# Rust Patterns — RTK Development Rules

RTK-specific Rust idioms and constraints. Applied to all code in this repository.

## Non-Negotiable RTK Rules

These override general Rust conventions:

1. **No async** — Zero `tokio`, `async-std`, `futures`. Single-threaded by design. Async adds 5-10ms startup.
2. **No `unwrap()` in production** — Use `.context("description")?`. Tests: use `expect("reason")`.
3. **Lazy regex** — Put fixed patterns reused across calls in `LazyLock<Regex>`; keep runtime-dependent patterns local.
4. **Fallback pattern** — If filter fails, execute raw command unchanged. Never block the user.
5. **Exit code propagation** — `std::process::exit(code)` if underlying command fails.

## Error Handling

### Always context, always anyhow

```rust
use anyhow::{Context, Result};

// ✅ Correct
fn read_config(path: &Path) -> Result<Config> {
    let content = fs::read_to_string(path)
        .with_context(|| format!("Failed to read config: {}", path.display()))?;
    toml::from_str(&content)
        .context("Failed to parse config TOML")
}

// ❌ Wrong — no context
fn read_config(path: &Path) -> Result<Config> {
    let content = fs::read_to_string(path)?;
    Ok(toml::from_str(&content)?)
}

// ❌ Wrong — panic in production
fn read_config(path: &Path) -> Config {
    let content = fs::read_to_string(path).unwrap();
    toml::from_str(&content).unwrap()
}
```

### Fallback pattern (mandatory for all filters)

```rust
pub fn run(args: MyArgs) -> Result<()> {
    let output = execute_command("mycmd", &args.to_cmd_args())
        .context("Failed to execute mycmd")?;

    let filtered = filter_output(&output.stdout)
        .unwrap_or_else(|e| {
            eprintln!("rtk: filter warning: {}", e);
            output.stdout.clone()  // Passthrough on failure
        });

    tracking::record("mycmd", &output.stdout, &filtered)?;
    print!("{}", filtered);

    if !output.status.success() {
        std::process::exit(output.status.code().unwrap_or(1));
    }
    Ok(())
}
```

## Regex — Use LazyLock for Fixed, Reused Patterns

```rust
use regex::Regex;
use std::sync::LazyLock;

static ERROR_RE: LazyLock<Regex> =
    LazyLock::new(|| Regex::new(r"^error\[").unwrap());
static HASH_RE: LazyLock<Regex> =
    LazyLock::new(|| Regex::new(r"^[0-9a-f]{7,40}").unwrap());

// ✅ Correct — regex compiled once at first use
fn is_error_line(line: &str) -> bool {
    ERROR_RE.is_match(line)
}

// ❌ Wrong — recompiles every call (kills performance)
fn is_error_line(line: &str) -> bool {
    let re = Regex::new(r"^error\[").unwrap();
    re.is_match(line)
}
```

Note: `LazyLock<Regex>` with `.unwrap()` in its initializer is an **established RTK pattern** — it's acceptable because a bad regex literal is a programming error caught at first use.

## Ownership — Borrow Over Clone

```rust
// ✅ Prefer borrows in filter functions
fn filter_lines<'a>(input: &'a str) -> Vec<&'a str> {
    input.lines()
        .filter(|line| !line.is_empty())
        .collect()
}

// ✅ Clone only when you need to own the data
fn filter_output(input: &str) -> String {
    input.lines()
        .filter(|line| !line.trim().is_empty())
        .collect::<Vec<_>>()
        .join("\n")
}

// ❌ Unnecessary clone
fn filter_output(input: &str) -> String {
    let owned = input.to_string();  // Clone for no reason
    owned.lines()
        .filter(|line| !line.is_empty())
        .collect::<Vec<_>>()
        .join("\n")
}
```

## Iterators Over Loops

```rust
// ✅ Iterator chain — idiomatic
let errors: Vec<&str> = output.lines()
    .filter(|l| l.starts_with("error"))
    .take(20)
    .collect();

// ❌ Manual loop — verbose
let mut errors = Vec::new();
for line in output.lines() {
    if line.starts_with("error") {
        errors.push(line);
        if errors.len() >= 20 { break; }
    }
}
```

## Struct Patterns

### Builder for complex args

```rust
// Use Builder when struct has >5 optional fields
pub struct FilterConfig {
    max_lines: usize,
    show_warnings: bool,
    strip_ansi: bool,
}

impl FilterConfig {
    pub fn new() -> Self {
        Self { max_lines: 100, show_warnings: false, strip_ansi: true }
    }
    pub fn max_lines(mut self, n: usize) -> Self { self.max_lines = n; self }
    pub fn show_warnings(mut self, v: bool) -> Self { self.show_warnings = v; self }
}

// Usage: FilterConfig::new().max_lines(50).show_warnings(true)
```

### Newtype for validation

```rust
// Newtype prevents misuse of raw strings
pub struct CommandName(String);

impl CommandName {
    pub fn new(name: &str) -> Result<Self> {
        if name.contains(';') || name.contains('|') {
            anyhow::bail!("Invalid command name: contains shell metacharacters");
        }
        Ok(Self(name.to_string()))
    }
}
```

## String Handling

```rust
// String: owned, heap-allocated
// &str: borrowed slice (prefer in function signatures)
// &String: almost never — use &str instead

fn process(input: &str) -> String {  // ✅ &str in, String out
    input.trim().to_uppercase()
}

fn process(input: &String) -> String {  // ❌ Unnecessary &String
    input.trim().to_uppercase()
}
```

## Match — Exhaustive and Explicit

```rust
// ✅ Exhaustive match with explicit cases
match result {
    Ok(output) => process(output),
    Err(e) => {
        eprintln!("rtk: filter warning: {}", e);
        fallback()
    }
}

// ❌ Silent swallow — catastrophic in RTK (user gets no output)
match result {
    Ok(output) => process(output),
    Err(_) => {}
}
```

## Module Structure

Every `*_cmd.rs` follows this pattern:

```rust
// 1. Imports
use anyhow::{Context, Result};
use regex::Regex;
use std::sync::LazyLock;

// 2. Types (args struct)
pub struct MyArgs { ... }

// 3. Lazy regexes
static MY_RE: LazyLock<Regex> = LazyLock::new(|| ...);

// 4. Public entry point
pub fn run(args: MyArgs) -> Result<()> { ... }

// 5. Private filter functions
fn filter_output(input: &str) -> Result<String> { ... }

// 6. Tests (always present)
#[cfg(test)]
mod tests {
    use super::*;
    fn count_tokens(s: &str) -> usize { s.split_whitespace().count() }
    // ... snapshot tests, savings tests
}
```

## Anti-Patterns (RTK-Specific)

| Pattern | Problem | Fix |
|---------|---------|-----|
| Fixed `Regex::new()` in hot function | Recompiles every call | `LazyLock<Regex>` |
| `unwrap()` in production | Panic breaks user workflow | `.context()?` |
| `tokio::main` or `async fn` | +5-10ms startup | Blocking I/O only |
| Silent match `Err(_) => {}` | User gets no output | Log warning + fallback |
| `println!` in filter path | Debug artifact in output | Remove or `eprintln!` |
| Returning early without exit code | CI/CD thinks command succeeded | `std::process::exit(code)` |
| `clone()` of large strings | Extra allocation in hot path | Borrow with `&str` |
