Warn if --jobs >8

Fixes #412

NEWS.md

@@ -6,6 +6,8 @@
 
 - Fixed: Support crates that use a non-default Cargo registry. Previously, `cargo metadata` failed with "registry index was not found."
 
+- Improved: Warn if `--jobs` is set higher than 8, which is likely to be too high.
+
 ## 25.0.1-pre3 2025-01-05
 
 - Fixed: Build arm64 binaries for macOS.

src/options.rs

@@ -321,6 +321,11 @@ impl Options {
             test_timeout_multiplier: args.timeout_multiplier.or(config.timeout_multiplier),
             test_tool: args.test_tool.or(config.test_tool).unwrap_or_default(),
         };
+        if let Some(jobs) = options.jobs {
+            if jobs >= 8 {
+                warn!("--jobs={jobs} is probably too high and may overload your machine: each job runs a separate `cargo` process, and cargo may internally start many threads and subprocesses; values <= 8 are usually safe");
+            }
+        }
         options.error_values.iter().for_each(|e| {
             if e.starts_with("Err(") {
                 warn!(

tests/jobs.rs

@@ -1,4 +1,4 @@
-// Copyright 2022-2023 Martin Pool.
+// Copyright 2022 - 2025 Martin Pool.
 
 //! Test handling of `--jobs` concurrency option.
 
@@ -43,3 +43,19 @@ fn jobs_option_accepted_and_causes_multiple_threads() {
         matches.len()
     );
 }
+
+#[test]
+fn warn_about_too_many_jobs() {
+    let testdata = copy_of_testdata("small_well_tested");
+    run()
+        .arg("mutants")
+        .arg("-d")
+        .arg(testdata.path())
+        .arg("-j40")
+        .arg("--shard=0/1000")
+        .assert()
+        .stderr(predicates::str::contains(
+            "WARN --jobs=40 is probably too high",
+        ))
+        .success();
+}