The Embedded Rust Book

In a bare metal environment no code has been loaded before your program. Without the software provided by an OS we can not load the standard library. Instead the program, along with the crates it uses, can only use the hardware (bare metal) to run. To prevent rust from loading the standard library use no_std. The platform-agnostic parts of the standard library are available through libcore. libcore also excludes things which are not always desirable in an embedded environment. One of these things is a memory allocator for dynamic memory allocation. If you require this or any other functionalities there are often crates which provide these.

The libstd Runtime

As mentioned before using libstd requires some sort of system integration, but this is not only because libstd is just providing a common way of accessing OS abstractions, it also provides a runtime. This runtime, among other things, takes care of setting up stack overflow protection, processing command line arguments, and spawning the main thread before a program's main function is invoked. This runtime also won't be available in a no_std environment.

#![no_std] is a crate-level attribute that indicates that the crate will link to the core-crate instead of the std-crate. The libcore crate in turn is a platform-agnostic subset of the std crate which makes no assumptions about the system the program will run on. As such, it provides APIs for language primitives like floats, strings and slices, as well as APIs that expose processor features like atomic operations and SIMD instructions. However it lacks APIs for anything that involves platform integration. Because of these properties no_std and libcore code can be used for any kind of bootstrapping (stage 0) code like bootloaders, firmware or kernels.

Overview

* Only if you use the alloc crate and use a suitable allocator like alloc-cortex-m.

** Only if you use the collections crate and configure a global default allocator.