std: add begin_unwind_fmt that reduces codesize for formatted fail!().

This ends up saving a single `call` instruction in the optimised code,
but saves a few hundred lines of non-optimised IR for `fn main() {
fail!("foo {}", "bar"); }` (comparing against the minimal generic
baseline from the parent commit).

src/libstd/macros.rs

@@ -48,8 +48,18 @@ macro_rules! fail(
         ::std::rt::begin_unwind($msg, file!(), line!())
     );
     ($fmt:expr, $($arg:tt)*) => (
-        ::std::rt::begin_unwind(format!($fmt, $($arg)*), file!(), line!())
-    )
+        {
+            // a closure can't have return type !, so we need a full
+            // function to pass to format_args!, *and* we need the
+            // file and line numbers right here; so an inner bare fn
+            // is our only choice.
+            #[inline]
+            fn run_fmt(fmt: &::std::fmt::Arguments) -> ! {
+                ::std::rt::begin_unwind_fmt(fmt, file!(), line!())
+            }
+            format_args!(run_fmt, $fmt, $($arg)*)
+        }
+        )
 )
 
 #[macro_export]

src/libstd/rt/mod.rs

@@ -69,7 +69,7 @@ use self::task::{Task, BlockedTask};
 pub use self::util::default_sched_threads;
 
 // Export unwinding facilities used by the failure macros
-pub use self::unwind::{begin_unwind, begin_unwind_raw};
+pub use self::unwind::{begin_unwind, begin_unwind_raw, begin_unwind_fmt};
 
 // FIXME: these probably shouldn't be public...
 #[doc(hidden)]

src/libstd/rt/unwind.rs

@@ -58,6 +58,7 @@
 use any::{Any, AnyRefExt};
 use c_str::CString;
 use cast;
+use fmt;
 use kinds::Send;
 use option::{Some, None, Option};
 use prelude::drop;
@@ -382,6 +383,17 @@ pub fn begin_unwind_raw(msg: *u8, file: *u8, line: uint) -> ! {
     begin_unwind(msg, file, line as uint)
 }
 
+/// The entry point for unwinding with a formatted message.
+///
+/// This is designed to reduce the amount of code required at the call
+/// site as much as possible (so that `fail!()` has as low an implact
+/// on (e.g.) the inlining of other functions as possible), by moving
+/// the actual formatting into this shared place.
+#[inline(never)] #[cold]
+pub fn begin_unwind_fmt(msg: &fmt::Arguments, file: &'static str, line: uint) -> ! {
+    begin_unwind_inner(~fmt::format(msg), file, line)
+}
+
 /// This is the entry point of unwinding for fail!() and assert!().
 #[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
 pub fn begin_unwind<M: Any + Send>(msg: M, file: &'static str, line: uint) -> ! {