attempt to register the helper macro so that it allows unstable things internally.

Unfortunately, i am having issues getting this to actually work.

I repeatedly see:

```
failures:
    [run-fail] run-fail/explicit-panic-msg.rs
```

Namely:

```
failures:

---- [run-fail] run-fail/explicit-panic-msg.rs stdout ----

error: compilation failed!
status: exit code: 101
command: x86_64-apple-darwin/stage2/bin/rustc /Users/fklock/Dev/Mozilla/rust-panic/src/test/run-fail/explicit-panic-msg.rs -L x86_64-apple-darwin/test/run-fail/ --target=x86_64-apple-darwin -L x86_64-apple-darwin/test/run-fail/explicit-panic-msg.stage2-x86_64-apple-darwinlibaux -C prefer-dynamic -o x86_64-apple-darwin/test/run-fail/explicit-panic-msg.stage2-x86_64-apple-darwin --cfg rtopt --cfg debug -L x86_64-apple-darwin/rt
stdout:
------------------------------------------

------------------------------------------
stderr:
------------------------------------------
<std macros>:2:26: 2:57 error: use of unstable library feature 'core': internal to format_args!
<std macros>:2 $ crate:: fmt:: format ( format_args ! ( $ ( $ arg ) * ) ) )
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of __unstable_rustc_ensure_not_fmt_string_literal!
<std macros>:4:1: 4:78 note: expansion site
<std macros>:1:1: 12:23 note: in expansion of panic!
/Users/fklock/Dev/Mozilla/rust-panic/src/test/run-fail/explicit-panic-msg.rs:18:5: 18:37 note: expansion site
<std macros>:2:26: 2:57 help: add #![feature(core)] to the crate attributes to enable
<std macros>:2:26: 2:57 error: use of unstable library feature 'core': internal to format_args!
<std macros>:2 $ crate:: fmt:: format ( format_args ! ( $ ( $ arg ) * ) ) )
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: in expansion of __unstable_rustc_ensure_not_fmt_string_literal!
<std macros>:4:1: 4:78 note: expansion site
<std macros>:1:1: 12:23 note: in expansion of panic!
/Users/fklock/Dev/Mozilla/rust-panic/src/test/run-fail/explicit-panic-msg.rs:18:5: 18:37 note: expansion site
<std macros>:2:26: 2:57 help: add #![feature(core)] to the crate attributes to enable
error: aborting due to 2 previous errors

------------------------------------------
```

and I have not yet figured out why this happens even now, nor
how to resolve it.

Author: pnkfelix

src/libsyntax/ext/base.rs

@@ -458,9 +458,14 @@ fn initial_syntax_expander_table<'feat>(ecfg: &expand::ExpansionConfig<'feat>)
     syntax_expanders.insert(intern("format_args"),
                             // format_args uses `unstable` things internally.
                             NormalTT(Box::new(ext::format::expand_format_args), None, true));
-    syntax_expanders.insert(intern("__unstable_rustc_ensure_not_fmt_string_literal"),
-                            builtin_normal_expander(
-                                    ext::format::ensure_not_fmt_string_literal));
+
+    // ensure_not_fmt_string_literal is passed `unstable` things to
+    // use in its expansion. (Of course, that should not actually
+    // force us to pass `true` to allow the use of unstable things;
+    // i.e. this is working around a bug elsewhere.)
+    let tt = NormalTT(Box::new(ext::format::ensure_not_fmt_string_literal), None, true);
+    syntax_expanders.insert(intern("__unstable_rustc_ensure_not_fmt_string_literal"), tt);
+
     syntax_expanders.insert(intern("env"),
                             builtin_normal_expander(
                                     ext::env::expand_env));