Auto merge of #30856 - mneumann:thread_local_extern, r=alexcrichton

This will correctly add the thread_local attribute to the external static variable ```errno```:

```rust
extern {
     #[thread_local]
     static errno: c_int;
}
```

Before this commit, the thread_local attribute is ignored. Fixes #30795.

Thanks @alexcrichton for pointing out the solution.

src/librustc_trans/trans/foreign.rs

@@ -39,6 +39,7 @@ use syntax::attr;
 use syntax::codemap::Span;
 use syntax::parse::token::{InternedString, special_idents};
 use syntax::ast;
+use syntax::attr::AttrMetaMethods;
 
 use rustc_front::print::pprust;
 use rustc_front::hir;
@@ -120,8 +121,8 @@ pub fn register_static(ccx: &CrateContext,
     let llty = type_of::type_of(ccx, ty);
 
     let ident = link_name(foreign_item);
-    match attr::first_attr_value_str_by_name(&foreign_item.attrs,
-                                             "linkage") {
+    let c = match attr::first_attr_value_str_by_name(&foreign_item.attrs,
+                                                     "linkage") {
         // If this is a static with a linkage specified, then we need to handle
         // it a little specially. The typesystem prevents things like &T and
         // extern "C" fn() from being non-null, so we can't just declare a
@@ -166,7 +167,16 @@ pub fn register_static(ccx: &CrateContext,
         }
         None => // Generate an external declaration.
             declare::declare_global(ccx, &ident[..], llty),
+    };
+
+    // Handle thread-local external statics.
+    for attr in foreign_item.attrs.iter() {
+        if attr.check_name("thread_local") {
+            llvm::set_thread_local(c, true);
+        }
     }
+
+    return c;
 }
 
 // only use this for foreign function ABIs and glue, use `get_extern_rust_fn` for Rust functions

src/test/auxiliary/thread-local-extern-static.rs

@@ -0,0 +1,17 @@
+// Copyright 2016 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.
+
+#![feature(thread_local)]
+#![feature(cfg_target_thread_local)]
+#![crate_type = "lib"]
+
+#[no_mangle]
+#[cfg_attr(target_thread_local, thread_local)]
+pub static FOO: u32 = 3;

src/test/run-pass/thread-local-extern-static.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+// ignore-windows
+// aux-build:thread-local-extern-static.rs
+
+#![feature(thread_local)]
+#![feature(cfg_target_thread_local)]
+
+extern crate thread_local_extern_static;
+
+extern {
+    #[cfg_attr(target_thread_local, thread_local)]
+    static FOO: u32;
+}
+
+fn main() {
+    assert_eq!(FOO, 3);
+}