Rust Best Practices

What it does

When your agent uses it

• Reviewing Rust code
• Tasks that involve API design

Example prompts

• “/rust-best-practices”

Workflow steps

5 steps, taken from the first numbered list in SKILL.md.

1. 1Leverage the type system — Use types to make invalid states unrepresentable
2. 2Embrace ownership — Work with the borrow checker, not against it
3. 3Explicit over implicit — Be clear about fallibility, mutability, and lifetimes
4. 4Zero-cost abstractions — Use iterators, generics, and traits without runtime cost
5. 5Fail fast, recover gracefully — Validate early, handle errors explicitly

What it can do on your machine

Read from SKILL.md and the folder at commit ed52af7. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  No scripts in the folder and no shell commands in SKILL.md (its code samples are rust).

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  Links to these hosts (documentation or services it may open):

  • microsoft.github.io
  • rust-lang.github.io
  • doc.rust-lang.org

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Rust Best Practices loads about 955 tokens when it runs, and up to ~1.8k if it reads all its reference files. Until then it costs about 54 tokens; SKILL.md has 118 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~54

When it runs · the whole SKILL.md, loaded when a task matches

~955

With references · SKILL.md plus every file in references/, read only if the agent opens them

~1.8k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from majiayu000/spellbook at commit ed52af7, republished under its MIT licence (© majiayu000). 118 words, ~955 tokens.

name

rust-best-practices

description

Use when writing, modifying, or reviewing Rust code. ALWAYS invoke before Rust edits; covers Microsoft Pragmatic Rust guidance for error handling, API design, performance, and idiomatic patterns.

use thiserror::Error;

#[derive(Error, Debug)]
pub enum MyError {
    #[error("IO error: {0}")]
    Io(#[from] std::io::Error),
    #[error("Parse error at line {line}: {message}")]
    Parse { line: usize, message: String },
    #[error("Not found: {0}")]
    NotFound(String),
}

use anyhow::{Context, Result};

fn main() -> Result<()> {
    let config = load_config()
        .context("Failed to load configuration")?;
    run_app(config)?;
    Ok(())
}

// ❌ BAD
pub fn get_item(index: usize) -> &Item {
    &self.items[index]  // Panics on out-of-bounds
}

// ✅ GOOD
pub fn get_item(&self, index: usize) -> Option<&Item> {
    self.items.get(index)
}

// ✅ GOOD - when you need Result
pub fn get_item(&self, index: usize) -> Result<&Item, Error> {
    self.items.get(index).ok_or(Error::NotFound(index))
}

pub struct Client {
    url: String,
    timeout: Duration,
    retries: u32,
}

impl Client {
    pub fn builder(url: impl Into<String>) -> ClientBuilder {
        ClientBuilder {
            url: url.into(),
            timeout: Duration::from_secs(30),
            retries: 3,
        }
    }
}

pub struct ClientBuilder {
    url: String,
    timeout: Duration,
    retries: u32,
}

impl ClientBuilder {
    pub fn timeout(mut self, timeout: Duration) -> Self {
        self.timeout = timeout;
        self
    }

    pub fn retries(mut self, retries: u32) -> Self {
        self.retries = retries;
        self
    }

    pub fn build(self) -> Client {
        Client {
            url: self.url,
            timeout: self.timeout,
            retries: self.retries,
        }
    }
}

// ❌ BAD - primitive obsession
fn create_user(name: String, email: String, age: u32) -> User { ... }

// ✅ GOOD - newtype wrappers
pub struct Username(String);
pub struct Email(String);
pub struct Age(u32);

impl Email {
    pub fn new(email: impl Into<String>) -> Result<Self, ValidationError> {
        let email = email.into();
        if email.contains('@') {
            Ok(Self(email))
        } else {
            Err(ValidationError::InvalidEmail)
        }
    }
}

fn create_user(name: Username, email: Email, age: Age) -> User { ... }

// ❌ BAD - overly specific
pub fn process(items: Vec<String>) { ... }

// ✅ GOOD - accept any iterable
pub fn process(items: impl IntoIterator<Item = impl AsRef<str>>) {
    for item in items {
        println!("{}", item.as_ref());
    }
}

© majiayu000, MIT. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 1 other file (references) in skills/rust-best-practices of majiayu000/spellbook.

• SKILL.md
• references/advanced.md

Open the folder on GitHubat commit ed52af7