test: verify cargo-run doesn't inherit jobserver

tests/testsuite/jobserver.rs

@@ -12,7 +12,7 @@ const EXE_CONTENT: &str = r#"
 use std::env;
 
 fn main() {
-    let var = env::var("CARGO_MAKEFLAGS").unwrap();
+    let var = env::var("CARGO_MAKEFLAGS").expect("no jobserver from env");
     let arg = var.split(' ')
                  .find(|p| p.starts_with("--jobserver"))
                 .unwrap();
@@ -105,6 +105,46 @@ all:
     p.process(make).env("CARGO", cargo_exe()).arg("-j2").run();
 }
 
+#[cargo_test]
+fn cargo_run_inherits_jobserver() {
+    let make = make_exe();
+    if Command::new(make).arg("--version").output().is_err() {
+        return;
+    }
+
+    let name = "cargo-jobserver-check";
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [package]
+                    name = "{name}"
+                    version = "0.0.1"
+                "#
+            ),
+        )
+        .file("src/main.rs", EXE_CONTENT)
+        .file(
+            "Makefile",
+            "\
+all:
+\t+$(CARGO) run
+",
+        )
+        .build();
+
+    p.process(make).env("CARGO", cargo_exe()).arg("-j2")
+        .with_status(2)
+        .with_stderr_contains("[..]no jobserver from env[..]")
+        .run();
+
+    p.cargo("run -j2")
+        .with_status(101)
+        .with_stderr_contains("[..]no jobserver from env[..]")
+        .run();
+}
+
 #[cargo_test]
 fn makes_jobserver_used() {
     let make = make_exe();