Show a better error when using --test with #[proc_macro_derive]

Author: keeperofdakeys

src/librustc_driver/driver.rs

@@ -709,10 +709,12 @@ pub fn phase_2_configure_and_expand<'a, F>(sess: &Session,
             let crate_types = sess.crate_types.borrow();
             let num_crate_types = crate_types.len();
             let is_proc_macro_crate = crate_types.contains(&config::CrateTypeProcMacro);
+            let is_test_crate = sess.opts.test;
             syntax_ext::proc_macro_registrar::modify(&sess.parse_sess,
                                                      &mut resolver,
                                                      krate,
                                                      is_proc_macro_crate,
+                                                     is_test_crate,
                                                      num_crate_types,
                                                      sess.diagnostic(),
                                                      &sess.features.borrow())

src/libsyntax_ext/proc_macro_registrar.rs

@@ -38,12 +38,14 @@ struct CollectCustomDerives<'a> {
     in_root: bool,
     handler: &'a errors::Handler,
     is_proc_macro_crate: bool,
+    is_test_crate: bool,
 }
 
 pub fn modify(sess: &ParseSess,
               resolver: &mut ::syntax::ext::base::Resolver,
               mut krate: ast::Crate,
               is_proc_macro_crate: bool,
+              is_test_crate: bool,
               num_crate_types: usize,
               handler: &errors::Handler,
               features: &Features) -> ast::Crate {
@@ -55,6 +57,7 @@ pub fn modify(sess: &ParseSess,
         in_root: true,
         handler: handler,
         is_proc_macro_crate: is_proc_macro_crate,
+        is_test_crate: is_test_crate,
     };
     visit::walk_crate(&mut collect, &krate);
 
@@ -137,6 +140,12 @@ impl<'a> Visitor for CollectCustomDerives<'a> {
                                              attributes found");
         }
 
+        if self.is_test_crate {
+            self.handler.span_err(attr.span(),
+                                  "`--test` cannot be used with proc-macro crates");
+            return;
+        }
+
         if !self.is_proc_macro_crate {
             self.handler.span_err(attr.span(),
                                   "the `#[proc_macro_derive]` attribute is \