Rollup merge of #107890 - obeis:mapping-to-unit, r=cjgillot

Lint against `Iterator::map` receiving a callable that returns `()`

Close #106991

Author: matthiaskrgr

core/src/iter/mod.rs

@@ -278,6 +278,7 @@
 //!
 //! ```
 //! # #![allow(unused_must_use)]
+//! # #![cfg_attr(not(bootstrap), allow(map_unit_fn))]
 //! let v = vec![1, 2, 3, 4, 5];
 //! v.iter().map(|x| println!("{x}"));
 //! ```

core/src/iter/traits/iterator.rs

@@ -789,6 +789,7 @@ pub trait Iterator {
     ///     println!("{x}");
     /// }
     /// ```
+    #[rustc_diagnostic_item = "IteratorMap"]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_do_not_const_check]