Harden the pre-tyctxt query system against accidental recomputation

compiler/rustc_data_structures/src/steal.rs

@@ -40,6 +40,11 @@ impl<T> Steal<T> {
         ReadGuard::map(borrow, |opt| opt.as_ref().unwrap())
     }
 
+    #[track_caller]
+    pub fn get_mut(&mut self) -> &mut T {
+        self.value.get_mut().as_mut().expect("attempt to read from stolen value")
+    }
+
     #[track_caller]
     pub fn steal(&self) -> T {
         let value_ref = &mut *self.value.try_write().expect("stealing value which is locked");