Disable thread support in libtest if RUST_TEST_THREADS=1

library/test/src/helpers/concurrency.rs

@@ -3,12 +3,30 @@
 use std::{env, num::NonZeroUsize, thread};
 
 pub fn get_concurrency() -> usize {
-    if let Ok(value) = env::var("RUST_TEST_THREADS") {
-        match value.parse::<NonZeroUsize>().ok() {
-            Some(n) => n.get(),
-            _ => panic!("RUST_TEST_THREADS is `{value}`, should be a positive integer."),
-        }
+    rust_test_threads_from_env()
+        .unwrap_or_else(|| thread::available_parallelism().map(|n| n.get()).unwrap_or(1))
+}
+
+pub fn supports_threads() -> bool {
+    if cfg!(target_os = "emscripten") || cfg!(target_family = "wasm") {
+        return false;
+    }
+
+    // Accommodate libraries that may rely on shared thread-local storage (e.g.
+    // integrating with old C libraries).
+    if let Some(1) = rust_test_threads_from_env() {
+        return false;
+    }
+
+    true
+}
+
+fn rust_test_threads_from_env() -> Option<usize> {
+    let value = env::var("RUST_TEST_THREADS").ok()?;
+
+    if let Ok(value) = value.parse::<NonZeroUsize>() {
+        Some(value.get())
     } else {
-        thread::available_parallelism().map(|n| n.get()).unwrap_or(1)
+        panic!("RUST_TEST_THREADS is `{value}`, should be a positive integer.", value = value)
     }
 }

library/test/src/lib.rs

@@ -82,7 +82,7 @@ mod tests;
 
 use core::any::Any;
 use event::{CompletedTest, TestEvent};
-use helpers::concurrency::get_concurrency;
+use helpers::concurrency::{get_concurrency, supports_threads};
 use helpers::exit_code::get_exit_code;
 use helpers::shuffle::{get_shuffle_seed, shuffle_tests};
 use options::RunStrategy;
@@ -592,8 +592,7 @@ pub fn run_test(
         // If the platform is single-threaded we're just going to run
         // the test synchronously, regardless of the concurrency
         // level.
-        let supports_threads = !cfg!(target_os = "emscripten") && !cfg!(target_family = "wasm");
-        if supports_threads {
+        if supports_threads() {
             let cfg = thread::Builder::new().name(name.as_slice().to_owned());
             let mut runtest = Arc::new(Mutex::new(Some(runtest)));
             let runtest2 = runtest.clone();