remove #[used] from the unstable book

it'll be documented in the reference

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

@@ -1,157 +0,0 @@
-# `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>
-```