auto merge of #17894 : steveklabnik/rust/fail_to_panic, r=aturon

This in-progress PR implements https://github.com/rust-lang/rust/issues/17489.

I made the code changes in this commit, next is to go through alllllllll the documentation and fix various things.

- Rename column headings as appropriate, `# Panics` for panic conditions and `# Errors` for `Result`s.
- clean up usage of words like 'fail' in error messages

Anything else to add to the list, @aturon ? I think I should leave the actual functions with names like `slice_or_fail` alone, since you'll get to those in your conventions work?

I'm submitting just the code bits now so that we can see it separately, and I also don't want to have to keep re-building rust over and over again if I don't have to :wink: 

Listing all the bits so I can remember as I go:

- [x] compiler-rt
- [x] compiletest
- [x] doc
- [x] driver
- [x] etc
- [x] grammar
- [x] jemalloc
- [x] liballoc
- [x] libarena
- [x] libbacktrace
- [x] libcollections
- [x] libcore
- [x] libcoretest
- [x] libdebug
- [x] libflate
- [x] libfmt_macros
- [x] libfourcc
- [x] libgetopts
- [x] libglob
- [x] libgraphviz
- [x] libgreen
- [x] libhexfloat
- [x] liblibc
- [x] liblog
- [x] libnative
- [x] libnum
- [x] librand
- [x] librbml
- [x] libregex
- [x] libregex_macros
- [x] librlibc
- [x] librustc
- [x] librustc_back
- [x] librustc_llvm
- [x] librustdoc
- [x] librustrt
- [x] libsemver
- [x] libserialize
- [x] libstd
- [x] libsync
- [x] libsyntax
- [x] libterm
- [x] libtest
- [x] libtime
- [x] libunicode
- [x] liburl
- [x] libuuid
- [x] llvm
- [x] rt
- [x] test

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 967ad3a897c..47ca66b0b49 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -171,7 +171,7 @@ pub trait Folder {
     }
 
     fn fold_mac(&mut self, _macro: Mac) -> Mac {
-        fail!("fold_mac disabled by default");
+        panic!("fold_mac disabled by default");
         // NB: see note about macros above.
         // if you really want a folder that
         // works on macros, use this
@@ -1404,7 +1404,7 @@ mod test {
                 let a_val = $a;
                 let b_val = $b;
                 if !(pred_val(a_val.as_slice(),b_val.as_slice())) {
-                    fail!("expected args satisfying {}, got {} and {}",
+                    panic!("expected args satisfying {}, got {} and {}",
                           $predname, a_val, b_val);
                 }
             }