Emacs: 26.0.90
OS: macOS 10.13.2

1. start Emacs with -Q
2. open a test file (like 1.c)
3. turn on auto-revert-mode by run global-auto-revert-mode
4. add auto-revert-use-notify as a watcher variable by debug-on-variable-change
5. open the test file with vim
6. modify the test file in vim and save it
7. Emacs enter debug with the following msg:

Debugger entered--setting auto-revert-use-notify in buffer 1.c to nil:
  debug--implement-debug-watch(auto-revert-use-notify nil set #<buffer 1.c>)
  auto-revert-notify-handler((13 stopped "/Users/jun/test/1.c"))
  file-notify--rm-descriptor(13)
  file-notify-rm-watch(13)
  #f(compiled-function (key value) #<bytecode 0x40a952e1>)(13 (#<buffer 1.c>))
  maphash(#f(compiled-function (key value) #<bytecode 0x40a952e1>) #<hash-table equal 7/65 0x40e7896d>)
  auto-revert-notify-rm-watch()
  auto-revert-notify-handler((13 stopped "/Users/jun/test/1.c"))
  file-notify--rm-descriptor(13)
  file-notify-rm-watch(13)
  file-notify-callback((13 (rename delete) "/Users/jun/test/1.c"))
  file-notify-handle-event((file-notify (13 (rename delete) "/Users/jun/test/1.c") file-notify-callback))
  funcall-interactively(file-notify-handle-event (file-notify (13 (rename delete) "/Users/jun/test/1.c") file-notify-callback))
  call-interactively(file-notify-handle-event nil [(file-notify (13 (rename delete) "/Users/jun/test/1.c") file-notify-callback)])
  command-execute(file-notify-handle-event nil [(file-notify (13 (rename delete) "/Users/jun/test/1.c") file-notify-callback)] t)