---
name: TDD Rust
slug: tdd-rust
category: Quality
description: TDD Rust guides Red-Green-Refactor development of RTK filters in Rust, using real command fixtures, token savings assertions, and insta snapshot tests. Use it when implementing new filter commands to enforce quality and output stability.
github: "https://github.com/rtk-ai/rtk/tree/develop/.claude/skills/tdd-rust"
language: Rust
stars: 75943
forks: 4776
install: "npx degit https://github.com/rtk-ai/rtk/tree/develop/.claude/skills/tdd-rust ~/.claude/skills/tdd-rust"
installs_to: ~/.claude/skills/tdd-rust
source_path: .claude/skills/tdd-rust/SKILL.md
collection_size: 12
category_size: 1354
collection_url: "https://dirskills.com/collections/rtk-ai/rtk"
added: 2026-08-13T07:36:22.046Z
last_synced: 2026-08-13T07:36:22.046Z
canonical_url: "https://dirskills.com/skills/tdd-rust"
---

# TDD Rust

TDD Rust guides Red-Green-Refactor development of RTK filters in Rust, using real command fixtures, token savings assertions, and insta snapshot tests. Use it when implementing new filter commands to enforce quality and output stability.

**Install:**

```bash
npx degit https://github.com/rtk-ai/rtk/tree/develop/.claude/skills/tdd-rust ~/.claude/skills/tdd-rust
```

## README

# RTK TDD Workflow

Enforce Red-Green-Refactor for all RTK filter development.

## The Loop

```
1. RED   — Write failing test with real fixture
2. GREEN — Implement minimum code to pass
3. REFACTOR — Clean up, verify still passing
4. SAVINGS — Verify ≥60% token reduction
5. SNAPSHOT — Lock output format with insta
```

## Step 1: Real Fixture First

Never write synthetic test data. Capture real command output:

```bash
# Capture real output from the actual command
git log -20 > tests/fixtures/git_log_raw.txt
cargo test 2>&1 > tests/fixtures/cargo_test_raw.txt
cargo clippy 2>&1 > tests/fixtures/cargo_clippy_raw.txt
gh pr view 42 > tests/fixtures/gh_pr_view_raw.txt

# For commands with ANSI codes — capture as-is
script -q /dev/null cargo test 2>&1 > tests/fixtures/cargo_test_ansi_raw.txt
```

Fixture naming: `tests/fixtures/<command>_raw.txt`

## Step 2: Write the Test (Red)

```rust
#[cfg(test)]
mod tests {
    use super::*;
    use insta::assert_snapshot;

    fn count_tokens(s: &str) -> usize {
        s.split_whitespace().count()
    }

    // Test 1: Output format (snapshot)
    #[test]
    fn test_filter_output_format() {
        let input = include_str!("../tests/fixtures/mycmd_raw.txt");
        let output = filter_mycmd(input).expect("filter should not fail");
        assert_snapshot!(output);
    }

    // Test 2: Token savings ≥60%
    #[test]
    fn test_token_savings() {
        let input = include_str!("../tests/fixtures/mycmd_raw.txt");
        let output = filter_mycmd(input).expect("filter should not fail");

        let input_tokens = count_tokens(input);
        let output_tokens = count_tokens(&output);
        let savings = 100.0 * (1.0 - output_tokens as f64 / input_tokens as f64);

        assert!(
            savings >= 60.0,
            "Expected ≥60% token savings, got {:.1}% ({} → {} tokens)",
            savings, input_tokens, output_tokens
        );
    }

    // Test 3: Edge cases
    #[test]
    fn test_empty_input() {
        let result = filter_mycmd("");
        assert!(result.is_ok());
        // Empty input = empty output OR passthrough, never panic
    }

    #[test]
    fn test_malformed_input() {
        let result = filter_mycmd("not valid command output\nrandom text\n");
        // Must not panic — either filter best-effort or return input unchanged
        assert!(result.is_ok());
    }
}
```

Run: `cargo test` → should fail (function doesn't exist yet).

## Step 3: Minimum Implementation (Green)

```rust
// src/mycmd_cmd.rs

use anyhow::{Context, Result};
use regex::Regex;
use std::sync::LazyLock;

static ERROR_RE: LazyLock<Regex> =
    LazyLock::new(|| Regex::new(r"^error").unwrap());

pub fn filter_mycmd(input: &str) -> Result<String> {
    if input.is_empty() {
        return Ok(String::new());
    }

    let filtered: Vec<&str> = input.lines()
        .filter(|line| ERROR_RE.is_match(line))
        .collect();

    Ok(filtered.join("\n"))
}
```

Run: `cargo test` → green.

