Add optional messages to the unreachable macro.

Closes #18842.

src/libstd/macros.rs

@@ -211,7 +211,15 @@ macro_rules! debug_assert_eq(
 /// ```
 #[macro_export]
 macro_rules! unreachable(
-    () => (panic!("internal error: entered unreachable code"))
+    () => ({
+        panic!("internal error: entered unreachable code")
+    });
+    ($msg:expr) => ({
+        unreachable!("{}", $msg)
+    });
+    ($fmt:expr, $($arg:tt)*) => ({
+        panic!(concat!("internal error: entered unreachable code: ", $fmt), $($arg)*)
+    });
 )
 
 /// A standardised placeholder for marking unfinished code. It panics with the

src/test/run-fail/unreachable-fmt-msg.rs

@@ -0,0 +1,14 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// error-pattern:internal error: entered unreachable code: 6 is not prime
+fn main() {
+    unreachable!("{} is not {}", 6u32, "prime");
+}

src/test/run-fail/unreachable-static-msg.rs

@@ -0,0 +1,12 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// error-pattern:internal error: entered unreachable code: uhoh
+fn main() { unreachable!("uhoh") }