Bolt ⚡

Lightning-fast Rust type checker for rapid development.

Get instant feedback on your Rust code. Bolt checks your source files in milliseconds by using stubs for external crates instead of compiling them.

bolt check .        # ~50ms  (your code only, stubs for deps)
cargo check         # 3+ min cold, ~2s cached

Install

cargo install bolt-rs   # Installs the 'bolt' binary

Or build from source:

git clone https://github.com/varun29ankuS/bolt.git
cd bolt && cargo build --release

Usage

# Check a file
bolt check src/main.rs

# Check entire project (auto-detects Cargo.toml)
bolt check .

# JSON output for tooling/IDE integration
bolt check . --format json

Why Bolt?

The problem: cargo check takes seconds to minutes. That breaks your flow.

The solution: A fast type checker optimized for the edit-check loop. Check your code in milliseconds, not seconds.

You write code
    ↓
bolt check .     ← ~50ms feedback
    ↓
Fix errors
    ↓
bolt check .     ← ~50ms feedback
    ↓
Ready? → cargo build --release

What Works

Feature	Status
Type checking	✅ Full
Borrow checking	✅ Async (non-blocking)
Watch mode	✅ bolt watch --run with auto-cancel
Cargo.toml detection	✅ Auto-finds entry point
External crates	✅ Stub resolution (serde, std, etc.)
Generics	✅ Full
Pattern matching	✅ Full
Closures	✅ Full
impl blocks	✅ Full
Error recovery	✅ Graceful fallback suggestions

Async Borrow Checking

Bolt's borrow checker runs in the background - your code executes immediately while safety checks happen concurrently. If issues are found, they're reported after execution (and block the next run).

bolt run main.rs    # Runs instantly, borrow check in background
                    # If errors found → shown after execution
                    # Next run blocks until you fix them

What Doesn't Work (Yet)

Feature	Workaround
Proc macros (#[derive])	Manual impl or use rustc
async/await	Use rustc for async code
Complex trait bounds	Bolt suggests rustc fallback

When Bolt can't handle something, it tells you:

Error: Bolt couldn't fully check this file
  💡 Try: rustc --edition 2021 src/lib.rs

Benchmarks

Scenario	Bolt	cargo check	Notes
Single file	~25-50ms	~150ms (cached)	Bolt is 3-6x faster
Project (cold)	<1s	3+ minutes	Bolt skips dependency compilation
Project (cached)	<1s	~1-3s	Similar for cached builds

Bolt checks your code only - it uses stubs for external crates instead of compiling them. This is why it's fast but may miss some cross-crate type errors.

Architecture

Source → Parse → HIR → TypeCheck → Run Code
                           ↓           ↑
                    External Stubs     │ (async)
                                       ↓
                                  BorrowCheck

Key innovations:

• Async Borrow Checking: Safety checks run in background - code executes immediately
• Ownership Ledger: Blockchain-inspired borrow tracking with full audit trail
• Stub Resolution: Type-check against external crates without compiling them
• Watch Mode: Concurrent compilation with automatic cancellation on new changes

Roadmap

Version	Focus	Status
0.1	Core type checking, async borrow checker, watch mode	✅ Current
0.2	Proc macro expansion (#[derive] support)	🔄 Next
0.3	Incremental checking (only re-check changed functions)	Planned
0.4	Hybrid build (bolt compile + cargo link)	Planned
1.0	IDE integrations, LSP server	Planned

Vision

Gradual compilation: Eventually, Bolt aims to compile your code incrementally at the function level - only recompiling what changed. Combined with async borrow checking and Cranelift's fast codegen, this could enable sub-10ms feedback loops.

Hybrid builds: Use Bolt for rapid iteration, then seamlessly hand off to cargo for production builds with full optimization.

Contributing

PRs welcome! The codebase is ~25k lines of clean, readable Rust.

src/
├── parser/      # syn-based parsing
├── typeck/      # Type inference
├── borrowck/    # Borrow checking + ownership ledger
├── extern_crates/ # External crate stubs
└── cli/         # Command line interface

License

MIT

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Bolt is not a rustc replacement. It's a fast feedback tool for development. Use rustc/cargo for production builds.