Doesn't take CPU affinity into account

[jonhoo]

While num_cpus() gives the correct number of physical CPUs, it does not take into account which of those CPU's the current process is actually allowed to use. It would be neat to be able to specify which of the two numbers to get.

For example, consider a simple num_cpus program that just prints the value of num_cpus::get();, and then exits:

$ nproc
80
$ num_cpus
80
$ taskset -c 0,1 nproc
2
$ taskset -c 0,1 num_cpus
80

If it's at all helpful, the source for nproc (which is included in coreutils) can be seen here.

[seanmonstar]

I'd welcome a PR!

[jonhoo]

Seems like unless we want to do some deep systems level hackery, we'll have to wait for rust-lang/rfcs#819