Auto merge of #51363 - japaric:stable-used, r=cramertj

stabilize #[used]

closes #40289

RFC for stabilization: rust-lang/rfcs#2386

r? @Centril

Where should this be documented? Currently the documentation is in the unstable book

Author: bors

src/doc/unstable-book/src/language-features/used.md

@@ -307,7 +307,7 @@
 #![feature(doc_cfg)]
 #![feature(doc_masked)]
 #![feature(doc_spotlight)]
-#![cfg_attr(windows, feature(used))]
+#![cfg_attr(all(windows, stage0), feature(used))]
 #![feature(doc_alias)]
 #![feature(doc_keyword)]
 #![feature(panic_info_message)]

src/libstd/lib.rs

@@ -349,9 +349,6 @@ declare_features! (
     // Allows the `try {...}` expression
     (active, try_blocks, "1.29.0", Some(31436), None),
 
-    // Used to preserve symbols (see llvm.used)
-    (active, used, "1.18.0", Some(40289), None),
-
     // Allows module-level inline assembly by way of global_asm!()
     (active, global_asm, "1.18.0", Some(35119), None),
 
@@ -677,6 +674,9 @@ declare_features! (
     // Allows all literals in attribute lists and values of key-value pairs.
     (accepted, attr_literals, "1.30.0", Some(34981), None),
     (accepted, panic_handler, "1.30.0", Some(44489), None),
+    // Used to preserve symbols (see llvm.used)
+    (accepted, used, "1.30.0", Some(40289), None),
+
 );
 
 // If you change this, please modify src/doc/unstable-book as well. You must
@@ -1071,10 +1071,7 @@ pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeG
                                   "unwind_attributes",
                                   "#[unwind] is experimental",
                                   cfg_fn!(unwind_attributes))),
-    ("used", Whitelisted, Gated(
-        Stability::Unstable, "used",
-        "the `#[used]` attribute is an experimental feature",
-        cfg_fn!(used))),
+    ("used", Whitelisted, Ungated),
 
     // used in resolve
     ("prelude_import", Whitelisted, Gated(Stability::Unstable,

src/libsyntax/feature_gate.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 #![crate_type = "lib"]
-#![feature(used)]
 
 #[used]
 static FOO: u32 = 0;

src/test/run-make-fulldeps/used/used.rs

@@ -1,4 +1,4 @@
-// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// 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.
 //
@@ -15,6 +15,5 @@
 
 #[used]
 fn foo() {}
-//~^^ ERROR the `#[used]` attribute is an experimental feature
 
 fn main() {}

src/test/ui/feature-gates/feature-gate-linker-flavor.rs

@@ -1,11 +1,8 @@
-error[E0658]: the `#[used]` attribute is an experimental feature (see issue #40289)
+error: attribute must be applied to a `static` variable
   --> $DIR/feature-gate-linker-flavor.rs:16:1
    |
 LL | #[used]
    | ^^^^^^^
-   |
-   = help: add #![feature(used)] to the crate attributes to enable
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0658`.

src/test/ui/feature-gates/feature-gate-linker-flavor.stderr

@@ -10,7 +10,6 @@
 
 // run-pass
 #![deny(dead_code)]
-#![feature(used)]
 
 #[used]
 static FOO: u32 = 0;

src/test/ui/feature-gates/feature-gate-used.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(used)]
-
 #[used]
 static FOO: u32 = 0; // OK