Rollup merge of rust-lang#124308 - CBSpeir:diagnostic-item-enumerate, r=compiler-errors

Add diagnostic item for `std::iter::Enumerate`

This adds a diagnostic item for `std::iter::Enumerate`.  The change will be used by the clippy `unused_enumerate_index` lint to move away from type paths to using diagnostic items.

see: rust-lang/rust-clippy#5393

Author: fmease

core/src/iter/adapters/enumerate.rs

@@ -15,6 +15,7 @@ use crate::ops::Try;
 #[derive(Clone, Debug)]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[cfg_attr(not(test), rustc_diagnostic_item = "Enumerate")]
 pub struct Enumerate<I> {
     iter: I,
     count: usize,