Make the libc "extra_traits" feature optional.

Make "libc/extra_traits" optional and enabled by default, so that it can
be disabled by --no-default-features. This fixes rustc-dep-of-std
builds, because extra_traits depends on std.

Cargo.toml

@@ -38,15 +38,15 @@ once_cell = { version = "1.5.2", optional = true }
 [target.'cfg(all(not(rustix_use_libc), not(miri), target_os = "linux", target_endian = "little", any(target_arch = "arm", all(target_arch = "aarch64", target_pointer_width = "64"), target_arch = "riscv64", all(rustix_use_experimental_asm, target_arch = "powerpc64"), all(rustix_use_experimental_asm, target_arch = "mips"), all(rustix_use_experimental_asm, target_arch = "mips32r6"), all(rustix_use_experimental_asm, target_arch = "mips64"), all(rustix_use_experimental_asm, target_arch = "mips64r6"), target_arch = "x86", all(target_arch = "x86_64", target_pointer_width = "64"))))'.dependencies]
 linux-raw-sys = { version = "0.4.12", default-features = false, features = ["general", "errno", "ioctl", "no_std", "elf"] }
 libc_errno = { package = "errno", version = "0.3.8", default-features = false, optional = true }
-libc = { version = "0.2.153", default-features = false, features = ["extra_traits"], optional = true }
+libc = { version = "0.2.153", default-features = false, optional = true }
 
 # Dependencies for platforms where only libc is supported:
 #
 # On all other Unix-family platforms, and under Miri, we always use the libc
 # backend, so enable its dependencies unconditionally.
 [target.'cfg(all(not(windows), any(rustix_use_libc, miri, not(all(target_os = "linux", target_endian = "little", any(target_arch = "arm", all(target_arch = "aarch64", target_pointer_width = "64"), target_arch = "riscv64", all(rustix_use_experimental_asm, target_arch = "powerpc64"), all(rustix_use_experimental_asm, target_arch = "mips"), all(rustix_use_experimental_asm, target_arch = "mips32r6"), all(rustix_use_experimental_asm, target_arch = "mips64"), all(rustix_use_experimental_asm, target_arch = "mips64r6"), target_arch = "x86", all(target_arch = "x86_64", target_pointer_width = "64")))))))'.dependencies]
 libc_errno = { package = "errno", version = "0.3.8", default-features = false }
-libc = { version = "0.2.153", default-features = false, features = ["extra_traits"] }
+libc = { version = "0.2.153", default-features = false }
 
 # Additional dependencies for Linux with the libc backend:
 #
@@ -119,14 +119,18 @@ targets = [
 # It turns out to be bizarrely awkward to obtain the aux values reliably and
 # efficiently on Linux from anywhere other than libc. We can do it, but most
 # users are better served by just using libc for this.
-default = ["std", "use-libc-auxv"]
+default = ["std", "use-libc-auxv", "libc-extra-traits"]
+
+# Enable `extra_traits` in libc types, to provide `Debug`, `Hash`, and other
+# trait impls for libc types.
+libc-extra-traits = ["libc?/extra_traits"]
 
 # This enables use of std. Disabling this enables `#![no_std]`, and requires
 # Rust 1.64 or newer.
 std = ["bitflags/std", "alloc", "libc?/std", "libc_errno?/std"]
 
 # Enable this to request the libc backend.
-use-libc = ["libc_errno", "libc"]
+use-libc = ["libc_errno", "libc", "libc-extra-traits"]
 
 # Enable `rustix::event::*`.
 event = []