Add tracking issue for the unwind attribute.

Author: Mark-Simulacrum

src/libsyntax/feature_gate.rs

@@ -233,8 +233,8 @@ declare_features! (
 
     // Allows `#[unwind(..)]`.
     //
-    // rustc internal for rust runtime
-    (active, unwind_attributes, "1.4.0", None, None),
+    // Permits specifying whether a function should permit unwinding or abort on unwind.
+    (active, unwind_attributes, "1.4.0", Some(58760), None),
 
     // Allows the use of `#[naked]` on functions.
     (active, naked_functions, "1.9.0", Some(32408), None),

src/test/ui/feature-gates/feature-gate-unwind-attributes.stderr

@@ -1,4 +1,4 @@
-error[E0658]: #[unwind] is experimental
+error[E0658]: #[unwind] is experimental (see issue #58760)
   --> $DIR/feature-gate-unwind-attributes.rs:11:5
    |
 LL |     #[unwind(allowed)] //~ ERROR #[unwind] is experimental