rustc: Handle #[linkage] anywhere in a crate

This commit updates the reachability pass of the compiler to seed the local
worklist with `#[linkage]`-like items anywhere in a crate, not just those
reachable from public items.

Closes #45165

Author: alexcrichton

src/librustc/middle/reachable.rs

@@ -336,6 +336,12 @@ struct CollectPrivateImplItemsVisitor<'a, 'tcx: 'a> {
 
 impl<'a, 'tcx: 'a> ItemLikeVisitor<'tcx> for CollectPrivateImplItemsVisitor<'a, 'tcx> {
     fn visit_item(&mut self, item: &hir::Item) {
+        // Anything which has custom linkage gets thrown on the worklist no
+        // matter where it is in the crate.
+        if attr::contains_name(&item.attrs, "linkage") {
+            self.worklist.push(item.id);
+        }
+
         // We need only trait impls here, not inherent impls, and only non-exported ones
         if let hir::ItemImpl(.., Some(ref trait_ref), _, ref impl_item_refs) = item.node {
             if !self.access_levels.is_reachable(item.id) {

src/test/run-pass/smallest-hello-world.rs

@@ -0,0 +1,19 @@
+// 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.
+
+// compile-flags: -Z thinlto -C codegen-units=2
+// min-llvm-version 4.0
+
+#![feature(allocator_api, global_allocator)]
+
+#[global_allocator]
+static A: std::heap::System = std::heap::System;
+
+fn main() {}