Rust essentials

Canonical: [docs/fcis-rust.md](../../docs/fcis-rust.md)

Goal

Every .rs change follows FCIS and the ponytail ladder.

Inputs / outputs

• In: current crate + the behaviour to change
• Out: Rust that compiles, no extra crates, illegal states unrepresentable

Reads / writes

• Read: Cargo.toml, files the task already opened, docs/fcis-rust.md in this pack
• Write: none unless a playbook opened the task

Approval

Adding a dependency, unsafe, or toolchain install.

RULES — no exceptions

1. Apply [docs/fcis-rust.md](../../docs/fcis-rust.md) (ladder + six FCIS rules). Do not restate them here.
2. Names: types UpperCamelCase, fns snakecase, consts SCREAMINGSNAKE; acronyms as words (HttpServer)
3. Never skip: trust-boundary parse, Result, // SAFETY:, one failing test for non-trivial logic

Example

// ❌
fn bind(port: u16) -> std::net::SocketAddr {
    format!("127.0.0.1:{port}").parse().unwrap()
}

// ✅
use std::net::{Ipv4Addr, SocketAddr, SocketAddrV4};
use std::num::NonZeroU16;

fn bind(port: NonZeroU16) -> SocketAddr {
    SocketAddr::V4(SocketAddrV4::new(Ipv4Addr::LOCALHOST, port.get()))
}

Steps

1. Load this skill + docs/fcis-rust.md
2. If the crate exists, run load-context first
3. Apply RULES to the diff; do not add types/traits “for later”

Validation

RED: cargo test <test_name> -- --exact. Done: commands in docs/skill-authoring.md (fmt, clippy --all-targets -D warnings, cargo test). No new deps. No unwrap on recoverable paths.

Pitfalls

Integration