Use std::thread::available_parallelism() instead of num_cpus dependency

(cherry picked from commit 936093f)

Author: andrewdavidmackenzie

rayon-core/Cargo.toml

@@ -18,7 +18,6 @@ name = "rayon_core"
 
 # Some dependencies may not be their latest version, in order to support older rustc.
 [dependencies]
-num_cpus = "1.2"
 crossbeam-channel = "0.5.0"
 crossbeam-deque = "0.8.1"
 crossbeam-utils = "0.8.0"

rayon-core/src/lib.rs

@@ -70,6 +70,7 @@ use std::fmt;
 use std::io;
 use std::marker::PhantomData;
 use std::str::FromStr;
+use std::thread;
 
 #[macro_use]
 mod log;
@@ -496,7 +497,8 @@ impl<S> ThreadPoolBuilder<S> {
                 .and_then(|s| usize::from_str(&s).ok())
             {
                 Some(x) if x > 0 => return x,
-                Some(x) if x == 0 => return num_cpus::get(),
+                Some(x) if x == 0 =>  return thread::available_parallelism()
+					.map(|n| n.get()).unwrap_or(1),
                 _ => {}
             }
 
@@ -506,7 +508,7 @@ impl<S> ThreadPoolBuilder<S> {
                 .and_then(|s| usize::from_str(&s).ok())
             {
                 Some(x) if x > 0 => x,
-                _ => num_cpus::get(),
+                _ => thread::available_parallelism().map(|n| n.get()).unwrap_or(1)
             }
         }
     }