Remove unnecessary fixme on new thread stack size

library/std/src/sys/pal/windows/thread.rs

@@ -26,11 +26,8 @@ impl Thread {
     pub unsafe fn new(stack: usize, p: Box<dyn FnOnce()>) -> io::Result<Thread> {
         let p = Box::into_raw(Box::new(p));
 
-        // FIXME On UNIX, we guard against stack sizes that are too small but
-        // that's because pthreads enforces that stacks are at least
-        // PTHREAD_STACK_MIN bytes big. Windows has no such lower limit, it's
-        // just that below a certain threshold you can't do anything useful.
-        // That threshold is application and architecture-specific, however.
+        // CreateThread rounds up values for the stack size to the nearest page size (at least 4kb).
+        // If a value of zero is given then the default stack size is used instead.
         let ret = c::CreateThread(
             ptr::null_mut(),
             stack,

library/std/src/thread/tests.rs

@@ -423,3 +423,16 @@ fn scope_join_race() {
         });
     }
 }
+
+// Test that the smallest value for stack_size works on Windows.
+#[cfg(windows)]
+#[test]
+fn test_minimal_thread_stack() {
+    use crate::sync::atomic::AtomicU8;
+    static COUNT: AtomicU8 = AtomicU8::new(0);
+
+    let builder = thread::Builder::new().stack_size(1);
+    let before = builder.spawn(|| COUNT.fetch_add(1, Ordering::Relaxed)).unwrap().join().unwrap();
+    assert_eq!(before, 0);
+    assert_eq!(COUNT.load(Ordering::Relaxed), 1);
+}