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]

1 files changed, 4 insertions, 2 deletions

diff --git a/src/libstd/rand/reader.rs b/src/libstd/rand/reader.rs
index 8ca1cec3e0a..4f220531237 100644
--- a/src/libstd/rand/reader.rs
+++ b/src/libstd/rand/reader.rs
@@ -18,7 +18,9 @@ use result::{Ok, Err};
 /// An RNG that reads random bytes straight from a `Reader`. This will
 /// work best with an infinite reader, but this is not required.
 ///
-/// It will fail if it there is insufficient data to fulfill a request.
+/// # Panics
+///
+/// It will panic if it there is insufficient data to fulfill a request.
 ///
 /// # Example
 ///
@@ -65,7 +67,7 @@ impl<R: Reader> Rng for ReaderRng<R> {
         if v.len() == 0 { return }
         match self.reader.read_at_least(v.len(), v) {
             Ok(_) => {}
-            Err(e) => fail!("ReaderRng.fill_bytes error: {}", e)
+            Err(e) => panic!("ReaderRng.fill_bytes error: {}", e)
         }
     }
 }