Switch back to available_parallelism

This reverts commit 8345cf5037506b38457483429fb113322c58f668. Since that
time, there are now multiple calls to get the number of CPUs, to handle
the `-j -NUM` case, so factor out a helper function.

Cargo.toml

@@ -44,7 +44,6 @@ libc = "0.2"
 log = "0.4.6"
 libgit2-sys = "0.13.2"
 memchr = "2.1.3"
-num_cpus = "1.0"
 opener = "0.5"
 os_info = "3.5.0"
 pathdiff = "0.2"

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)]
@@ -45,6 +46,12 @@ pub struct BuildConfig {
     pub timing_outputs: Vec<TimingOutput>,
 }
 
+fn default_parallelism() -> CargoResult<u32> {
+    Ok(available_parallelism()
+        .context("failed to determine the amount of parallelism available")?
+        .get() as u32)
+}
+
 impl BuildConfig {
     /// Parses all config files to learn about build configuration. Currently
     /// configured options are:
@@ -71,9 +78,9 @@ impl BuildConfig {
             )?;
         }
         let jobs = match jobs.or(cfg.jobs) {
-            None => ::num_cpus::get() as u32,
+            None => default_parallelism()?,
             Some(0) => anyhow::bail!("jobs may not be 0"),
-            Some(j) if j < 0 => (::num_cpus::get() as i32 + j).max(1) as u32,
+            Some(j) if j < 0 => (default_parallelism()? as i32 + j).max(1) as u32,
             Some(j) => j as u32,
         };
 

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.jobs(),
-            num_cpus::get(),
+            num_cpus,
             self.start_str,
             total_time,
             rustc_info,