rewrite libtest-thread-limit to rmake

Author: Oneirical

src/tools/tidy/src/allowed_run_make_makefiles.txt

@@ -24,7 +24,6 @@ run-make/jobserver-error/Makefile
 run-make/libs-through-symlinks/Makefile
 run-make/libtest-json/Makefile
 run-make/libtest-junit/Makefile
-run-make/libtest-thread-limit/Makefile
 run-make/link-cfg/Makefile
 run-make/long-linker-command-lines-cmd-exe/Makefile
 run-make/long-linker-command-lines/Makefile

tests/run-make/libtest-thread-limit/Makefile

@@ -0,0 +1,19 @@
+// libtest used to panic if it hit the thread limit. This often resulted in spurious test failures
+// (thread 'main' panicked at 'called Result::unwrap() on an Err value: Os
+// { code: 11, kind: WouldBlock, message: "Resource temporarily unavailable" }' ...
+// error: test failed, to rerun pass '--lib').
+// Since the fix in #81546, the test should continue to run synchronously
+// if it runs out of threads. Therefore, this test's final execution step
+// should succeed without an error.
+// See https://github.com/rust-lang/rust/pull/81546
+
+//@ only-linux
+// Reason: the `ulimit` utility is used
+
+use run_make_support::{cmd, run, rustc};
+
+fn main() {
+    rustc().input("test.rs").arg("--test").run();
+    cmd("ulimit").arg("-p").arg("0").run();
+    run("test");
+}

tests/run-make/libtest-thread-limit/rmake.rs

@@ -4,6 +4,7 @@
 // See https://github.com/rust-lang/rust/pull/105123
 
 //@ only-macos
+// Reason: this test exercises an OSX-specific issue
 
 use run_make_support::{cmd, rustc};