Use available_parallelism instead of num_cpus

Cargo.toml

@@ -43,7 +43,6 @@ libc = "0.2"
 log = "0.4.6"
 libgit2-sys = "0.12.24"
 memchr = "2.1.3"
-num_cpus = "1.0"
 opener = "0.5"
 os_info = "3.0.7"
 percent-encoding = "2.0"

src/cargo/core/compiler/build_config.rs

@@ -1,11 +1,12 @@
 use crate::core::compiler::CompileKind;
 use crate::util::interning::InternedString;
 use crate::util::{CargoResult, Config, RustfixDiagnosticServer};
-use anyhow::bail;
+use anyhow::{bail, Context as _};
 use cargo_util::ProcessBuilder;
 use serde::ser;
 use std::cell::RefCell;
 use std::path::PathBuf;
+use std::thread::available_parallelism;
 
 /// Configuration information for a rustc build.
 #[derive(Debug)]
@@ -70,7 +71,12 @@ impl BuildConfig {
                  its environment, ignoring the `-j` parameter",
             )?;
         }
-        let jobs = jobs.or(cfg.jobs).unwrap_or(::num_cpus::get() as u32);
+        let jobs = match jobs.or(cfg.jobs) {
+            Some(j) => j,
+            None => available_parallelism()
+                .context("failed to determine the amount of parallelism available")?
+                .get() as u32,
+        };
         if jobs == 0 {
             anyhow::bail!("jobs may not be 0");
         }

src/cargo/core/compiler/timings.rs

@@ -13,6 +13,7 @@ use anyhow::Context as _;
 use cargo_util::paths;
 use std::collections::HashMap;
 use std::io::{BufWriter, Write};
+use std::thread::available_parallelism;
 use std::time::{Duration, Instant, SystemTime};
 
 pub struct Timings<'cfg> {
@@ -380,6 +381,9 @@ impl<'cfg> Timings<'cfg> {
         };
         let total_time = format!("{:.1}s{}", duration, time_human);
         let max_concurrency = self.concurrency.iter().map(|c| c.active).max().unwrap();
+        let num_cpus = available_parallelism()
+            .map(|x| x.get().to_string())
+            .unwrap_or_else(|_| "n/a".into());
         let max_rustc_concurrency = self
             .concurrency
             .iter()
@@ -442,7 +446,7 @@ impl<'cfg> Timings<'cfg> {
             self.total_fresh + self.total_dirty,
             max_concurrency,
             bcx.build_config.jobs,
-            num_cpus::get(),
+            num_cpus,
             self.start_str,
             total_time,
             rustc_info,