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, 3 insertions, 3 deletions

diff --git a/src/liballoc/arc.rs b/src/liballoc/arc.rs
index cc6f2d76eaf..f543826fe01 100644
--- a/src/liballoc/arc.rs
+++ b/src/liballoc/arc.rs
@@ -227,9 +227,9 @@ impl<T: Sync + Send> Drop for Arc<T> {
 impl<T: Sync + Send> Weak<T> {
     /// Attempts to upgrade this weak reference to a strong reference.
     ///
-    /// This method will fail to upgrade this reference if the strong reference
-    /// count has already reached 0, but if there are still other active strong
-    /// references this function will return a new strong reference to the data.
+    /// This method will not upgrade this reference if the strong reference count has already
+    /// reached 0, but if there are still other active strong references this function will return
+    /// a new strong reference to the data.
     pub fn upgrade(&self) -> Option<Arc<T>> {
         // We use a CAS loop to increment the strong count instead of a
         // fetch_add because once the count hits 0 is must never be above 0.