Feature-gate #![no_std]

Fixes #21833.

[breaking-change]

Author: Keegan McAllister

src/doc/reference.md

@@ -2467,6 +2467,12 @@ The currently implemented features of the reference compiler are:
 
 * `associated_types` - Allows type aliases in traits. Experimental.
 
+* `no_std` - Allows the `#![no_std]` crate attribute, which disables the implicit
+             `extern crate std`. This typically requires use of the unstable APIs
+             behind the libstd "facade", such as libcore and libcollections. It
+             may also cause problems when using syntax extensions, including
+             `#[derive]`.
+
 If a feature is promoted to a language feature, then all existing programs will
 start to receive compilation warnings about #[feature] directives which enabled
 the new feature (because the directive is no longer necessary). However, if a

src/doc/trpl/unsafe.md

@@ -433,6 +433,7 @@ attribute attached to the crate.
 ```ignore
 // a minimal library
 #![crate_type="lib"]
+#![feature(no_std)]
 #![no_std]
 # // fn main() {} tricked you, rustdoc!
 ```
@@ -446,8 +447,8 @@ The function marked `#[start]` is passed the command line parameters
 in the same format as C:
 
 ```
+#![feature(lang_items, start, no_std)]
 #![no_std]
-#![feature(lang_items, start)]
 
 // Pull in the system libc library for what crt0.o likely requires
 extern crate libc;
@@ -473,6 +474,7 @@ correct ABI and the correct name, which requires overriding the
 compiler's name mangling too:
 
 ```ignore
+#![feature(no_std)]
 #![no_std]
 #![no_main]
 #![feature(lang_items, start)]
@@ -528,8 +530,8 @@ As an example, here is a program that will calculate the dot product of two
 vectors provided from C, using idiomatic Rust practices.
 
 ```
+#![feature(lang_items, start, no_std)]
 #![no_std]
-#![feature(lang_items, start)]
 
 # extern crate libc;
 extern crate core;
@@ -652,8 +654,8 @@ and one for deallocation. A freestanding program that uses the `Box`
 sugar for dynamic allocations via `malloc` and `free`:
 
 ```
+#![feature(lang_items, box_syntax, start, no_std)]
 #![no_std]
-#![feature(lang_items, box_syntax, start)]
 
 extern crate libc;
 

src/liballoc/lib.rs

@@ -65,6 +65,7 @@
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",
        html_root_url = "http://doc.rust-lang.org/nightly/")]
 
+#![feature(no_std)]
 #![no_std]
 #![feature(lang_items, unsafe_destructor)]
 #![feature(box_syntax)]

src/libcollections/lib.rs

@@ -33,6 +33,7 @@
 #![cfg_attr(test, feature(test))]
 #![cfg_attr(test, allow(deprecated))] // rand
 
+#![feature(no_std)]
 #![no_std]
 
 #[macro_use]

src/libcore/lib.rs

@@ -56,6 +56,7 @@
        html_root_url = "http://doc.rust-lang.org/nightly/",
        html_playground_url = "http://play.rust-lang.org/")]
 
+#![feature(no_std)]
 #![no_std]
 #![allow(raw_pointer_derive)]
 #![deny(missing_docs)]

src/liblibc/lib.rs

@@ -16,6 +16,7 @@
 #![cfg_attr(not(feature = "cargo-build"), staged_api)]
 #![cfg_attr(not(feature = "cargo-build"), feature(core))]
 #![feature(int_uint)]
+#![feature(no_std)]
 #![no_std]
 #![doc(html_logo_url = "http://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png",
        html_favicon_url = "http://www.rust-lang.org/favicon.ico",

src/librand/lib.rs

@@ -23,6 +23,7 @@
        html_root_url = "http://doc.rust-lang.org/nightly/",
        html_playground_url = "http://play.rust-lang.org/")]
 #![feature(int_uint)]
+#![feature(no_std)]
 #![no_std]
 #![unstable(feature = "rand")]
 #![feature(staged_api)]

src/librustc_bitflags/lib.rs

@@ -12,6 +12,7 @@
 #![feature(staged_api)]
 #![staged_api]
 #![crate_type = "rlib"]
+#![feature(no_std)]
 #![no_std]
 #![unstable(feature = "rustc_private")]
 

src/librustc_driver/test.rs

@@ -43,7 +43,7 @@ struct RH<'a> {
     sub: &'a [RH<'a>]
 }
 
-static EMPTY_SOURCE_STR: &'static str = "#![no_std]";
+static EMPTY_SOURCE_STR: &'static str = "#![feature(no_std)] #![no_std]";
 
 struct ExpectErrorEmitter {
     messages: Vec<String>

src/libstd/lib.rs

@@ -124,6 +124,7 @@
 #![cfg_attr(test, feature(test))]
 
 // Don't link to std. We are std.
+#![feature(no_std)]
 #![no_std]
 
 #![deny(missing_docs)]