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

   1 % Using Rust Without the Standard Library
   2
   3 Rust’s standard library provides a lot of useful functionality, but assumes
   4 support for various features of its host system: threads, networking, heap
   5 allocation, and others. There are systems that do not have these features,
   6 however, and Rust can work with those too! To do so, we tell Rust that we
   7 don’t want to use the standard library via an attribute: `#![no_std]`.
   8
   9 > Note: This feature is technically stable, but there are some caveats. For
  10 > one, you can build a `#![no_std]` _library_ on stable, but not a _binary_.
  11 > For details on binaries without the standard library, see [the nightly
  12 > chapter on `#![no_std]`](no-stdlib.html)
  13
  14 To use `#![no_std]`, add it to your crate root:
  15
  16 ```rust
  17 #![no_std]
  18
  19 fn plus_one(x: i32) -> i32 {
  20     x + 1
  21 }
  22 ```
  23
  24 Much of the functionality that’s exposed in the standard library is also
  25 available via the [`core` crate](../core/index.html). When we’re using the
  26 standard library, Rust automatically brings `std` into scope, allowing you to
  27 use its features without an explicit import. By the same token, when using
  28 `#![no_std]`, Rust will bring `core` into scope for you, as well as [its
  29 prelude](../core/prelude/v1/index.html). This means that a lot of code will Just
  30 Work:
  31
  32 ```rust
  33 #![no_std]
  34
  35 fn may_fail(failure: bool) -> Result<(), &'static str> {
  36     if failure {
  37         Err("this didn’t work!")
  38     } else {
  39         Ok(())
  40     }
  41 }
  42 ```