documenation for abort and unwind

Author: Charisee

src/SUMMARY.md

@@ -155,6 +155,7 @@
 
 - [Error handling](error.md)
     - [`panic`](error/panic.md)
+    - [`abort` & `unwind`](error/abort_unwind.md)
     - [`Option` & `unwrap`](error/option_unwrap.md)
         - [Unpacking options with `?`](error/option_unwrap/question_mark.md)
         - [Combinators: `map`](error/option_unwrap/map.md)

src/error/abort_unwind.md

@@ -0,0 +1,53 @@
+# `abort` and `unwind`
+
+The previous section illustrates the error handling mechanism `panic`.  The `cfg_panic` feature makes it possible to execute different code depending on the panic strategy. The current values available are `unwind` and `abort`. 
+
+
+Building on the prior lemonade example, we explicitly use the panic strategy to execise different lines of code.  
+
+```rust,editable,ignore,mdbook-runnable
+#![feature(cfg_panic)]
+
+fn drink(beverage: &str) {
+   // You shouldn't drink too much sugary beverages.
+   if beverage == "lemonade" {
+     if cfg!(panic="abort"){ println!("This is not your party. Run!!!!");}
+     else{ println!("Spit it out!!!!");}
+   }
+   else{ println!("Some refreshing {} is all I need.", beverage); }
+}
+
+fn main() {
+    drink("water");
+    drink("lemonade");
+}
+```
+
+Here is another example focusing on rewriting `drink()` and explicitly use the `unwind` keyword.
+
+```rust,editable,ignore
+#![feature(cfg_panic)]
+
+#[cfg(panic = "unwind")]
+fn ah(){ println!("Spit it out!!!!");}
+
+#[cfg(not(panic="unwind"))]
+fn ah(){ println!("This is not your party. Run!!!!");}
+
+fn drink(beverage: &str){
+   if beverage == "lemonade"{ ah();}
+   else{println!("Some refreshing {} is all I need.", beverage);}
+}
+
+fn main() {
+    drink("water");
+    drink("lemonade");
+}
+```
+
+The panic strategy can be set from the command line by using `abort` or `unwind`.
+
+```rust,editable,ignore
+rustc  lemonade.rc -C panic=abort
+```
+