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/libsyntax/codemap.rs

@@ -425,7 +425,7 @@ impl CodeMap {
 
         // FIXME #8256: this used to be an assert but whatever precondition
         // it's testing isn't true for all spans in the AST, so to allow the
-        // caller to not have to fail (and it can't catch it since the CodeMap
+        // caller to not have to panic (and it can't catch it since the CodeMap
         // isn't sendable), return None
         if begin.fm.start_pos != end.fm.start_pos {
             None
@@ -441,7 +441,7 @@ impl CodeMap {
                 return fm.clone();
             }
         }
-        fail!("asking for {} which we don't know about", filename);
+        panic!("asking for {} which we don't know about", filename);
     }
 
     pub fn lookup_byte_offset(&self, bpos: BytePos) -> FileMapAndBytePos {
@@ -503,13 +503,13 @@ impl CodeMap {
                 break;
             }
             if a == 0 {
-                fail!("position {} does not resolve to a source location",
+                panic!("position {} does not resolve to a source location",
                       pos.to_uint());
             }
             a -= 1;
         }
         if a >= len {
-            fail!("position {} does not resolve to a source location",
+            panic!("position {} does not resolve to a source location",
                   pos.to_uint())
         }