Add section about diverging functions

Author: msehnout

src/SUMMARY.md

@@ -70,6 +70,7 @@
             - [Iterator::any](fn/closures/closure_examples/iter_any.md)
             - [Iterator::find](fn/closures/closure_examples/iter_find.md)
     - [Higher Order Functions](fn/hof.md)
+    - [Diverging functions](fn/diverging.md)
 
 - [Modules](mod.md)
     - [Visibility](mod/visibility.md)

src/fn/diverging.md

@@ -0,0 +1,50 @@
+# Diverging functions
+
+Diverging functions never return. Their return type in Rust is denoted by `!`, which is an empty type. As opposed to all the other types, this one cannot be instantiated, because the set of all possible values this type can have is empty. Note, that it is different from the `()` type, which has exactly one possible value.
+
+
+For example, this functions returns as usual, although there is no information in the return value.
+```rust
+fn main() {
+    fn some_fn() {
+        ()
+    }
+
+    let a: () = some_fn();
+    println!("This functions returns and you can see this line.")
+}
+```
+
+As opposed to this function, which will never return the control back to the caller.
+```rust
+#![feature(never_type)]
+
+fn main() {
+    let x: ! = panic!("This call never returns.");
+    println!("You will never see this line!");
+}
+```
+
+Although this might seem like an abstract concept, it is in fact very useful and often handy. The main advantage of this type is that it can be cast to any other one and therefore used at places where an exact type is required, for instance in `match` branches. This allows us to write code like this:
+
+```rust
+fn main() {
+    fn sum_odd_numbers(up_to: u32) -> u32 {
+        let mut acc = 0;
+        for i in 0..up_to {
+            let addition: u32 = match i%2 == 1 {
+                true => i,
+                false => continue,
+            };
+            acc += addition;
+        }
+        acc
+    }
+    println!("Sum of odd numbers up to 9 (excluding): {}", sum_odd_numbers(9));
+}
+```
+
+It is also the return type of functions that loop forever (e.g. `loop {}`) like network servers or functions that terminates the process (e.g. `exit()`).
+
+
+

src/macro/syntax.md

@@ -0,0 +1 @@
+# Syntax