Don't panic if an attribute macro fails to resolve at crate root

Adds temporary regression test; this ideally should work as-is (rust-lang#41430)

Closes rust-lang#41211

src/libsyntax/ext/expand.rs

@@ -205,6 +205,8 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
         module.directory.pop();
         self.cx.current_expansion.module = Rc::new(module);
 
+        let orig_mod_span = krate.module.inner;
+
         let krate_item = Expansion::Items(SmallVector::one(P(ast::Item {
             attrs: krate.attrs,
             span: krate.span,
@@ -214,11 +216,19 @@ impl<'a, 'b> MacroExpander<'a, 'b> {
             vis: ast::Visibility::Public,
         })));
 
-        match self.expand(krate_item).make_items().pop().unwrap().unwrap() {
-            ast::Item { attrs, node: ast::ItemKind::Mod(module), .. } => {
+        match self.expand(krate_item).make_items().pop().map(P::unwrap) {
+            Some(ast::Item { attrs, node: ast::ItemKind::Mod(module), .. }) => {
                 krate.attrs = attrs;
                 krate.module = module;
             },
+            None => {
+                // Resolution failed so we return an empty expansion
+                krate.attrs = vec![];
+                krate.module = ast::Mod {
+                    inner: orig_mod_span,
+                    items: vec![],
+                };
+            },
             _ => unreachable!(),
         };
 

src/test/compile-fail-fulldeps/proc-macro/auxiliary/issue-41211.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 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.
+
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+#![feature(proc_macro)]
+
+extern crate proc_macro;
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn emit_unchanged(_args: TokenStream, input: TokenStream) -> TokenStream {
+    input
+}

src/test/compile-fail-fulldeps/proc-macro/issue-41211.rs

@@ -0,0 +1,22 @@
+// Copyright 2017 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:issue-41211.rs
+
+// FIXME: https://github.com/rust-lang/rust/issues/41430
+// This is a temporary regression test for the ICE reported in #41211
+
+#![feature(proc_macro)]
+#![emit_unchanged]
+//~^ ERROR: cannot find attribute macro `emit_unchanged` in this scope
+extern crate issue_41211;
+use issue_41211::emit_unchanged;
+
+fn main() {}