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

@@ -238,9 +238,10 @@ mod inprocess_jobserver {
             // 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.
+
+            let mut parallelism = std::thread::available_parallelism()
+                .map(|v| u32::from(v))
+                .unwrap_or(4);
             if let Ok(amt) = var("NUM_JOBS") {
                 if let Ok(amt) = amt.parse() {
                     parallelism = amt;