Rollup merge of #35701 - matthew-piziak:rangefull-example-error, r=steveklabnik

explicitly show how iterating over `..` fails

I've also removed the `main()` wrapper, which I believe is extraneous.
LMK if that's incorrect.

src/libcore/ops.rs

@@ -1467,17 +1467,30 @@ pub trait IndexMut<Idx: ?Sized>: Index<Idx> {
 ///
 /// # Examples
 ///
-/// ```
-/// fn main() {
-///     assert_eq!((..), std::ops::RangeFull);
+/// The `..` syntax is a `RangeFull`:
 ///
-///     let arr = [0, 1, 2, 3];
-///     assert_eq!(arr[ .. ], [0,1,2,3]);  // RangeFull
-///     assert_eq!(arr[ ..3], [0,1,2  ]);
-///     assert_eq!(arr[1.. ], [  1,2,3]);
-///     assert_eq!(arr[1..3], [  1,2  ]);
+/// ```
+/// assert_eq!((..), std::ops::RangeFull);
+/// ```
+///
+/// It does not have an `IntoIterator` implementation, so you can't use it in a
+/// `for` loop directly. This won't compile:
+///
+/// ```ignore
+/// for i in .. {
+///    // ...
 /// }
 /// ```
+///
+/// Used as a slicing index, `RangeFull` produces the full array as a slice.
+///
+/// ```
+/// let arr = [0, 1, 2, 3];
+/// assert_eq!(arr[ .. ], [0,1,2,3]);  // RangeFull
+/// assert_eq!(arr[ ..3], [0,1,2  ]);
+/// assert_eq!(arr[1.. ], [  1,2,3]);
+/// assert_eq!(arr[1..3], [  1,2  ]);
+/// ```
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct RangeFull;