Add thread spawn with proper trait checks.

rust/lib/mammoth/src/thread.rs

@@ -42,3 +42,43 @@ impl Thread {
         syscall::thread_wait(&self.cap)
     }
 }
+
+pub struct JoinHandle {
+    cap: Capability,
+}
+
+impl JoinHandle {
+    pub fn join(&self) -> Result<(), zion::ZError> {
+        syscall::thread_wait(&self.cap)
+    }
+}
+
+#[no_mangle]
+extern "C" fn entry_point(func: *mut c_void) -> ! {
+    unsafe {
+        Box::from_raw(func as *mut Box<dyn FnOnce()>)();
+    }
+
+    syscall::thread_exit()
+}
+
+pub fn spawn<F>(f: F) -> JoinHandle
+where
+    F: FnOnce() + Send + 'static,
+{
+    // This is very tricky.
+    // If we have been passed a closure that doesn't capture
+    // anything it will be 0 size and creating a Box of it
+    // will create a pointer with address 0x1.
+    // So we create this "main" closure that captures f to get around this.
+    // Also somehow having the explicit type annotation here is important.
+    let main: Box<dyn FnOnce()> = Box::new(move || {
+        f();
+    });
+    let raw_main = Box::into_raw(Box::new(main));
+    let proc_cap = Capability::take_copy(unsafe { crate::init::SELF_PROC_CAP }).unwrap();
+    let cap = syscall::thread_create(&proc_cap).unwrap();
+    syscall::thread_start(&cap, entry_point as u64, raw_main as u64, 0).unwrap();
+
+    JoinHandle { cap }
+}