Add Steal::is_stolen()

compiler/rustc_data_structures/src/steal.rs

@@ -51,6 +51,12 @@ pub fn steal(&self) -> T {
         let value = value_ref.take();
         value.expect("attempt to steal from stolen value")
     }
+
+    /// Writers of rustc drivers often encounter stealing issues. This function makes it possible to
+    /// handle these errors gracefully. This is not used within rustc as the time of writing.
+    pub fn is_stolen(&self) -> bool {
+        self.value.borrow().is_none()
+    }
 }
 
 impl<CTX, T: HashStable<CTX>> HashStable<CTX> for Steal<T> {