auto merge of #10539 : alexcrichton/rust/external-linkage, r=pcwalton

If a function is marked as external, then it's likely desired for use with some
native library, so we're not really accomplishing a whole lot by internalizing
all of these symbols.

src/test/run-pass/extern-fn-reachable.rs

@@ -0,0 +1,39 @@
+// Copyright 2013 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.
+
+// xfail-fast
+// xfail-linux apparently dlsym doesn't work on program symbols?
+// xfail-android apparently dlsym doesn't work on program symbols?
+// xfail-freebsd apparently dlsym doesn't work on program symbols?
+
+use std::unstable::dynamic_lib::DynamicLibrary;
+
+#[no_mangle] pub extern "C" fn fun1() {}
+#[no_mangle] extern "C" fn fun2() {}
+
+mod foo {
+    #[no_mangle] pub extern "C" fn fun3() {}
+}
+pub mod bar {
+    #[no_mangle] pub extern "C" fn fun4() {}
+}
+
+#[no_mangle] pub fn fun5() {}
+
+fn main() {
+    unsafe {
+        let a = DynamicLibrary::open(None).unwrap();
+        assert!(a.symbol::<int>("fun1").is_ok());
+        assert!(a.symbol::<int>("fun2").is_err());
+        assert!(a.symbol::<int>("fun3").is_err());
+        assert!(a.symbol::<int>("fun4").is_ok());
+        assert!(a.symbol::<int>("fun5").is_err());
+    }
+}