Give a tracking-issue number for unsized tuple coercion.

Author: qnighy

src/doc/unstable-book/src/language-features/unsized-tuple-coercion.md

@@ -1,8 +1,8 @@
 # `unsized_tuple_coercion`
 
-The tracking issue for this feature is: [#XXXXX]
+The tracking issue for this feature is: [#42877]
 
-[#XXXXX]: https://github.com/rust-lang/rust/issues/XXXXX
+[#42877]: https://github.com/rust-lang/rust/issues/42877
 
 ------------------------
 

src/libsyntax/feature_gate.rs

@@ -359,7 +359,7 @@ declare_features! (
     (active, allow_fail, "1.19.0", Some(42219)),
 
     // Allows unsized tuple coercion.
-    (active, unsized_tuple_coercion, "1.20.0", None),
+    (active, unsized_tuple_coercion, "1.20.0", Some(42877)),
 );
 
 declare_features! (