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

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

-# `used`
-
-The tracking issue for this feature
-is: [40289](https://github.com/rust-lang/rust/issues/40289).
-
-------------------------
-
-The `#[used]` attribute can be applied to `static` variables to prevent the Rust
-compiler from optimizing them away even if they appear to be unused by the crate
-(appear to be "dead code").
-
-``` rust
-#![feature(used)]
-
-#[used]
-static FOO: i32 = 1;
-
-static BAR: i32 = 2;
-
-fn main() {}
-```
-
-If you compile this program into an object file, you'll see that `FOO` makes it
-to the object file but `BAR` doesn't. Neither static variable is used by the
-program.
-
-``` text
-$ rustc -C opt-level=3 --emit=obj used.rs
-
-$ nm -C used.o
-0000000000000000 T main
-                 U std::rt::lang_start
-0000000000000000 r used::FOO
-0000000000000000 t used::main
-```
-
-Note that the *linker* knows nothing about the `#[used]` attribute and will
-remove `#[used]` symbols if they are not referenced by other parts of the
-program:
-
-``` text
-$ rustc -C opt-level=3 used.rs
-
-$ nm -C used | grep FOO
-```
-
-"This doesn't sound too useful then!" you may think but keep reading.
-
-To preserve the symbols all the way to the final binary, you'll need the
-cooperation of the linker. Here's one example:
-
-The ELF standard defines two special sections, `.init_array` and
-`.pre_init_array`, that may contain function pointers which will be executed
-*before* the `main` function is invoked. The linker will preserve symbols placed
-in these sections (at least when linking programs that target the `*-*-linux-*`
-targets).
-
-``` rust,ignore
-#![feature(used)]
-
-extern "C" fn before_main() {
-    println!("Hello, world!");
-}
-
-#[link_section = ".init_array"]
-#[used]
-static INIT_ARRAY: [extern "C" fn(); 1] = [before_main];
-
-fn main() {}
-```
-
-So, `#[used]` and `#[link_section]` can be combined to obtain "life before
-main".
-
-``` text
-$ rustc -C opt-level=3 before-main.rs
-
-$ ./before-main
-Hello, world!
-```
-
-Another example: ARM Cortex-M microcontrollers need their reset handler, a
-pointer to the function that will executed right after the microcontroller is
-turned on, to be placed near the start of their FLASH memory to boot properly.
-
-This condition can be met using `#[used]` and `#[link_section]` plus a linker
-script.
-
-``` rust,ignore
-#![feature(panic_handler)]
-#![feature(used)]
-#![no_main]
-#![no_std]
-
-use core::panic::PanicInfo;
-
-extern "C" fn reset_handler() -> ! {
-    loop {}
-}
-
-#[link_section = ".reset_handler"]
-#[used]
-static RESET_HANDLER: extern "C" fn() -> ! = reset_handler;
-
-#[panic_handler]
-fn panic(info: &PanicInfo) -> ! {
-    loop {}
-}
-```
-
-``` text
-MEMORY
-{
-  FLASH : ORIGIN = 0x08000000, LENGTH = 128K
-  RAM : ORIGIN = 0x20000000, LENGTH = 20K
-}
-
-SECTIONS
-{
-  .text ORIGIN(FLASH) :
-  {
-    /* Vector table */
-    LONG(ORIGIN(RAM) + LENGTH(RAM)); /* initial SP value */
-    KEEP(*(.reset_handler));
-
-    /* Omitted: The rest of the vector table */
-
-    *(.text.*);
-  } > FLASH
-
-  /DISCARD/ :
-  {
-    /* Unused unwinding stuff */
-    *(.ARM.exidx.*)
-  }
-}
-```
-
-``` text
-$ xargo rustc --target thumbv7m-none-eabi --release -- \
-    -C link-arg=-Tlink.x -C link-arg=-nostartfiles
-
-$ arm-none-eabi-objdump -Cd target/thumbv7m-none-eabi/release/app
-./target/thumbv7m-none-eabi/release/app:     file format elf32-littlearm
-
-
-Disassembly of section .text:
-
-08000000 <app::RESET_HANDLER-0x4>:
- 8000000:       20005000        .word   0x20005000
-
-08000004 <app::RESET_HANDLER>:
- 8000004:       08000009                                ....
-
-08000008 <app::reset_handler>:
- 8000008:       e7fe            b.n     8000008 <app::reset_handler>
-```

src/libstd/lib.rs

@@ -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/libsyntax/feature_gate.rs

@@ -349,9 +349,6 @@ pub fn walk_feature_fields<F>(&self, mut f: F)
     // 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 @@ pub fn walk_feature_fields<F>(&self, mut f: F)
     // 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 fn is_builtin_attr(attr: &ast::Attribute) -> bool {
                                   "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/test/run-make-fulldeps/used/used.rs

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

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

-// 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.stderr

-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-used.rs

-// 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.
-
-#[used]
-fn foo() {}
-//~^^ ERROR the `#[used]` attribute is an experimental feature
-
-fn main() {}

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

-error[E0658]: the `#[used]` attribute is an experimental feature (see issue #40289)
-  --> $DIR/feature-gate-used.rs:11:1
-   |
-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/run-pass/issues/issue-41628.rs

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

src/test/ui/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
 

src/test/ui/used.stderr

 error: attribute must be applied to a `static` variable
-  --> $DIR/used.rs:16:1
+  --> $DIR/used.rs:14:1
    |
 LL | #[used] //~ ERROR attribute must be applied to a `static` variable
    | ^^^^^^^
 
 error: attribute must be applied to a `static` variable
-  --> $DIR/used.rs:19:1
+  --> $DIR/used.rs:17:1
    |
 LL | #[used] //~ ERROR attribute must be applied to a `static` variable
    | ^^^^^^^
 
 error: attribute must be applied to a `static` variable
-  --> $DIR/used.rs:22:1
+  --> $DIR/used.rs:20:1
    |
 LL | #[used] //~ ERROR attribute must be applied to a `static` variable
    | ^^^^^^^
 
 error: attribute must be applied to a `static` variable
-  --> $DIR/used.rs:25:1
+  --> $DIR/used.rs:23:1
    |
 LL | #[used] //~ ERROR attribute must be applied to a `static` variable
    | ^^^^^^^