Tell the compiler to tell us that deriving is dead.

I'm beginning to suspect it's impossible to avoid accidentally writing
`#[deriving]` at least once in every program, and it results in
non-intuitive error messages: "Foo doesn't have any method in scope
`clone`" despite there being a `#[deriv...(Clone)]` attribute!

Also, lots of documentation around the internet uses `#[deriving]` so
providing this guidance is very helpful (lots of people ask in #rust
about this error).

Fixes #21166.

src/libsyntax/ext/base.rs

@@ -467,6 +467,8 @@ fn initial_syntax_expander_table(ecfg: &expand::ExpansionConfig) -> SyntaxEnv {
                                     ext::log_syntax::expand_syntax_ext));
     syntax_expanders.insert(intern("derive"),
                             Decorator(box ext::deriving::expand_meta_derive));
+    syntax_expanders.insert(intern("deriving"),
+                            Decorator(box ext::deriving::expand_deprecated_deriving));
 
     if ecfg.enable_quotes {
         // Quasi-quoting expanders