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, 2 insertions, 2 deletions

diff --git a/src/doc/guide-testing.md b/src/doc/guide-testing.md
index 07813855d9b..9d15f55f33f 100644
--- a/src/doc/guide-testing.md
+++ b/src/doc/guide-testing.md
@@ -49,7 +49,7 @@ value. To run the tests in a crate, it must be compiled with the
 `--test` flag: `rustc myprogram.rs --test -o myprogram-tests`. Running
 the resulting executable will run all the tests in the crate. A test
 is considered successful if its function returns; if the task running
-the test fails, through a call to `fail!`, a failed `assert`, or some
+the test fails, through a call to `panic!`, a failed `assert`, or some
 other (`assert_eq`, ...) means, then the test fails.
 
 When compiling a crate with the `--test` flag `--cfg test` is also
@@ -77,7 +77,7 @@ test on windows you can write `#[cfg_attr(windows, ignore)]`.
 
 Tests that are intended to fail can be annotated with the
 `should_fail` attribute. The test will be run, and if it causes its
-task to fail then the test will be counted as successful; otherwise it
+task to panic then the test will be counted as successful; otherwise it
 will be counted as a failure. For example:
 
 ~~~test_harness