In nix 0.26.3, the bitflags crate was updated which is technically a breaking change.
For example, the function nix::fcntl::OFlag::from_bits_unchecked no longer exist.

(Note: while we were using that function in our project, we're going to port to 0.27 anyway so we're not going to be impacted by this change, but i just wanted to report the issue just in case you think this is worth fixing. I can suggest either reverting the change, or add the missing functions in libc_bitflags macro. )