Rust 1.29.2

# Using Rust without the Standard Library

There is a new edition of the book and this is an old link.

> 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.

This particular chapter has moved to the Unstable Book.

- In the Unstable Rust Book: `lang_items` — Writing an executable without stdlib
- In the first edition: Ch 4.12 — Using Rust without the Standard Library