no-std

name	no-std
description	no_std Guide guidance for Fortress Rollback. Use when Writing no_std code, embedded targets, core vs alloc decisions.

no_std Guide

The std / core / alloc Split

Layer	Contains	Requires
core	Primitives, Option, Result, Iterator, PhantomData	Nothing
alloc	Vec, String, Box, Rc, Arc, BTreeMap, format!	Global allocator
std	fs, net, thread, time, HashMap, Mutex, io	Operating system

Note: HashMap/HashSet are NOT in alloc (need randomness). Use BTreeMap or hashbrown.

Basic Structure

#![cfg_attr(not(feature = "std"), no_std)]

#[cfg(all(not(feature = "std"), feature = "alloc"))]
extern crate alloc;

Prelude Pattern

// src/prelude.rs
pub use core::cmp::{max, min, Ordering};
pub use core::fmt;
pub use core::mem;

#[cfg(feature = "alloc")]
pub use alloc::{boxed::Box, string::{String, ToString}, vec, vec::Vec};

#[cfg(feature = "std")]
pub use std::collections::{HashMap, HashSet};

#[cfg(all(feature = "alloc", not(feature = "std")))]
pub use hashbrown::{HashMap, HashSet};

Feature Flag Setup

[features]
default = ["std"]
std = ["alloc", "dep:parking_lot"]
alloc = []

[dependencies]
# Always: default-features = false
serde = { version = "1", default-features = false, features = ["derive", "alloc"] }
hashbrown = { version = "0.14", optional = true, default-features = false, features = ["alloc"] }

Feature Hygiene

#[cfg(all(feature = "tokio", not(feature = "std")))]
compile_error!("tokio feature requires std");

Common std Replacements

std	no_std + alloc	no_std (no heap)
Vec<T>	alloc::vec::Vec<T>	heapless::Vec<T, N>
String	alloc::string::String	heapless::String<N>
HashMap	hashbrown::HashMap	heapless::FnvIndexMap
Box<T>	alloc::boxed::Box<T>	Stack or arena
Mutex	spin::Mutex	critical-section

Time

Accept time from outside -- caller is responsible for timing. This enforces determinism.

Random Numbers

Use seeded PRNG (rand_xoshiro::Xoshiro256PlusPlus). Use BTreeMap or hashbrown for deterministic iteration.

Synchronization

// no_std: spin-locks or critical-section
use spin::Mutex;
use alloc::sync::Arc; // Arc is in alloc!

Error Handling Without std

std::error::Error is NOT in core (before Rust 1.81).

impl core::fmt::Display for Error { /* ... */ }

#[cfg(feature = "std")]
impl std::error::Error for Error {}

// Rust 1.81+: core::error::Error exists

Conditional Compilation

#[cfg(feature = "std")]
mod std_networking;

#[cfg(target_arch = "wasm32")]
fn get_timestamp() -> u64 { /* js_sys */ }

#[cfg(all(feature = "alloc", not(feature = "std")))]
use hashbrown::HashMap;

Testing

# Verify no_std compilation
cargo check --no-default-features --features alloc
cargo check --no-default-features  # bare no_std
cargo check --target wasm32-unknown-unknown --no-default-features --features alloc

Tests always have std available (#[cfg(test)] runs with std).

Popular no_std Crates

Serialization

serde (default-features=false), postcard, rkyv

Data Structures

heapless, arrayvec, smallvec, hashbrown, indexmap

Utilities

bitflags, bytemuck, byteorder, num-traits, static_assertions

Hashing

ahash, rustc-hash, fnv, sha2, blake3

Random

rand_core, rand_xoshiro, rand_chacha

Embedded

embedded-hal, critical-section, portable-atomic, defmt

Checklist

• #![cfg_attr(not(feature = "std"), no_std)] in lib.rs
• extern crate alloc; when using alloc feature
• Prelude module for consistent imports
• core::/alloc:: instead of std::
• hashbrown instead of std::collections::HashMap
• std::error::Error impl conditional on feature
• default-features = false for ALL dependencies
• CI job verifying no_std compilation
• Test on wasm32-unknown-unknown
• Avoid std::time, std::thread, std::net in core logic
• Document which features require std vs alloc