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/libregex/parse.rs

@@ -117,7 +117,7 @@ impl BuildAst {
     fn flags(&self) -> Flags {
         match *self {
             Paren(flags, _, _) => flags,
-            _ => fail!("Cannot get flags from {}", self),
+            _ => panic!("Cannot get flags from {}", self),
         }
     }
 
@@ -125,7 +125,7 @@ impl BuildAst {
         match *self {
             Paren(_, 0, _) => None,
             Paren(_, c, _) => Some(c),
-            _ => fail!("Cannot get capture group from {}", self),
+            _ => panic!("Cannot get capture group from {}", self),
         }
     }
 
@@ -139,7 +139,7 @@ impl BuildAst {
                     Some(name.clone())
                 }
             }
-            _ => fail!("Cannot get capture name from {}", self),
+            _ => panic!("Cannot get capture name from {}", self),
         }
     }
 
@@ -153,7 +153,7 @@ impl BuildAst {
     fn unwrap(self) -> Result<Ast, Error> {
         match self {
             Expr(x) => Ok(x),
-            _ => fail!("Tried to unwrap non-AST item: {}", self),
+            _ => panic!("Tried to unwrap non-AST item: {}", self),
         }
     }
 }
@@ -321,7 +321,7 @@ impl<'a> Parser<'a> {
         }
         let rep: Repeater = match c {
             '?' => ZeroOne, '*' => ZeroMore, '+' => OneMore,
-            _ => fail!("Not a valid repeater operator."),
+            _ => panic!("Not a valid repeater operator."),
         };
 
         match self.peek(1) {
@@ -393,7 +393,7 @@ impl<'a> Parser<'a> {
                             continue
                         }
                         Some(ast) =>
-                            fail!("Expected Class AST but got '{}'", ast),
+                            panic!("Expected Class AST but got '{}'", ast),
                         // Just drop down and try to add as a regular character.
                         None => {},
                     },
@@ -408,7 +408,7 @@ impl<'a> Parser<'a> {
                             return self.err(
                                 "\\A, \\z, \\b and \\B are not valid escape \
                                  sequences inside a character class."),
-                        ast => fail!("Unexpected AST item '{}'", ast),
+                        ast => panic!("Unexpected AST item '{}'", ast),
                     }
                 }
                 _ => {},