Move some #[no_std] info to stable book.

This feature is partially stabilized, so describe each part in the appropriate place.

Author: steveklabnik

src/doc/book/SUMMARY.md

@@ -51,6 +51,7 @@
     * [FFI](ffi.md)
     * [Borrow and AsRef](borrow-and-asref.md)
     * [Release Channels](release-channels.md)
+    * [Using Rust without the standard library](using-rust-without-the-standard-library.md)
 * [Nightly Rust](nightly-rust.md)
     * [Compiler Plugins](compiler-plugins.md)
     * [Inline Assembly](inline-assembly.md)

src/doc/book/no-stdlib.md

@@ -1,8 +1,15 @@
 % No stdlib
 
-By default, `std` is linked to every Rust crate. In some contexts,
-this is undesirable, and can be avoided with the `#![no_std]`
-attribute attached to the crate.
+Rust’s standard library provides a lot of useful functionality, but assumes
+support for various features of its host system: threads, networking, heap
+allocation, and others. There are systems that do not have these features,
+however, and Rust can work with those too! To do so, we tell Rust that we
+don’t want to use the standard library via an attribute: `#![no_std]`.
+
+> Note: This feature is technically stable, but there are some caveats. For
+> one, you can build a `#![no_std]` _library_ on stable, but not a _binary_.
+> For details on libraries without the standard library, see [the chapter on
+> `#![no_std]`](using-rust-without-the-standard-library.html)
 
 Obviously there's more to life than just libraries: one can use
 `#[no_std]` with an executable, controlling the entry point is
@@ -77,89 +84,3 @@ personality function (see the
 information), but crates which do not trigger a panic can be assured
 that this function is never called. The second function, `panic_fmt`, is
 also used by the failure mechanisms of the compiler.
-
-## Using libcore
-
-> **Note**: the core library's structure is unstable, and it is recommended to
-> use the standard library instead wherever possible.
-
-With the above techniques, we've got a bare-metal executable running some Rust
-code. There is a good deal of functionality provided by the standard library,
-however, that is necessary to be productive in Rust. If the standard library is
-not sufficient, then [libcore](../core/index.html) is designed to be used
-instead.
-
-The core library has very few dependencies and is much more portable than the
-standard library itself. Additionally, the core library has most of the
-necessary functionality for writing idiomatic and effective Rust code. When
-using `#![no_std]`, Rust will automatically inject the `core` crate, like
-we do for `std` when we’re using it.
-
-As an example, here is a program that will calculate the dot product of two
-vectors provided from C, using idiomatic Rust practices.
-
-```rust
-# #![feature(libc)]
-#![feature(lang_items)]
-#![feature(start)]
-#![feature(raw)]
-#![no_std]
-
-extern crate libc;
-
-use core::mem;
-
-#[no_mangle]
-pub extern fn dot_product(a: *const u32, a_len: u32,
-                          b: *const u32, b_len: u32) -> u32 {
-    use core::raw::Slice;
-
-    // Convert the provided arrays into Rust slices.
-    // The core::raw module guarantees that the Slice
-    // structure has the same memory layout as a &[T]
-    // slice.
-    //
-    // This is an unsafe operation because the compiler
-    // cannot tell the pointers are valid.
-    let (a_slice, b_slice): (&[u32], &[u32]) = unsafe {
-        mem::transmute((
-            Slice { data: a, len: a_len as usize },
-            Slice { data: b, len: b_len as usize },
-        ))
-    };
-
-    // Iterate over the slices, collecting the result
-    let mut ret = 0;
-    for (i, j) in a_slice.iter().zip(b_slice.iter()) {
-        ret += (*i) * (*j);
-    }
-    return ret;
-}
-
-#[lang = "panic_fmt"]
-extern fn panic_fmt(args: &core::fmt::Arguments,
-                    file: &str,
-                    line: u32) -> ! {
-    loop {}
-}
-
-#[lang = "eh_personality"] extern fn eh_personality() {}
-# #[start] fn start(argc: isize, argv: *const *const u8) -> isize { 0 }
-# #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
-# #[no_mangle] pub extern fn rust_eh_register_frames () {}
-# #[no_mangle] pub extern fn rust_eh_unregister_frames () {}
-# fn main() {}
-```
-
-Note that there is one lang item here whose signature differs from the examples
-above, `panic_fmt`. This must be defined by consumers of libcore because the
-core library declares panics, but it does not define it. The `panic_fmt`
-lang item is this crate's definition of panic, and it must be guaranteed to
-never return.
-
-As can be seen in this example, the core library is intended to provide the
-power of Rust in all circumstances, regardless of platform requirements. Further
-libraries, such as liballoc, add functionality to libcore which make other
-platform-specific assumptions, but continue to be more portable than the
-standard library itself.
-

src/doc/book/using-rust-without-the-standard-library.md

@@ -0,0 +1,41 @@
+% Using Rust Without the Standard Library
+
+Rust’s standard library provides a lot of useful functionality, but assumes
+support for various features of its host system: threads, networking, heap
+allocation, and others. There are systems that do not have these features,
+however, and Rust can work with those too! To do so, we tell Rust that we
+don’t want to use the standard library via an attribute: `#![no_std]`.
+
+> Note: This feature is technically stable, but there are some caveats. For
+> one, you can build a `#![no_std]` _library_ on stable, but not a _binary_.
+> For details on binaries without the standard library, see [the nightly
+> chapter on `#![no_std]`](no-stdlib.html)
+
+To use `#![no_std]`, add a it to your crate root:
+
+```rust
+#![no_std]
+
+fn plus_one(x: i32) -> i32 {
+    x + 1
+}
+```
+
+Much of the functionality that’s exposed in the standard library is also
+available via the [`core` crate](../core/). When we’re using the standard
+library, Rust automatically brings `std` into scope, allowing you to use
+its features without an explicit import. By the same token, when using
+`!#[no_std]`, Rust will bring `core` into scope for you, as well as [its
+prelude](../core/prelude/v1/). This means that a lot of code will Just Work:
+
+```rust
+#![no_std]
+
+fn may_fail(failure: bool) -> Result<(), &'static str> {
+    if failure {
+        Err("this didn’t work!")
+    } else {
+        Ok(())
+    }
+}
+```