# Rust Essentials

> MANDATORY before any .rs write. FCIS, ponytail ladder, parse-at-boundary, Result + ?, iterators, match, naming. Trigger: rust, FCIS, idiomatic rust, ownership intro, Result, newtype, clippy.

- Skill: `igmarin/rust-essentials` (Agent Skill)
- Install (CLI): `npx skillmds@latest add igmarin/rust-essentials`
- Raw SKILL.md: https://api.skillmd.com/api/skills/igmarin/rust-essentials/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: MIT
- Author: igmarin (https://skillmd.com/u/igmarin)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/igmarin/rust-essentials

---


# 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 `snake_case`, consts `SCREAMING_SNAKE`; acronyms as words (`HttpServer`)
3. **Never skip:** trust-boundary parse, `Result`, `// SAFETY:`, one failing test for non-trivial logic

## Example

```rust
// ❌
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

| ❌ | ✅ |
|----|----|
| New crate for what `std` does | `std` / existing dep |
| `bool` pairs for states | `enum` |
| Clone to please borrowck | restructure or borrow |
| Essay comments | the type and the test |

## Integration

| Predecessor | This | Successor |
|-------------|------|-----------|
| `load-context` | rust-essentials | `ownership-borrowing`, `type-driven-design`, `error-handling` |
| playbook `tdd` | rust-essentials | implementation after HITL |