Add '@' subpattern binding and examples to manual and tutorial

Author: Armavica

doc/rust.md

@@ -2939,6 +2939,27 @@ This can be changed to bind to a reference by
 using the `ref` keyword,
 or to a mutable reference using `ref mut`.
 
+Subpatterns can also be bound to variables by the use of the syntax
+`variable @ pattern`.
+For example:
+
+~~~~
+enum List { Nil, Cons(uint, ~List) }
+
+fn is_sorted(list: &List) -> bool {
+    match *list {
+        Nil | Cons(_, ~Nil) => true,
+        Cons(x, ref r @ ~Cons(y, _)) => (x <= y) && is_sorted(*r)
+    }
+}
+
+fn main() {
+    let a = Cons(6, ~Cons(7, ~Cons(42, ~Nil)));
+    assert!(is_sorted(&a));
+}
+
+~~~~
+
 Patterns can also dereference pointers by using the `&`,
 `~` or `@` symbols, as appropriate. For example, these two matches
 on `x: &int` are equivalent:

doc/tutorial.md

@@ -520,6 +520,16 @@ to the value of the matched value inside of the arm's action. Thus, `(0.0,
 y)` matches any tuple whose first element is zero, and binds `y` to
 the second element. `(x, y)` matches any two-element tuple, and binds both
 elements to variables.
+A subpattern can also be bound to a variable, using `variable @ pattern`. For
+example:
+
+~~~~
+# let age = 23;
+match age {
+    a @ 0..20 => println!("{} years old", a),
+    _ => println!("older than 21")
+}
+~~~~
 
 Any `match` arm can have a guard clause (written `if EXPR`), called a
 *pattern guard*, which is an expression of type `bool` that