Forbid '#[macro_use] extern crate' outside the crate root

Author: Keegan McAllister

src/librustc/plugin/load.rs

@@ -20,6 +20,7 @@ use std::dynamic_lib::DynamicLibrary;
 use std::collections::HashSet;
 use syntax::ast;
 use syntax::attr;
+use syntax::codemap::Span;
 use syntax::parse::token;
 use syntax::ptr::P;
 use syntax::visit;
@@ -45,6 +46,7 @@ pub struct Plugins {
 
 struct PluginLoader<'a> {
     sess: &'a Session,
+    span_whitelist: HashSet<Span>,
     reader: CrateReader<'a>,
     plugins: Plugins,
 }
@@ -54,6 +56,7 @@ impl<'a> PluginLoader<'a> {
         PluginLoader {
             sess: sess,
             reader: CrateReader::new(sess),
+            span_whitelist: HashSet::new(),
             plugins: Plugins {
                 macros: vec!(),
                 registrars: vec!(),
@@ -66,6 +69,14 @@ impl<'a> PluginLoader<'a> {
 pub fn load_plugins(sess: &Session, krate: &ast::Crate,
                     addl_plugins: Option<Plugins>) -> Plugins {
     let mut loader = PluginLoader::new(sess);
+
+    // We need to error on `#[macro_use] extern crate` when it isn't at the
+    // crate root, because `$crate` won't work properly. Identify these by
+    // spans, because the crate map isn't set up yet.
+    for vi in krate.module.view_items.iter() {
+        loader.span_whitelist.insert(vi.span);
+    }
+
     visit::walk_crate(&mut loader, krate);
 
     let mut plugins = loader.plugins;
@@ -158,6 +169,11 @@ impl<'a, 'v> Visitor<'v> for PluginLoader<'a> {
         };
         let load_registrar = plugin_attr.is_some();
 
+        if load_macros && !self.span_whitelist.contains(&vi.span) {
+            self.sess.span_err(vi.span, "an `extern crate` loading macros must be at \
+                                         the crate root");
+        }
+
         if load_macros || load_registrar {
             let pmd = self.reader.read_plugin_metadata(vi);
             if load_macros {

src/test/compile-fail/lint-stability.rs

@@ -19,13 +19,14 @@
 #![deny(experimental)]
 #![allow(dead_code)]
 
+#[macro_use]
+extern crate lint_stability; //~ ERROR: use of unmarked item
+
 mod cross_crate {
     extern crate stability_cfg1;
     extern crate stability_cfg2; //~ ERROR: use of experimental item
 
-    #[macro_use]
-    extern crate lint_stability; //~ ERROR: use of unmarked item
-    use self::lint_stability::*;
+    use lint_stability::*;
 
     fn test() {
         let foo = MethodTester;

src/test/compile-fail/macro-crate-nonterminal-non-root.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 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:macro_crate_nonterminal.rs
+// ignore-stage1
+
+mod foo {
+    #[macro_use]
+    extern crate macro_crate_nonterminal;  //~ ERROR must be at the crate root
+}
+
+fn main() {
+}