hoare skill

---
name: hoare-rust-origins
description: Write Rust code informed by Graydon Hoare's original vision for Rust. Emphasizes memory safety without GC, practical systems programming, and learning from C/C++ mistakes. Use when designing safe systems code or understanding Rust's fundamental choices.
---

# Graydon Hoare Style Guide

## Overview

Graydon Hoare created Rust in 2006 as a personal project at Mozilla, driven by frustration with memory bugs in Firefox. His goal: a language as fast as C++ but safe by default. Rust's core innovation—ownership-based memory management—came from this vision.

## Core Philosophy

> "Rust is a systems programming language focused on safety, speed, and concurrency."

> "Memory safety and thread safety are the same problem, approached from different angles."

Hoare designed Rust to **eliminate entire classes of bugs** that plague C and C++: use-after-free, double-free, data races, null pointer dereferences.

## Design Principles

1. **Safety by Default**: Unsafe operations require explicit `unsafe` blocks.

2. **No Garbage Collector**: Memory management through ownership, not runtime overhead.

3. **Zero-Cost Abstractions**: Safe code should be as fast as unsafe code.

4. **Compiler as Ally**: The compiler catches bugs before runtime.

## When Writing Code

### Always

- Let the borrow checker guide your design
- Prefer stack allocation over heap when possible
- Use `Option<T>` instead of null pointers
- Use `Result<T, E>` for fallible operations
- Make illegal states unrepresentable via types
- Think about ownership before writing code

### Never

- Use `unsafe` without a clear safety comment
- Leak memory (even though Rust allows it with `mem::forget`)
- Ignore compiler warnings—they're often future errors
- Use `.unwrap()` in library code (only in tests/examples)
- Create self-referential structs without understanding pinning

### Prefer

- `&str` over `String` for function parameters
- `&[T]` over `Vec<T>` for read-only access
- `impl Trait` over boxed trait objects when possible
- Iterators over index-based loops
- Pattern matching over if-else chains

## Code Patterns

### Ownership: The Foundation

```rust
// Ownership moves by default
fn main() {
    let s1 = String::from("hello");
    let s2 = s1;  // s1 is MOVED to s2
    // println!("{}", s1);  // ERROR: s1 no longer valid
    println!("{}", s2);  // OK
}

// Borrowing: temporary access without taking ownership
fn print_length(s: &String) {  // Borrows s
    println!("Length: {}", s.len());
}  // s goes out of scope, but since it's borrowed, nothing happens

fn main() {
    let s = String::from("hello");
    print_length(&s);  // Lend s
    println!("{}", s);  // s still valid!
}
```

### Option Instead of Null

```rust
// BAD: Null pointer (not possible in safe Rust anyway)
// char* find(const char* haystack, char needle);  // C: returns NULL if not found

// GOOD: Option makes absence explicit
fn find(haystack: &str, needle: char) -> Option<usize> {
    haystack.chars().position(|c| c == needle)
}

fn main() {
    let text = "hello";
    match find(text, 'l') {
        Some(index) => println!("Found at {}", index),
        None => println!("Not found"),
    }
    
    // Or use combinators
    let index = find(text, 'l').unwrap_or(0);
    
    // Or the ? operator
    fn process(text: &str) -> Option<usize> {
        let index = find(text, 'l')?;  // Returns None if find returns None
        Some(index + 1)
    }
}
```

### Result for Error Handling

```rust
use std::fs::File;
use std::io::{self, Read};

// Errors are values, not exceptions
fn read_file(path: &str) -> Result<String, io::Error> {
    let mut file = File::open(path)?;  // ? propagates error
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    Ok(contents)
}

// Handle errors explicitly
fn main() {
    match read_file("config.txt") {
        Ok(contents) => println!("{}", contents),
        Err(e) => eprintln!("Error reading file: {}", e),
    }
}
```

### Making Illegal States Unrepresentable

```rust
// BAD: Runtime checks needed
struct Connection {
    is_connected: bool,
    socket: Option<Socket>,
}

impl Connection {
    fn send(&self, data: &[u8]) {
        if self.is_connected {  // Runtime check!
            self.socket.as_ref().unwrap().write(data);
        }
    }
}

// GOOD: Type system enforces valid states
struct Disconnected;
struct Connected { socket: Socket }

impl Disconnected {
    fn connect(self, addr: &str) -> Result<Connected, Error> {
        let socket = Socket::connect(addr)?;
        Ok(Connected { socket })
    }
}

impl Connected {
    fn send(&mut self, data: &[u8]) -> Result<(), Error> {
        self.socket.write(data)  // Always valid!
    }
    
    fn disconnect(self) -> Disconnected {
        // socket dropped here
        Disconnected
    }
}
```

### Zero-Cost Abstractions

```rust
// High-level iterator code...
let sum: i32 = (0..1000)
    .filter(|n| n % 2 == 0)
    .map(|n| n * n)
    .sum();

// ...compiles to the same machine code as:
let mut sum = 0i32;
for n in 0..1000 {
    if n % 2 == 0 {
        sum += n * n;
    }
}

// Abstractions have no runtime cost
```

## Mental Model

Hoare designed Rust by asking:

1. **What bugs killed us in C++?** Memory corruption, data races, null pointers
2. **Can the compiler catch these?** Yes, with ownership tracking
3. **What's the performance cost?** Zero—it's all at compile time
4. **Is this teachable?** The borrow checker is strict but consistent

## The Ownership Rules

1. Each value has exactly one owner
2. When the owner goes out of scope, the value is dropped
3. You can have either:
   - One mutable reference (`&mut T`), OR
   - Any number of immutable references (`&T`)
4. References must always be valid (no dangling)

These rules, enforced at compile time, prevent:
- Use-after-free
- Double-free
- Data races
- Null pointer dereferences