Deprecate custom_derive

Has a custom deprecation since deprecating features is not supported and is a pain to implement

Author: nrc

src/libsyntax/feature_gate.rs

@@ -897,7 +897,12 @@ pub const EXPLAIN_ALLOW_INTERNAL_UNSTABLE: &'static str =
     "allow_internal_unstable side-steps feature gating and stability checks";
 
 pub const EXPLAIN_CUSTOM_DERIVE: &'static str =
-    "`#[derive]` for custom traits is not stable enough for use and is subject to change";
+    "`#[derive]` for custom traits is not stable enough for use. It is deprecated and will \
+     be removed in v1.15";
+
+pub const EXPLAIN_DEPR_CUSTOM_DERIVE: &'static str =
+    "`#[derive]` for custom traits is deprecated and will be removed in v1.15. Prefer using \
+     procedural macro custom derive";
 
 pub const EXPLAIN_DERIVE_UNDERSCORE: &'static str =
     "attributes of the form `#[derive_*]` are reserved for the compiler";

src/libsyntax_ext/deriving/mod.rs

@@ -175,6 +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);
             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));