Avoid false positive `unused_extern_crates`.

src/librustc_resolve/build_reduced_graph.rs

@@ -513,6 +513,7 @@ impl<'b> Resolver<'b> {
                                   legacy_imports: LegacyMacroImports,
                                   allow_shadowing: bool) {
         let import_macro = |this: &mut Self, name, ext: Rc<_>, span| {
+            this.used_crates.insert(module.def_id().unwrap().krate);
             if let SyntaxExtension::NormalTT(..) = *ext {
                 this.macro_names.insert(name);
             }

src/test/compile-fail/lint-unused-extern-crate.rs

@@ -26,6 +26,8 @@ extern crate rand; // no error, the use marks it as used
 
 extern crate lint_unused_extern_crate as other; // no error, the use * marks it as used
 
+#[macro_use] extern crate core; // no error, the `#[macro_use]` marks it as used
+
 #[allow(unused_imports)]
 use rand::isaac::IsaacRng;