api/buffer: Add nvim_buf_get_changedtick method

src/nvim/api/buffer.c

@@ -411,10 +411,13 @@ end:
 
 /// Gets a buffer-scoped (b:) variable.
 ///
-/// @param buffer     Buffer handle
+/// @note Use nvim_buf_get_changedtick() for “variable” `b:changedtick`.
-/// @param name       Variable name
+///       This method only gets variables from b: dictionary.
-/// @param[out] err   Error details, if any
+///
-/// @return Variable value
+/// @param buffer The buffer handle
+/// @param name The variable name
+/// @param[out] err Details of an error that may have occurred
+/// @return The variable value
 Object nvim_buf_get_var(Buffer buffer, String name, Error *err)
 {
   buf_T *buf = find_buffer_by_handle(buffer, err);
@@ -426,6 +429,22 @@ Object nvim_buf_get_var(Buffer buffer, String name, Error *err)
   return dict_get_value(buf->b_vars, name, err);
 }
 
+/// Gets a changed tick of a buffer
+///
+/// @param[in]  buffer  The buffer handle.
+///
+/// @return `b:changedtick` value.
+Integer nvim_buf_get_changedtick(Buffer buffer, Error *err)
+{
+  const buf_T *const buf = find_buffer_by_handle(buffer, err);
+
+  if (!buf) {
+    return -1;
+  }
+
+  return buf->b_changedtick;
+}
+
 /// Sets a buffer-scoped (b:) variable
 ///
 /// @param buffer     Buffer handle

test/functional/api/buffer_spec.lua

@@ -2,7 +2,9 @@ local helpers = require('test.functional.helpers')(after_each)
 local clear, nvim, buffer = helpers.clear, helpers.nvim, helpers.buffer
 local curbuf, curwin, eq = helpers.curbuf, helpers.curwin, helpers.eq
 local curbufmeths, ok = helpers.curbufmeths, helpers.ok
-local funcs, request = helpers.funcs, helpers.request
+local funcs = helpers.funcs
+local request = helpers.request
+local neq = helpers.neq
 local NIL = helpers.NIL
 
 describe('api/buf', function()
@@ -249,6 +251,17 @@ describe('api/buf', function()
       eq(1, funcs.exists('b:lua'))
       curbufmeths.del_var('lua')
       eq(0, funcs.exists('b:lua'))
+      curbuf('set_var', 'changedtick', true)
+      eq(true, curbuf('get_var', 'changedtick'))
+      neq(true, nvim('eval', 'b:changedtick'))
+    end)
+  end)
+
+  describe('get_changedtick', function()
+    it('works', function()
+      eq(2, curbufmeths.get_changedtick())
+      curbufmeths.set_lines(0, 1, false, {'abc\0', '\0def', 'ghi'})
+      eq(3, curbufmeths.get_changedtick())
     end)
 
     it('buffer_set_var returns the old value', function()