Rename fail! to panic!

https://github.com/rust-lang/rfcs/pull/221

The current terminology of "task failure" often causes problems when
writing or speaking about code. You often want to talk about the
possibility of an operation that returns a Result "failing", but cannot
because of the ambiguity with task failure. Instead, you have to speak
of "the failing case" or "when the operation does not succeed" or other
circumlocutions.

Likewise, we use a "Failure" header in rustdoc to describe when
operations may fail the task, but it would often be helpful to separate
out a section describing the "Err-producing" case.

We have been steadily moving away from task failure and toward Result as
an error-handling mechanism, so we should optimize our terminology
accordingly: Result-producing functions should be easy to describe.

To update your code, rename any call to `fail!` to `panic!` instead.
Assuming you have not created your own macro named `panic!`, this
will work on UNIX based systems:

    grep -lZR 'fail!' . | xargs -0 -l sed -i -e 's/fail!/panic!/g'

You can of course also do this by hand.

[breaking-change]

src/libserialize/json.rs

@@ -1229,7 +1229,7 @@ impl Stack {
         let len = self.stack.len();
         let idx = match *self.stack.last().unwrap() {
             InternalIndex(i) => { i + 1 }
-            _ => { fail!(); }
+            _ => { panic!(); }
         };
         *self.stack.get_mut(len - 1) = InternalIndex(idx);
     }
@@ -1814,7 +1814,7 @@ impl<T: Iterator<char>> Builder<T> {
         match self.token {
             None => {}
             Some(Error(e)) => { return Err(e); }
-            ref tok => { fail!("unexpected token {}", tok.clone()); }
+            ref tok => { panic!("unexpected token {}", tok.clone()); }
         }
         result
     }
@@ -1874,7 +1874,7 @@ impl<T: Iterator<char>> Builder<T> {
             }
             let key = match self.parser.stack().top() {
                 Some(Key(k)) => { k.to_string() }
-                _ => { fail!("invalid state"); }
+                _ => { panic!("invalid state"); }
             };
             match self.build_value() {
                 Ok(value) => { values.insert(key, value); }
@@ -3015,9 +3015,9 @@ mod tests {
             Ok(json) => Decodable::decode(&mut Decoder::new(json))
         };
         match res {
-            Ok(_) => fail!("`{}` parsed & decoded ok, expecting error `{}`",
+            Ok(_) => panic!("`{}` parsed & decoded ok, expecting error `{}`",
                               to_parse, expected),
-            Err(ParseError(e)) => fail!("`{}` is not valid json: {}",
+            Err(ParseError(e)) => panic!("`{}` is not valid json: {}",
                                            to_parse, e),
             Err(e) => {
                 assert_eq!(e, expected);
@@ -3226,7 +3226,7 @@ mod tests {
         let bytes = mem_buf.unwrap();
         let json_str = from_utf8(bytes.as_slice()).unwrap();
         match from_str(json_str) {
-            Err(_) => fail!("Unable to parse json_str: {}", json_str),
+            Err(_) => panic!("Unable to parse json_str: {}", json_str),
             _ => {} // it parsed and we are good to go
         }
     }
@@ -3247,7 +3247,7 @@ mod tests {
         let bytes = mem_buf.unwrap();
         let json_str = from_utf8(bytes.as_slice()).unwrap();
         match from_str(json_str) {
-            Err(_) => fail!("Unable to parse json_str: {}", json_str),
+            Err(_) => panic!("Unable to parse json_str: {}", json_str),
             _ => {} // it parsed and we are good to go
         }
     }
@@ -3315,7 +3315,7 @@ mod tests {
         use Decodable;
         let json_str = "{\"1\":true}";
         let json_obj = match from_str(json_str) {
-            Err(_) => fail!("Unable to parse json_str: {}", json_str),
+            Err(_) => panic!("Unable to parse json_str: {}", json_str),
             Ok(o) => o
         };
         let mut decoder = Decoder::new(json_obj);
@@ -3328,7 +3328,7 @@ mod tests {
         use Decodable;
         let json_str = "{\"a\":true}";
         let json_obj = match from_str(json_str) {
-            Err(_) => fail!("Unable to parse json_str: {}", json_str),
+            Err(_) => panic!("Unable to parse json_str: {}", json_str),
             Ok(o) => o
         };
         let mut decoder = Decoder::new(json_obj);
@@ -3347,7 +3347,7 @@ mod tests {
             };
             let (ref expected_evt, ref expected_stack) = expected[i];
             if !parser.stack().is_equal_to(expected_stack.as_slice()) {
-                fail!("Parser stack is not equal to {}", expected_stack);
+                panic!("Parser stack is not equal to {}", expected_stack);
             }
             assert_eq!(&evt, expected_evt);
             i+=1;