feat(complete): CompleteDone reason "cancel", "discard" #32600

Problem: there is no way to distinguish between user's explicit
  completion stop/cancel and other automated reasons.

Solution: update "cancel" reason to be set only on explicit CTRL-e, and
  set intentionally vague "discard" otherwise.

runtime/doc/autocmd.txt

@@ -470,11 +470,11 @@ CompleteDone			After Insert mode completion is done.  Either
 				    reason	Reason for completion being
 						done. Can be one of:
 						- "accept": completion was
-						  accepted using |complete_CTRL-Y|.
-						- "cancel": completion was cancelled
-						  using |complete_CTRL-E|, pressing
-						  a non-keyword character, or
-						  triggering a new completion.
+						  accepted by |complete_CTRL-Y|.
+						- "cancel": completion was
+						  stopped by |complete_CTRL-E.
+						- "discard": completion was
+						  ended for other reason.
 
 							*CursorHold*
 CursorHold			When the user doesn't press a key for the time

src/nvim/insexpand.c

@@ -581,7 +581,8 @@ static void do_autocmd_completedone(int c, int mode, char *word)
   tv_dict_add_str(v_event, S_LEN("complete_word"), word != NULL ? word : "");
   tv_dict_add_str(v_event, S_LEN("complete_type"), mode_str != NULL ? mode_str : "");
 
-  tv_dict_add_str(v_event, S_LEN("reason"), (c == Ctrl_Y ? "accept" : "cancel"));
+  tv_dict_add_str(v_event, S_LEN("reason"),
+                  (c == Ctrl_Y ? "accept" : (c == Ctrl_E ? "cancel" : "discard")));
   tv_dict_set_keys_readonly(v_event);
 
   ins_apply_autocmds(EVENT_COMPLETEDONE);

test/functional/autocmd/completedone_spec.lua

@@ -23,18 +23,26 @@ describe('CompleteDone', function()
       feed('<C-y>')
       eq('accept', eval('g:donereason'))
     end)
-    describe('cancel', function()
-      it('on <C-e>', function()
-        feed('<C-e>')
-        eq('cancel', eval('g:donereason'))
-      end)
+
+    it('cancel', function()
+      feed('<C-e>')
+      eq('cancel', eval('g:donereason'))
+    end)
+
+    describe('discard', function()
       it('on non-keyword character', function()
-        feed('<Esc>')
-        eq('cancel', eval('g:donereason'))
+        feed('<Space>')
+        eq('discard', eval('g:donereason'))
       end)
+
+      it('on mode change', function()
+        feed('<Esc>')
+        eq('discard', eval('g:donereason'))
+      end)
+
       it('when overridden by another complete()', function()
         call('complete', call('col', '.'), { 'bar', 'baz' })
-        eq('cancel', eval('g:donereason'))
+        eq('discard', eval('g:donereason'))
       end)
     end)
   end)