rust-no-std

Rust no_std

Purpose

Guide agents through #![no_std] Rust development: what core and alloc provide vs std, implementing custom global allocators, panic handler selection for embedded targets, and strategies for testing no_std crates on the host machine.

When to Use

Use this skill when writing or debugging #![no_std] Rust code — library crates for embedded targets, or bare-metal firmware that cannot link against std. For the full embedded development workflow (probe-rs flashing, defmt logging, RTIC), use skills/embedded/embedded-rust. For cross-compilation target setup, use skills/rust/rust-cross. This skill focuses specifically on the no_std / core / alloc boundary and panic handler selection.

Examples

• "I need a parser crate that works without std" → structure with #![no_std], feature-gate alloc APIs, use borrowed slices for core API
• "How do I use Vec in a no_std environment?" → add alloc feature, provide a global allocator (e.g., linked-list-allocator), use alloc::vec::Vec
• "How do I test my no_std crate on my laptop?" → use #![cfg_attr(not(test), no_std)] to allow std in test mode, or cargo test --target x86_64-unknown-linux-gnu

Workflow

1. no_std crate structure

// src/lib.rs
#![no_std]

// core is always available (no OS needed)
use core::fmt;
use core::mem;
use core::slice;

// alloc: heap collections — requires a global allocator
#[cfg(feature = "alloc")]
extern crate alloc;
#[cfg(feature = "alloc")]
use alloc::{vec::Vec, string::String, boxed::Box, format};

pub fn add(a: u32, b: u32) -> u32 {
    a + b
}

# Cargo.toml
[features]
default = []
alloc = []       # opt-in to heap allocation

[dependencies]
# no_std-compatible dependencies only

2. core vs alloc vs std

Crate	Requires OS	Requires heap	Provides
core	No	No	Primitives, traits, iter, fmt, mem, ptr, slice, option, result
alloc	No	Yes (allocator)	Vec, String, Box, Arc, Rc, HashMap (requires global allocator)
std	Yes	Yes	All of core + alloc + OS APIs (threads, files, sockets, env)