Rollup merge of rust-lang#81939 - kper:fixing-81584-allocate-in-iter, r=davidtwco

Add suggestion `.collect()` for iterators in iterators

Closes rust-lang#81584

```
error[E0515]: cannot return value referencing function parameter `y`
 --> main3.rs:4:38
  |
4 | ...                   .map(|y| y.iter().map(|x| x + 1))
  |                                -^^^^^^^^^^^^^^^^^^^^^^
  |                                |
  |                                returns a value referencing data owned by the current function
  |                                `y` is borrowed here
  |                                help: Maybe use `.collect()` to allocate the iterator
```

Added the suggestion: `help: Maybe use `.collect()` to allocate the iterator`

Author: m-ou-se

core/src/iter/traits/iterator.rs

@@ -93,6 +93,7 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
     message = "`{Self}` is not an iterator"
 )]
 #[doc(spotlight)]
+#[rustc_diagnostic_item = "Iterator"]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub trait Iterator {
     /// The type of the elements being iterated over.