std_detect has `cfg(feature = "use_std")` but no such feature exists.

[thomcc]

In std_detect::detect, the non-x86{,_64} code for linux / freebsd is behind feature = "use_std":

stdarch/crates/std_detect/src/detect/mod.rs

Lines 101 to 104 in 016eff9

	} else if #[cfg(all(target_os = "linux", feature = "use_std"))] {
	#[path = "os/linux/mod.rs"]
	mod os;
	} else if #[cfg(all(target_os = "freebsd", feature = "use_std"))] {

I believe this is intended to be feature = "std_detect_file_io"? Or maybe not, because the linux module further cordons off the cpuinfo reading under that, implying it expects that it may exist without this std_detect_file_io:

stdarch/crates/std_detect/src/detect/os/linux/mod.rs

Lines 5 to 6 in e451716

	#[cfg(feature = "std_detect_file_io")]
	mod cpuinfo;

AFAICT this code is just totally dead, and it's a bit of a large amount of dead code to have sitting in the tree... and I can't find an old commit that even had a use_std feature, so I'm probably just wrong and am unaware of some mode that this code is compiled in — I know about "as a module of libstd", but is there anything else I should be aware of?

If not, even if the code is not production ready, it seems like it should stay compiling.

Anyway, I was gonna submit a PR with this to see if the CI passed at least, but I became worried that if it landed, it would regress #850, which added one of these lines, so I figured I'd ask first.

[Lokathor]

It's possible that this gets set by somewhere else that builds this crate as a submodule.

[bjorn3]

libstd doesn't enable it

[bjorn3]

It was introduced in 4c66d96, but even rust-lang/rust#58373, which is the first stdarch update since that commit doesn't enable it.