Rollup merge of rust-lang#66038 - jdxcode:char-len, r=alexcrichton

doc(str): show example of chars().count() under len()

the docs are great at explaining that .len() isn't like in other
languages but stops short of explaining how to get the character length.

src/liballoc/string.rs

@@ -1402,16 +1402,21 @@ impl String {
         &mut self.vec
     }
 
-    /// Returns the length of this `String`, in bytes.
+    /// Returns the length of this `String`, in bytes, not [`char`]s or
+    /// graphemes. In other words, it may not be what a human considers the
+    /// length of the string.
     ///
     /// # Examples
     ///
     /// Basic usage:
     ///
     /// ```
     /// let a = String::from("foo");
-    ///
     /// assert_eq!(a.len(), 3);
+    ///
+    /// let fancy_f = String::from("ƒoo");
+    /// assert_eq!(fancy_f.len(), 4);
+    /// assert_eq!(fancy_f.chars().count(), 3);
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/str/mod.rs

@@ -2085,8 +2085,8 @@ impl str {
     /// let len = "foo".len();
     /// assert_eq!(3, len);
     ///
-    /// let len = "ƒoo".len(); // fancy f!
-    /// assert_eq!(4, len);
+    /// assert_eq!("ƒoo".len(), 4); // fancy f!
+    /// assert_eq!("ƒoo".chars().count(), 3);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]