Add parentheses around expression arguments to ..

This makes it easier for humans to parse, and improves the result of
potential future automatic formatting.

Author: joshtriplett

core/src/iter/traits/iterator.rs

@@ -807,7 +807,7 @@ pub trait Iterator {
     /// might be preferable to keep a functional style with longer iterators:
     ///
     /// ```
-    /// (0..5).flat_map(|x| x * 100 .. x * 110)
+    /// (0..5).flat_map(|x| (x * 100)..(x * 110))
     ///       .enumerate()
     ///       .filter(|&(i, x)| (i + x) % 3 == 0)
     ///       .for_each(|(i, x)| println!("{i}:{x}"));