Rollup merge of #50330 - japaric:used, r=nagisa

check that #[used] is used only on statics

this attribute has no effect on other items. This makes the implementation match what's described in the RFC.

cc #40289

r? @nagisa

src/librustc/hir/check_attr.rs

@@ -31,6 +31,7 @@ enum Target {
     Expression,
     Statement,
     Closure,
+    Static,
     Other,
 }
 
@@ -43,6 +44,7 @@ impl Target {
             hir::ItemEnum(..) => Target::Enum,
             hir::ItemConst(..) => Target::Const,
             hir::ItemForeignMod(..) => Target::ForeignMod,
+            hir::ItemStatic(..) => Target::Static,
             _ => Target::Other,
         }
     }
@@ -102,6 +104,7 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
         }
 
         self.check_repr(item, target);
+        self.check_used(item, target);
     }
 
     /// Check if an `#[inline]` is applied to a function or a closure.
@@ -305,6 +308,15 @@ impl<'a, 'tcx> CheckAttrVisitor<'a, 'tcx> {
             }
         }
     }
+
+    fn check_used(&self, item: &hir::Item, target: Target) {
+        for attr in &item.attrs {
+            if attr.name().map(|name| name == "used").unwrap_or(false) && target != Target::Static {
+                self.tcx.sess
+                    .span_err(attr.span, "attribute must be applied to a `static` variable");
+            }
+        }
+    }
 }
 
 impl<'a, 'tcx> Visitor<'tcx> for CheckAttrVisitor<'a, 'tcx> {

src/test/compile-fail/used.rs

@@ -0,0 +1,28 @@
+// Copyright 2018 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(used)]
+
+#[used]
+static FOO: u32 = 0; // OK
+
+#[used] //~ ERROR attribute must be applied to a `static` variable
+fn foo() {}
+
+#[used] //~ ERROR attribute must be applied to a `static` variable
+struct Foo {}
+
+#[used] //~ ERROR attribute must be applied to a `static` variable
+trait Bar {}
+
+#[used] //~ ERROR attribute must be applied to a `static` variable
+impl Bar for Foo {}
+
+fn main() {}