deprecation message for custom derive

Author: nrc

src/libsyntax_ext/deriving/mod.rs

@@ -175,7 +175,7 @@ pub fn expand_derive(cx: &mut ExtCtxt,
                                            feature_gate::GateIssue::Language,
                                            feature_gate::EXPLAIN_CUSTOM_DERIVE);
         } else {
-            cx.span_warn(titem.span, EXPLAIN_DEPR_CUSTOM_DERIVE);
+            cx.span_warn(titem.span, feature_gate::EXPLAIN_DEPR_CUSTOM_DERIVE);
             let name = intern_and_get_ident(&format!("derive_{}", tname));
             let mitem = cx.meta_word(titem.span, name);
             new_attributes.push(cx.attribute(mitem.span, mitem));

src/test/compile-fail/deriving-meta-unknown-trait.rs

@@ -8,8 +8,10 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// ignore-tidy-linelength
+
 #[derive(Eqr)]
-//~^ ERROR `#[derive]` for custom traits is not stable enough for use and is subject to change
+//~^ ERROR `#[derive]` for custom traits is not stable enough for use. It is deprecated and will be removed in v1.15 (see issue #29644)
 struct Foo;
 
 pub fn main() {}