api: Replace set_var(name, NIL) with del_var(name)

src/nvim/api/buffer.c

@@ -423,7 +423,7 @@ Object buffer_get_var(Buffer buffer, String name, Error *err)
   return dict_get_value(buf->b_vars, name, err);
 }
 
-/// Sets a buffer-scoped (b:) variable. 'nil' value deletes the variable.
+/// Sets a buffer-scoped (b:) variable
 ///
 /// @param buffer The buffer handle
 /// @param name The variable name
@@ -438,7 +438,24 @@ Object buffer_set_var(Buffer buffer, String name, Object value, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(buf->b_vars, name, value, err);
+  return dict_set_value(buf->b_vars, name, value, false, err);
+}
+
+/// Removes a buffer-scoped (b:) variable
+///
+/// @param buffer The buffer handle
+/// @param name The variable name
+/// @param[out] err Details of an error that may have occurred
+/// @return The old value
+Object buffer_del_var(Buffer buffer, String name, Error *err)
+{
+  buf_T *buf = find_buffer_by_handle(buffer, err);
+
+  if (!buf) {
+    return (Object) OBJECT_INIT;
+  }
+
+  return dict_set_value(buf->b_vars, name, NIL, true, err);
 }
 
 /// Gets a buffer option value

src/nvim/api/private/helpers.c

@@ -90,14 +90,17 @@ Object dict_get_value(dict_T *dict, String key, Error *err)
 }
 
 /// Set a value in a dict. Objects are recursively expanded into their
-/// vimscript equivalents. Passing 'nil' as value deletes the key.
+/// vimscript equivalents.
 ///
 /// @param dict The vimscript dict
 /// @param key The key
 /// @param value The new value
+/// @param del Delete key in place of setting it. Argument `value` is ignored in
+///            this case.
 /// @param[out] err Details of an error that may have occurred
 /// @return the old value, if any
-Object dict_set_value(dict_T *dict, String key, Object value, Error *err)
+Object dict_set_value(dict_T *dict, String key, Object value, bool del,
+                      Error *err)
 {
   Object rv = OBJECT_INIT;
 
@@ -118,7 +121,7 @@ Object dict_set_value(dict_T *dict, String key, Object value, Error *err)
 
   dictitem_T *di = dict_find(dict, (uint8_t *)key.data, (int)key.size);
 
-  if (value.type == kObjectTypeNil) {
+  if (del) {
     // Delete the key
     if (di == NULL) {
       // Doesn't exist, fail

src/nvim/api/tabpage.c

@@ -54,7 +54,7 @@ Object tabpage_get_var(Tabpage tabpage, String name, Error *err)
   return dict_get_value(tab->tp_vars, name, err);
 }
 
-/// Sets a tab-scoped (t:) variable. 'nil' value deletes the variable.
+/// Sets a tab-scoped (t:) variable
 ///
 /// @param tabpage handle
 /// @param name The variable name
@@ -69,7 +69,24 @@ Object tabpage_set_var(Tabpage tabpage, String name, Object value, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(tab->tp_vars, name, value, err);
+  return dict_set_value(tab->tp_vars, name, value, false, err);
+}
+
+/// Removes a tab-scoped (t:) variable
+///
+/// @param tabpage handle
+/// @param name The variable name
+/// @param[out] err Details of an error that may have occurred
+/// @return The tab page handle
+Object tabpage_del_var(Tabpage tabpage, String name, Error *err)
+{
+  tabpage_T *tab = find_tab_by_handle(tabpage, err);
+
+  if (!tab) {
+    return (Object) OBJECT_INIT;
+  }
+
+  return dict_set_value(tab->tp_vars, name, NIL, true, err);
 }
 
 /// Gets the current window in a tab page

src/nvim/api/vim.c

@@ -331,7 +331,7 @@ Object vim_get_var(String name, Error *err)
   return dict_get_value(&globvardict, name, err);
 }
 
-/// Sets a global variable. Passing 'nil' as value deletes the variable.
+/// Sets a global variable
 ///
 /// @param name The variable name
 /// @param value The variable value
@@ -339,7 +339,17 @@ Object vim_get_var(String name, Error *err)
 /// @return the old value if any
 Object vim_set_var(String name, Object value, Error *err)
 {
-  return dict_set_value(&globvardict, name, value, err);
+  return dict_set_value(&globvardict, name, value, false, err);
+}
+
+/// Removes a global variable
+///
+/// @param name The variable name
+/// @param[out] err Details of an error that may have occurred
+/// @return the old value if any
+Object vim_del_var(String name, Error *err)
+{
+  return dict_set_value(&globvardict, name, NIL, true, err);
 }
 
 /// Gets a vim variable

src/nvim/api/window.c

@@ -197,7 +197,7 @@ Object window_get_var(Window window, String name, Error *err)
   return dict_get_value(win->w_vars, name, err);
 }
 
-/// Sets a window-scoped (w:) variable. 'nil' value deletes the variable.
+/// Sets a window-scoped (w:) variable
 ///
 /// @param window The window handle
 /// @param name The variable name
@@ -212,7 +212,24 @@ Object window_set_var(Window window, String name, Object value, Error *err)
     return (Object) OBJECT_INIT;
   }
 
-  return dict_set_value(win->w_vars, name, value, err);
+  return dict_set_value(win->w_vars, name, value, false, err);
+}
+
+/// Removes a window-scoped (w:) variable
+///
+/// @param window The window handle
+/// @param name The variable name
+/// @param[out] err Details of an error that may have occurred
+/// @return The old value
+Object window_del_var(Window window, String name, Error *err)
+{
+  win_T *win = find_window_by_handle(window, err);
+
+  if (!win) {
+    return (Object) OBJECT_INIT;
+  }
+
+  return dict_set_value(win->w_vars, name, NIL, true, err);
 }
 
 /// Gets a window option value