rustc: Forbid plugin_registrar in only rlib form

If a plugin registrar is available, the library must be found in dylib form, not
just in rlib form.

Closes #15475

src/librustc/metadata/creader.rs

@@ -446,7 +446,7 @@ impl<'a> PluginMetadataReader<'a> {
             should_match_name: true,
         };
         let library = match load_ctxt.maybe_load_library_crate() {
-            Some (l) => l,
+            Some(l) => l,
             None if is_cross => {
                 // try loading from target crates (only valid if there are
                 // no syntax extensions)
@@ -473,6 +473,14 @@ impl<'a> PluginMetadataReader<'a> {
         let registrar = decoder::get_plugin_registrar_fn(library.metadata.as_slice()).map(|id| {
             decoder::get_symbol(library.metadata.as_slice(), id)
         });
+        if library.dylib.is_none() && registrar.is_some() {
+            let message = format!("plugin crate `{}` only found in rlib format, \
+                                   but must be available in dylib format",
+                                  info.ident);
+            self.env.sess.span_err(krate.span, message.as_slice());
+            // No need to abort because the loading code will just ignore this
+            // empty dylib.
+        }
         let pc = PluginMetadata {
             lib: library.dylib.clone(),
             macros: macros,

src/test/auxiliary/rlib_crate_test.rs

@@ -0,0 +1,21 @@
+// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// no-prefer-dynamic
+
+#![crate_type = "rlib"]
+#![feature(plugin_registrar)]
+
+extern crate rustc;
+
+use rustc::plugin::Registry;
+
+#[plugin_registrar]
+pub fn plugin_registrar(_: &mut Registry) {}

src/test/compile-fail-fulldeps/macro-crate-rlib.rs

@@ -0,0 +1,20 @@
+// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:rlib_crate_test.rs
+// ignore-stage1
+// ignore-tidy-linelength
+// ignore-android
+
+#![feature(phase)]
+#[phase(plugin)] extern crate rlib_crate_test;
+//~^ ERROR: plugin crate `rlib_crate_test` only found in rlib format, but must be available in dylib format
+
+fn main() {}