Reexport num_cpus in std::os. Closes #14707

[japaric]

I'm not too confident about my approach regarding the handling of the docs, so here goes my train of thought:

The first commit does the reexport. But the side effect is that two num_cpus entries appear in the docs: std::os::num_cpus and std::rt::num_cpus, but we only need the former one.

The second commit deals with the duplicated entry. Is my first time using rustdoc, but I expected that a #[doc(hidden)] attribute on the pub use self::util::num_cpus would remove the duplicated entry - this didn't work and this seems like a bug to me (or it could be my inexperience with rustdoc). The only way I found to remove the duplicated entry was to make the util module public (this may not be acceptable), in addition to the #[doc(hidden)] attribute.

The third commit makes sure that other docs are kept intact (i.e. that the std::rt::util module doesn't appear in the docs).

cc @brson who requested the reexport, and @alexcrichton, who has been working on rustdoc lately.

[alexcrichton]

Perhaps the num_cpus function could just move to the std::os module? I would like to keep std::rt::util private rather than making it public but hidden from the documentation.

I also don't think that doc(hidden) is recognized by rustdoc right now. Does it inline the documentation for you?

[japaric]

Perhaps the num_cpus function could just move to the std::os module?

I thought about doing this in the first place, but a reexport seemed more conservative than a move. If it's ok to move the function, I can try that in another PR.

I also don't think that doc(hidden) is recognized by rustdoc right now. Does it inline the documentation for you?

I probably worded that bad, doc(hidden) does hide the docs in some cases. What I meant to say is that I tried several (all?) combinations of doc attributes (hidden, no_inline, inline) on the reexports (pub use ...) in the first commit, but I could only get one of these two outcomes:

• Both std::rt::num_cpus and std::os::num_cpus appear in the docs.
• num_cpus doesn't appear in the docs at all.

But I couldn't the desired outcome: Only std::os::num_cpus in the docs - until I changed the visibility of the util module to public.

EDIT: Adding doc(hidden)/doc(no_inline) to the pub use self::util::num_cpus in the first commit, does not hide std::rt::num_cpus from the docs.

[alexcrichton]

Let's move num_cpus to the os module in std, but it's ok to do it as part of this PR rather than opening another.

[japaric]

Let's move num_cpus to the os module in std

@alexcrichton done