Add magic constant to encode unstability of ensure_not_fmt_string_literal!

Author: pnkfelix

src/libcollections/fmt.rs

@@ -446,3 +446,6 @@ pub fn format(args: Arguments) -> string::String {
     let _ = write!(&mut output, "{}", args);
     output
 }
+
+#[unstable(feature = "ensure_not_fmt_string_literal")]
+pub use ::core::fmt::ENSURE_NOT_FMT_STRING_LITERAL_IS_UNSTABLE;

src/libcollections/lib.rs

@@ -41,6 +41,7 @@
 #![feature(slice_patterns)]
 #![feature(debug_builders)]
 #![feature(utf8_error)]
+#![feature(ensure_not_fmt_string_literal)] // (referenced in expansions)
 #![cfg_attr(test, feature(rand, rustc_private, test, hash, collections))]
 #![cfg_attr(test, allow(deprecated))] // rand
 

src/libcore/fmt/mod.rs

@@ -1084,5 +1084,16 @@ impl<'b, T: Debug> Debug for RefMut<'b, T> {
     }
 }
 
+/// This constant is a trick to force `ensure_not_fmt_string_literal!`
+/// to be treated as unstable whenever it occurs outside a macro
+/// marked with `#[allow_internal_unstable]`.
+///
+/// This constant really should not ever be stabilized; if we ever
+/// decide to stabilize the `ensure_not_fmt_string_literal!` macro
+/// itself, then we should remove its use of this constant (and then
+/// remove this constant).
+#[unstable(feature = "ensure_not_fmt_string_literal")]
+pub const ENSURE_NOT_FMT_STRING_LITERAL_IS_UNSTABLE: () = ();
+
 // If you expected tests to be here, look instead at the run-pass/ifmt.rs test,
 // it's a lot easier than creating all of the rt::Piece structures here.