The `feature!` macro does not work when encountering `cfg_if!()`

[SteveLauC]

$ cargo +nightly --version
cargo 1.86.0-nightly (0e3d73849 2025-02-01)

$ cargo +nightly c --all-features
    Checking nix v0.29.0 (/home/steve/Documents/workspace/nix)
error: unused attribute `<cfg_attr>`
   --> src/macros.rs:9:13
    |
9   |               #[cfg_attr(docsrs, doc(cfg($meta)))]
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
   ::: src/fcntl.rs:235:1
    |
235 | / feature! {
236 | | #![feature = "fs"]
237 | |
238 | | /// open or create a file for reading, writing or executing
...   |
444 | | }
    | |_- in this macro invocation
    |
note: the built-in attribute `<cfg_attr>` will be ignored, since it's applied to the macro invocation `cfg_if::cfg_if`
   --> src/fcntl.rs:293:1
    |
293 | cfg_if::cfg_if! {
    | ^^^^^^^^^^^^^^
note: the lint level is defined here
   --> src/lib.rs:51:9
    |
51  | #![deny(unused)]
    |         ^^^^^^
    = note: `#[deny(unused_attributes)]` implied by `#[deny(unused)]`
    = note: this error originates in the macro `feature` (in Nightly builds, run with -Z macro-backtrace for more info)

error: unused attribute `<cfg_attr>`
   --> src/macros.rs:9:13
    |
9   |               #[cfg_attr(docsrs, doc(cfg($meta)))]
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
   ::: src/sys/signal.rs:387:1
    |
387 | / feature! {
388 | | #![feature = "signal"]
389 | |
390 | | #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
...   |
434 | | }
    | |_- in this macro invocation
    |
note: the built-in attribute `<cfg_attr>` will be ignored, since it's applied to the macro invocation `cfg_if`
   --> src/sys/signal.rs:425:1
    |
425 | cfg_if! {
    | ^^^^^^
    = note: this error originates in the macro `feature` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `nix` (lib) due to 2 previous errors

It shows that 2 uses of features!() actually do not work, and it is right:

1. fcntl.rs

   nix/src/fcntl.rs

   Lines 293 to 413 in e4895d3

   	cfg_if::cfg_if! {
   	if #[cfg(target_os = "linux")] {
   	libc_bitflags! {
   	/// Path resolution flags.
   	///
   	/// See [path resolution(7)](https://man7.org/linux/man-pages/man7/path_resolution.7.html)
   	/// for details of the resolution process.
   	pub struct ResolveFlag: libc::c_ulonglong {
   	/// Do not permit the path resolution to succeed if any component of
   	/// the resolution is not a descendant of the directory indicated by
   	/// dirfd. This causes absolute symbolic links (and absolute values of
   	/// pathname) to be rejected.
   	RESOLVE_BENEATH;
   	/// Treat the directory referred to by dirfd as the root directory
   	/// while resolving pathname.
   	RESOLVE_IN_ROOT;
   	/// Disallow all magic-link resolution during path resolution. Magic
   	/// links are symbolic link-like objects that are most notably found
   	/// in proc(5); examples include `/proc/[pid]/exe` and `/proc/[pid]/fd/*`.
   	///
   	/// See symlink(7) for more details.
   	RESOLVE_NO_MAGICLINKS;
   	/// Disallow resolution of symbolic links during path resolution. This
   	/// option implies RESOLVE_NO_MAGICLINKS.
   	RESOLVE_NO_SYMLINKS;
   	/// Disallow traversal of mount points during path resolution (including
   	/// all bind mounts).
   	RESOLVE_NO_XDEV;
   	}
   	}
   	/// Specifies how [`openat2()`] should open a pathname.
   	///
   	/// # Reference
   	///
   	/// * [Linux](https://man7.org/linux/man-pages/man2/open_how.2type.html)
   	#[repr(transparent)]
   	#[derive(Clone, Copy, Debug)]
   	pub struct OpenHow(libc::open_how);
   	impl OpenHow {
   	/// Create a new zero-filled `open_how`.
   	pub fn new() -> Self {
   	// safety: according to the man page, open_how MUST be zero-initialized
   	// on init so that unknown fields are also zeroed.
   	Self(unsafe {
   	std::mem::MaybeUninit::zeroed().assume_init()
   	})
   	}
   	/// Set the open flags used to open a file, completely overwriting any
   	/// existing flags.
   	pub fn flags(mut self, flags: OFlag) -> Self {
   	let flags = flags.bits() as libc::c_ulonglong;
   	self.0.flags = flags;
   	self
   	}
   	/// Set the file mode new files will be created with, overwriting any
   	/// existing flags.
   	pub fn mode(mut self, mode: Mode) -> Self {
   	let mode = mode.bits() as libc::c_ulonglong;
   	self.0.mode = mode;
   	self
   	}
   	/// Set resolve flags, completely overwriting any existing flags.
   	///
   	/// See [ResolveFlag] for more detail.
   	pub fn resolve(mut self, resolve: ResolveFlag) -> Self {
   	let resolve = resolve.bits();
   	self.0.resolve = resolve;
   	self
   	}
   	}
   	// safety: default isn't derivable because libc::open_how must be zeroed
   	impl Default for OpenHow {
   	fn default() -> Self {
   	Self::new()
   	}
   	}
   	/// Open or create a file for reading, writing or executing.
   	///
   	/// `openat2` is an extension of the [`openat`] function that allows the caller
   	/// to control how path resolution happens.
   	///
   	/// # See also
   	///
   	/// [openat2](https://man7.org/linux/man-pages/man2/openat2.2.html)
   	pub fn openat2<P: ?Sized + NixPath, Fd: std::os::fd::AsFd>(
   	dirfd: Fd,
   	path: &P,
   	mut how: OpenHow,
   	) -> Result<OwnedFd> {
   	use std::os::fd::AsRawFd;
   	use std::os::fd::FromRawFd;
   	let fd = path.with_nix_path(|cstr| unsafe {
   	libc::syscall(
   	libc::SYS_openat2,
   	dirfd.as_fd().as_raw_fd(),
   	cstr.as_ptr(),
   	&mut how as *mut OpenHow,
   	std::mem::size_of::<libc::open_how>(),
   	)
   	})? as RawFd;
   	Errno::result(fd)?;
   	// SAFETY:
   	//
   	// `openat2(2)` should return a valid owned fd on success
   	Ok( unsafe { OwnedFd::from_raw_fd(fd) } )
   	}
   	}
   	}

2. sys/signal.rs

   nix/src/sys/signal.rs

   Lines 425 to 433 in e4895d3

   	cfg_if! {
   	if #[cfg(target_os = "redox")] {
   	type SaFlags_t = libc::c_ulong;
   	} else if #[cfg(target_env = "uclibc")] {
   	type SaFlags_t = libc::c_ulong;
   	} else {
   	type SaFlags_t = libc::c_int;
   	}
   	}

[SteveLauC]

I am going to allow this warning for now to make #2610's CI pass, but we need a solution for this

[Berrysoft]

Maybe just wait for rustc to fix: rust-lang/rust#138779

[SteveLauC]

Maybe just wait for rustc to fix: rust-lang/rust#138779

Yeah, thanks for letting me know this! :)

[SteveLauC]

Close as this has been fixed