Auto merge of #42155 - seanmonstar:unimplemented, r=sfackler

core: allow messages in unimplemented!() macro

This makes `unimplemented!()` match `unreachable!()`, allowing a message and possible formatting to be provided to better explain what and/or why something is not implemented.

I've used this myself in hyper for a while, include the type and method name, to better help while prototyping new modules, like `unimplemented!("Conn::poll_complete")`, or `unimplemented!("Conn::poll; state={:?}", state)`.

src/libcore/macros.rs

@@ -550,7 +550,8 @@ macro_rules! unreachable {
 #[macro_export]
 #[stable(feature = "rust1", since = "1.0.0")]
 macro_rules! unimplemented {
-    () => (panic!("not yet implemented"))
+    () => (panic!("not yet implemented"));
+    ($($arg:tt)+) => (panic!("not yet implemented: {}", format_args!($($arg)*)));
 }
 
 /// Built-in macros to the compiler itself.