The bitflags macro, expanded in the prescence of a definition of the type/value Ok errors.
Reproduction code:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3fda3e36c7c6a57e0f7a83c84e56df20

Interestingly, the relevant function, the fmt function from the Debug impl, does use ::bitflags::_core::fmt::Result, however, it merely returns the value Ok(()).