This generally gives a better experience with bindings generators,
possibly other tooling.
A possible risk is that some generators may not represent unknown
codes correctly.
Rust bindgen by default generates suitable code:
* a type alias nix_err = c_int
* individual constants for the known enum values
It does _not_ generate a closed type that can only hold the values
that were known at code generation time.
If this proves to be a problem, we could instead split the type:
`typedef int nix_err;` for return values
`enum nix_known_err` for code generation.
This would complicate the interface, so let's not do it unless it
is shown to be needed.
Inspired by
010ff57ebb
From the original PR:
> We do not have any of these warnings appearing at the moment, but
> it seems like a good idea to enable [[nodiscard]] checking anyway.
> Once we start introducing more functions with must-use conditions we will
> need such checking, and the rust stdlib has proven them very useful.
The documentation "solved" this by specifying a precondition, but
let's just make it more robust, and not leak irrelevant messages
that might linger.
We don't clear the message when clearing the status, in order to
keep clearing fast; see last_err field doc.
See https://github.com/NixOS/nix/pull/8699#discussion_r1554312181
Casting a function pointer to `void*` is undefined behavior in the C
spec, since there are platforms with different sizes for these two kinds
of pointers. A safe alternative might be `void (*callback)()`
