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2. Engine written in Rust, shared across shells via UniFFI

Date: 2026-06-27

Status

Accepted

Context

Under ADR-0001 the engine may be consumed first by SwiftUI and later by Tauri. The engine is the hard ~60% of the work and must not be rewritten per platform.

Decisions

Core Engine in Rust

Decision: Write the core engine as a UI-agnostic Rust library crate.

Rationale: - Rust is reusable by SwiftUI (via UniFFI) and native to Tauri. - Gives memory safety and performance suited to the project budgets.

Swift Binding via UniFFI

Decision: Expose the Rust engine to the SwiftUI shell via UniFFI bindings.

Rationale: - Automates boilerplate bridge code, exposing a safe and clean Swift API.

Consequences

  • The cross-platform/Tauri pivot only requires rebuilding the shell + UI; the engine logic carries over untouched.
  • Adds an FFI boundary on macOS (managed via UniFFI) and a learning investment if Rust is new — offset by strong reuse.
  • Alternatives Considered:
  • C++ engine (works with Swift, but awkward in the Tauri leg).
  • TypeScript-only (cannot meet performance budgets).