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From Rust to Zig: What changed my mind
The Roc compiler project successfully migrated 300,000 lines of code from Rust to Zig, citing faster builds and granular memory management as the primary drivers.
What: After 18 months, the Roc team achieved feature parity with their Rust-based compiler using Zig. They found that Zig’s comptime features, explicit control over memory allocators, and ability to handle per-module arenas provided a better fit for their specific compiler architecture than Rust's global-allocator-centric ecosystem.
Why it matters: This highlights that Rust's 'one-size-fits-all' safety model and build overhead can become burdensome for niche systems projects that require highly specific memory layouts and zero-allocation primitives.
Takeaway: Consider Zig if your project requires custom memory management, zero-parse deserialization, or extremely tight feedback loops for large-scale systems builds.
Deep dive
- Roc rewrote its compiler from Rust to Zig to address build-time and architectural constraints.
- Zig allows for granular per-module allocators, which was a requirement for Roc's performance needs.
- The team implemented 'zero-parse deserialization' to load compiler data structures directly from disk into memory.
- Rust's build times (even with incremental improvements) were slower than Zig's
-fincrementalrebuilds in practice. - The project experienced few memory-safety bugs in both versions, suggesting that in a compiler context, the borrow checker's impact is often offset by the complexity of unsafe code.
- Roc's team values Zig's
comptimefor metaprogramming, which replaced some of the complexity found in Rust macros.
Decoder
- Comptime: A Zig feature that allows code to be executed during compilation, often used for metaprogramming and specialized code generation.
- Defunctionalization: A technique for transforming higher-order functions into data types, aiding in compiler optimization.
- Zero-parse deserialization: A technique where data is stored on disk in the same memory layout used by the application, allowing it to be loaded into memory without conversion or parsing.
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