Use std::thread::available_parallelism for determining the default number of jobs

The function was stabilized in 1.59, while the current MSRV is 1.63.

Author: Kobzol

src/parallel/job_token.rs

@@ -233,19 +233,14 @@ mod inprocess_jobserver {
     impl JobServer {
         pub(super) fn new() -> Self {
             // Use `NUM_JOBS` if set (it's configured by Cargo) and otherwise
-            // just fall back to a semi-reasonable number.
-            //
-            // Note that we could use `num_cpus` here but it's an extra
-            // dependency that will almost never be used, so
-            // it's generally not too worth it.
-            let mut parallelism = 4;
-            // TODO: Use std::thread::available_parallelism as an upper bound
-            // when MSRV is bumped.
-            if let Ok(amt) = var("NUM_JOBS") {
-                if let Ok(amt) = amt.parse() {
-                    parallelism = amt;
-                }
-            }
+            // just fall back to the number of cores on the local machine.
+
+            let num_jobs = var("NUM_JOBS").ok().and_then(|j| j.parse::<u32>().ok());
+            let parallelism = num_jobs.unwrap_or_else(|| {
+                std::thread::available_parallelism()
+                    .map(|v| usize::from(v) as u32)
+                    .unwrap_or(4)
+            });
 
             Self(AtomicU32::new(parallelism))
         }