auto merge of #11607 : alexcrichton/rust/issue-9957, r=cmr

For `use` statements, this means disallowing qualifiers when in functions and
disallowing `priv` outside of functions.

For `extern mod` statements, this means disallowing everything everywhere. It
may have been envisioned for `pub extern mod foo` to be a thing, but it
currently doesn't do anything (resolve doesn't pick it up), so better to err on
the side of forwards-compatibility and forbid it entirely for now.

Closes #9957

src/test/compile-fail/issue-9957.rs

@@ -0,0 +1,23 @@
+// Copyright 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.
+
+pub extern mod std; //~ ERROR: `pub` visibility is not allowed
+priv extern mod std; //~ ERROR: unnecessary visibility qualifier
+extern mod std;
+
+pub use std::bool;
+priv use std::bool; //~ ERROR: unnecessary visibility qualifier
+use std::bool;
+
+fn main() {
+    pub use std::bool; //~ ERROR: imports in functions are never reachable
+    priv use std::bool; //~ ERROR: unnecessary visibility qualifier
+    use std::bool;
+}