save_b_p_ul;        // Restore 'undolevels'
+    buf->b_p_ma = cp_bufinfo.save_b_p_ma;        // Restore 'modifiable'
   }
 
   for (size_t i = 0; i < cpinfo->win_info.size; i++) {
@@ -2704,7 +2707,6 @@ static int command_line_changed(CommandLineState *s)
       && current_sctx.sc_sid == 0    // only if interactive
       && *p_icm != NUL       // 'inccommand' is set
       && !exmode_active      // not in ex mode
-      && curbuf->b_p_ma      // buffer is modifiable
       && cmdline_star == 0   // not typing a password
       && !vpeekc_any()
       && cmdpreview_may_show(s)) {

test/functional/ui/inccommand_user_spec.lua

@@ -253,6 +253,50 @@ describe("'inccommand' for user commands", function()
     ]]
   end)
 
+  it("can preview 'nomodifiable' buffer", function()
+    exec_lua([[
+      vim.api.nvim_create_user_command("PreviewTest", function() end, {
+        preview = function(ev)
+          vim.bo.modifiable = true
+          vim.api.nvim_buf_set_lines(0, 0, -1, false, {"cats"})
+          return 2
+        end,
+      })
+    ]])
+    command('set inccommand=split')
+
+    command('set nomodifiable')
+    eq(false, api.nvim_get_option_value('modifiable', { buf = 0 }))
+
+    feed(':PreviewTest')
+
+    screen:expect([[
+      cats                                    |
+      {1:~                                       }|*8
+      {3:[No Name] [+]                           }|
+                                              |
+      {1:~                                       }|*4
+      {2:[Preview]                               }|
+      :PreviewTest^                            |
+    ]])
+    feed('<Esc>')
+    screen:expect([[
+        text on line 1                        |
+        more text on line 2                   |
+        oh no, even more text                 |
+        will the text ever stop               |
+        oh well                               |
+        did the text stop                     |
+        why won't it stop                     |
+        make the text stop                    |
+      ^                                        |
+      {1:~                                       }|*7
+                                              |
+    ]])
+
+    eq(false, api.nvim_get_option_value('modifiable', { buf = 0 }))
+  end)
+
   it('works with inccommand=nosplit', function()
     command('set inccommand=nosplit')
     feed(':Replace text cats')