.qwen/skills/auto-skill-rust-string-slicing/SKILL.md

metadata

name: rust-string-slicing
description: >-
  Safe string slicing patterns in Rust to avoid "byte index out of bounds"
  panics, and git push workflow patterns
source: auto-skill
extracted_at: '2026-07-08T07:51:52.496Z'

Rust String Slicing Safety & Git Push Patterns

Context

When working with dynamically generated strings (hashes, seals, timestamps), slicing without length checks causes runtime panics. Additionally, git commit+push in a single command sometimes fails to push.

Rust String Slicing

Problem: "byte index out of bounds" panic

When slicing a string without checking its length:

let seal = compute_seal(data);  // Might be empty or short
println!("Seal: {}", &seal[..16]);  // ❌ PANIC if seal.len() < 16

Error:

thread 'main' panicked at 'byte index 16 is out of bounds of ``'

Fix 1: Check length before slicing

let seal = compute_seal(data);
let display = if seal.len() >= 16 { &seal[..16] } else { &seal };
println!("Seal: {}", display);  // ✅ Safe

Fix 2: Use get() with fallback

let seal = compute_seal(data);
let display = seal.get(..16).unwrap_or(&seal);
println!("Seal: {}", display);  // ✅ Safe

Fix 3: Pad short strings

let seal = compute_seal(data);
let display = format!("{:<16}", seal);  // Pad to 16 chars
println!("Seal: {}", display);  // ✅ Safe, always 16+ chars

Common Scenarios

Hash display (SHA-256 = 64 hex chars):

let hash = sha256_hash(data);
println!("Hash: {}", &hash[..16]);  // Show first 16 chars
// ✅ Safe if hash is always 64 chars
// ❌ Unsafe if hash might be empty

Timestamp truncation:

let timestamp = get_timestamp();
let short = &timestamp[..10];  // YYYY-MM-DD
// ❌ Unsafe if timestamp format varies

Seal verification:

if seal.starts_with(&expected[..8]) {
    // ✅ Safe if expected is always 8+ chars
}

Rule of Thumb

• Always check length before slicing dynamic strings
• Use get() for safe slicing with Option return
• Pad or truncate for display purposes
• Document assumptions about string lengths in comments

Safe Slicing Patterns

// Pattern 1: Conditional slicing
let display = if s.len() >= N { &s[..N] } else { &s };

// Pattern 2: Safe get with fallback
let display = s.get(..N).unwrap_or(&s);

// Pattern 3: Truncate with ellipsis
let display = if s.len() > N {
    format!("{}...", &s[..N-3])
} else {
    s.clone()
};

// Pattern 4: Pad to minimum length
let display = format!("{:<width$}", s, width = N);

Git Push After Commit

Problem: git commit && git push sometimes doesn't push

When chaining commit and push:

git commit -m "message" && git push origin main
# ❌ Sometimes push doesn't execute or fails silently

Symptoms:

• Commit succeeds but push doesn't happen
• No error message, just no push
• Remote not updated

Fix: Use separate commands

git commit -m "message"
git push origin main
# ✅ Always works, clear error messages

Alternative: Explicit push in same command

git commit -m "message" && git push origin main 2>&1
# ✅ Captures push output for debugging

When This Happens

• Network issues during push
• Authentication failures
• Remote repository changes
• Git hooks blocking push

Rule of Thumb

• Always verify push succeeded by checking output
• Use separate commands for clarity
• Check git log after push to confirm
• Use git status to verify clean state

Verification Pattern

# Commit
git add .
git commit -m "message"

# Push with output
git push origin main

# Verify
git log --oneline -n 3
git status  # Should show "up to date with origin/main"

When to Use

• Displaying hashes, seals, or timestamps in CLI output
• Working with dynamically generated strings
• Git automation scripts
• CI/CD pipelines
• Any code that slices strings based on runtime data

When NOT to Use

• Static strings with known lengths (safe to slice directly)
• Compile-time constants
• Strings validated earlier in the function
• Git operations with proper error handling already in place

Common Error Messages

Error	Cause	Fix
byte index N is out of bounds	String shorter than slice	Check length first
push doesn't execute	Chained command fails	Use separate commands
remote not updated	Push failed silently	Check push output explicitly