rtk: Skill for Claude Code

RTK Rust Design Patterns

Patterns that apply to RTK's filter module architecture. Focused on CLI tool patterns, not web/service patterns.

Pattern 1: Newtype (Type Safety)

Use when: wrapping primitive types to prevent misuse (command names, paths, token counts).

// Without Newtype — easy to mix up
fn track(input_tokens: usize, output_tokens: usize) { ... }
track(output_tokens, input_tokens);  // Silent bug!

// With Newtype — compile error on swap
pub struct InputTokens(pub usize);
pub struct OutputTokens(pub usize);
fn track(input: InputTokens, output: OutputTokens) { ... }
track(OutputTokens(100), InputTokens(400));  // Compile error ✅

// Practical RTK example: command name validation
pub struct CommandName(String);
impl CommandName {
    pub fn new(s: &str) -> Result<Self> {
        if s.contains(';') || s.contains('|') || s.contains('`') {
            anyhow::bail!("Invalid command name: shell metacharacters");
        }
        Ok(Self(s.to_string()))
    }
    pub fn as_str(&self) -> &str { &self.0 }
}

Pattern 2: Builder (Complex Configuration)

Use when: a struct has 4+ optional fields, many with defaults.

#[derive(Default)]
pub struct FilterConfig {
    max_lines: Option<usize>,
    strip_ansi: bool,
    show_warnings: bool,
    truncate_at: Option<usize>,
}

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

// Usage — readable, no positional arg confusion
let config = FilterConfig::new()
    .max_lines(50)
    .strip_ansi(true)
    .show_warnings(false);

When NOT to use Builder: if the struct has 1-3 fields with obvious meaning. Over-engineering for simple cases.

Pattern 3: State Machine (Parser/Filter Flows)

Use when: parsing multi-section output (test results, build output) where context changes behavior.