Document and test minimal stack size on Windows

Author: ChrisDenton

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

@@ -26,6 +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));
 
+        // 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

@@ -4,7 +4,7 @@ use crate::mem;
 use crate::panic::panic_any;
 use crate::result;
 use crate::sync::{
-    atomic::{AtomicBool, Ordering},
+    atomic::{AtomicBool, AtomicU8, Ordering},
     mpsc::{channel, Sender},
     Arc, Barrier,
 };
@@ -423,3 +423,14 @@ fn scope_join_race() {
         });
     }
 }
+
+// Test that the smallest value for stack_size works on Windows.
+#[cfg(windows)]
+#[test]
+fn test_minimal_thread_stack() {
+    static COUNT: AtomicU8 = AtomicU8::new(0);
+
+    let builder = thread::Builder::new().stack_size(1);
+    builder.spawn(move || COUNT.fetch_add(1, Ordering::Relaxed)).unwrap().join().unwrap();
+    assert_eq!(COUNT.load(Ordering::Relaxed), 1);
+}