## Step 4: Accept Snapshot

```bash
# First run creates the snapshot
cargo test test_filter_output_format

# Review what was captured
cargo insta review
# Press 'a' to accept

# Snapshot saved to src/snapshots/mycmd_cmd__tests__test_filter_output_format.snap
```

## Step 5: Wire to main.rs (Integration)

```rust
// src/main.rs
mod mycmd_cmd;

#[derive(Subcommand)]
pub enum Commands {
    // ... existing commands ...
    Mycmd(MycmdArgs),
}

// In match:
Commands::Mycmd(args) => mycmd_cmd::run(args),
```

```rust
// src/mycmd_cmd.rs — add run() function
pub fn run(args: MycmdArgs) -> Result<()> {
    let output = execute_command("mycmd", &args.to_vec())
        .context("Failed to execute mycmd")?;

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

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

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

## Step 6: Quality Gate

```bash
cargo fmt --all && cargo clippy --all-targets && cargo test
```

All 3 must pass. Zero clippy warnings.

## Arrange-Act-Assert Pattern

```rust
#[test]
fn test_filters_only_errors() {
    // Arrange
    let input = "info: starting build\nerror[E0001]: undefined\nwarning: unused\n";

    // Act
    let output = filter_mycmd(input).expect("should succeed");

    // Assert
    assert!(output.contains("error[E0001]"), "Should keep error lines");
    assert!(!output.contains("info:"), "Should drop info lines");
    assert!(!output.contains("warning:"), "Should drop warning lines");
}
```

## RTK-Specific Test Patterns

### Test ANSI stripping

```rust
#[test]
fn test_strips_ansi_codes() {
    let input = "\x1b[32mSuccess\x1b[0m\n\x1b[31merror: failed\x1b[0m\n";
    let output = filter_mycmd(input).expect("should succeed");
    assert!(!output.contains("\x1b["), "ANSI codes should be stripped");
    assert!(output.contains("error: failed"), "Content should be preserved");
}
```

### Test fallback behavior

```rust
#[test]
fn test_filter_handles_unexpected_format() {
    // Give it something completely unexpected
    let input = "completely unexpected\x00binary\xff data";
    // Should not panic — returns Ok() with either empty or passthrough
    let result = filter_mycmd(input);
    assert!(result.is_ok(), "Filter must not panic on unexpected input");
}
```

### Test savings at multiple sizes

```rust
#[test]
fn test_savings_large_output() {
    // 1000-line fixture → must still hit ≥60%
    let large_input: String = (0..1000)
        .map(|i| format!("info: processing item {}\n", i))
        .collect();
    let output = filter_mycmd(&large_input).expect("should succeed");

    let savings = 100.0 * (1.0 - count_tokens(&output) as f64 / count_tokens(&large_input) as f64);
    assert!(savings >= 60.0, "Large output savings: {:.1}%", savings);
}
```

## What "Done" Looks Like

Checklist before moving on:

- [ ] `tests/fixtures/<cmd>_raw.txt` — real command output
- [ ] `filter_<cmd>()` function returns `Result<String>`
- [ ] Snapshot test passes and accepted via `cargo insta review`
- [ ] Token savings test: ≥60% verified
- [ ] Empty input test: no panic
- [ ] Malformed input test: no panic
- [ ] `run()` function with fallback pattern
- [ ] Registered in `main.rs` Commands enum
- [ ] `cargo fmt --all && cargo clippy --all-targets && cargo test` — all green

## Never Do This

```rust
// ❌ Synthetic fixture data
let input = "fake error: something went wrong";  // Not real cargo output

// ❌ Missing savings test
#[test]
fn test_filter() {
    let output = filter_mycmd(input);
    assert!(!output.is_empty());  // No savings verification
}

// ❌ unwrap() in production code
let filtered = filter_mycmd(input).unwrap();  // Panic in prod

// ❌ Regex inside the filter function
fn filter_mycmd(input: &str) -> Result<String> {
    let re = Regex::new(r"^error").unwrap();  // Recompiles every call
    ...
}
```