Add diagnostic item for std::iter::Iterator::enumerate

Author: CBSpeir

core/src/iter/traits/iterator.rs

@@ -974,6 +974,7 @@ pub trait Iterator {
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_do_not_const_check]
+    #[cfg_attr(not(test), rustc_diagnostic_item = "enumerate_method")]
     fn enumerate(self) -> Enumerate<Self>
     where
         Self: Sized,