Adding catch_panic anti-pattern

Author: liamzdenek

README.md

@@ -56,7 +56,7 @@ language.
 
 ### Anti-patterns
 
-* TODO thread + catch_panic for exceptions
+* [thread + catch_panic for exceptions](anti_patterns/catch_panic.md)
 * TODO Clone to satisfy the borrow checker
 * [Deref polymorphism](anti_patterns/deref.md)
 * TODO Matching all fields of a struct (back compat)

anti_patterns/catch_panic.md

@@ -0,0 +1,54 @@
+# panic::catch_unwind for exceptions
+
+## Description
+
+This antipattern arises when the method for catching (`panic::catch_unwind`) an
+unexpected problem (implementation bug) is used to handle an expected problem,
+such as a missing file.
+
+## Example
+
+```rust
+use std::io::prelude::*;
+use std::fs::File;
+use std::panic;
+
+fn main() {
+    // panic::catch_unwind catches any panic that occurs within the
+    // function passed to it
+    let result = panic::catch_unwind(|| {
+        let mut file_result = File::open("foo.txt");
+        file_result.unwrap(); // causes a panic if the file is not found
+    });
+
+    // the program continues running despite the panic
+    println!("potential error: {:?}", result);
+    assert!(result.is_err());
+}
+```
+
+
+## Motivation
+
+In rust, there are two ways an operation can fail: An expected problem, like a
+file not being found, or a HTTP request timing out. Or, an unexpected problem,
+such as an index being out of bounds, or an assert!() failing. These are
+unexpected because they are bugs that should not happen. It would not make sense
+to handle an error for QuickSort returning an incorrectly unsorted array, as
+this should not happen.
+
+There are scenarios where using panic::catch_unwind is the correct choice, eg, a
+web server implementation wants to save an unwinding thread in order to send a
+valid response if the route for that request (as in: logic outside of the web server
+implementor's control) is producing a panic.
+
+Expected errors should not result in stack unwinding. Instead, expected errors
+should be handled through the Result and Option types. [The Rust Book's chapter
+on Error Handling](https://doc.rust-lang.org/book/error-handling.html) elaborates further on this.
+
+## See also
+
+[The Rust Book: Error Handling](https://doc.rust-lang.org/book/error-handling.html)
+[Rust 1.9 announcement, which contains a description of this antipattern](http://blog.rust-lang.org/2016/05/26/Rust-1.9.html)
+[Result documentation](http://doc.rust-lang.org/std/result/enum.Result.html)
+[panic::catch_unwind documentation](https://doc.rust-lang.org/std/panic/fn.catch_unwind.html)