Remove core::panicking::panic's dependence on str's Display::fmt impl

Display::fmt for str calls into Formatter::pad, which is modest in size
and also pulls in string-related functions for its truncation and padding
abilities.  For size-critical programs (e.g. embedded), this call site
may be the only reason Formatter::pad is linked into the output.

src/libcore/panicking.rs

@@ -35,8 +35,14 @@ use fmt;
 #[cold] #[inline(never)] // this is the slow path, always
 #[lang="panic"]
 pub fn panic(expr_file_line: &(&'static str, &'static str, u32)) -> ! {
+    // Use Arguments::new_v1 instead of format_args!("{}", expr) to potentially
+    // reduce size overhead. The format_args! macro uses str's Display trait to
+    // write expr, which calls Formatter::pad, which must accommodate string
+    // truncation and padding (even though none is used here). Using
+    // Arguments::new_v1 may allow the compiler to omit Formatter::pad from the
+    // output binary, saving up to a few kilobytes.
     let (expr, file, line) = *expr_file_line;
-    panic_fmt(format_args!("{}", expr), &(file, line))
+    panic_fmt(fmt::Arguments::new_v1(&[expr], &[]), &(file, line))
 }
 
 #[cold] #[inline(never)]