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 😉 

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

src/libregex/parse.rs

@@ -117,7 +117,7 @@ impl BuildAst {
     fn flags(&self) -> Flags {
         match *self {
             Paren(flags, _, _) => flags,
-            _ => fail!("Cannot get flags from {}", self),
+            _ => panic!("Cannot get flags from {}", self),
         }
     }
 
@@ -125,7 +125,7 @@ impl BuildAst {
         match *self {
             Paren(_, 0, _) => None,
             Paren(_, c, _) => Some(c),
-            _ => fail!("Cannot get capture group from {}", self),
+            _ => panic!("Cannot get capture group from {}", self),
         }
     }
 
@@ -139,7 +139,7 @@ impl BuildAst {
                     Some(name.clone())
                 }
             }
-            _ => fail!("Cannot get capture name from {}", self),
+            _ => panic!("Cannot get capture name from {}", self),
         }
     }
 
@@ -153,7 +153,7 @@ impl BuildAst {
     fn unwrap(self) -> Result<Ast, Error> {
         match self {
             Expr(x) => Ok(x),
-            _ => fail!("Tried to unwrap non-AST item: {}", self),
+            _ => panic!("Tried to unwrap non-AST item: {}", self),
         }
     }
 }
@@ -321,7 +321,7 @@ impl<'a> Parser<'a> {
         }
         let rep: Repeater = match c {
             '?' => ZeroOne, '*' => ZeroMore, '+' => OneMore,
-            _ => fail!("Not a valid repeater operator."),
+            _ => panic!("Not a valid repeater operator."),
         };
 
         match self.peek(1) {
@@ -389,7 +389,7 @@ impl<'a> Parser<'a> {
                             continue
                         }
                         Some(ast) =>
-                            fail!("Expected Class AST but got '{}'", ast),
+                            panic!("Expected Class AST but got '{}'", ast),
                         // Just drop down and try to add as a regular character.
                         None => {},
                     },
@@ -404,7 +404,7 @@ impl<'a> Parser<'a> {
                             return self.err(
                                 "\\A, \\z, \\b and \\B are not valid escape \
                                  sequences inside a character class."),
-                        ast => fail!("Unexpected AST item '{}'", ast),
+                        ast => panic!("Unexpected AST item '{}'", ast),
                     }
                 }
                 ']' if ranges.len() > 0 || alts.len() > 0 => {