Rollup merge of rust-lang#124542 - CBSpeir:diagnostic-item-enumerate-method, r=scottmcm

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

Adds a diagnostic item for the `std::iter:Iterator::enumerate` trait method. This change, along with PR rust-lang#124308, will be used by the clippy `unused_enumerate_index` lint to move away from paths to using diagnostic items.

see: rust-lang/rust-clippy#5393

Author: matthiaskrgr

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,