Rust: use try_borrow where applicable.

artiq/runtime.rs/src/lib.rs

@@ -1,5 +1,5 @@
 #![no_std]
-#![feature(libc, borrow_state, const_fn)]
+#![feature(libc, borrow_state, const_fn, try_borrow)]
 
 #[macro_use]
 extern crate std_artiq as std;

artiq/runtime.rs/src/sched.rs

@@ -68,17 +68,16 @@ impl ThreadHandle {
     }
 
     pub fn terminated(&self) -> bool {
-        match self.0.borrow_state() {
-            BorrowState::Unused => self.0.borrow().terminated(),
-            _ => false // the running thread hasn't terminated
+        match self.0.try_borrow() {
+            Ok(thread) => thread.terminated(),
+            Err(_) => false // the running thread hasn't terminated
         }
     }
 
     pub fn interrupt(&self) {
-        // FIXME: use try_borrow() instead once it's available
-        match self.0.borrow_state() {
-            BorrowState::Unused => self.0.borrow_mut().interrupt(),
-            _ => panic!("cannot interrupt the running thread")
+        match self.0.try_borrow_mut() {
+            Ok(mut thread) => thread.interrupt(),
+            Err(_) => panic!("cannot interrupt the running thread")
         }
     }
 }