Implementation language and runtime choice
Rust reference core
ADR 0001 selects Rust for document validation, operations, codecs, evaluation and language bindings. The existing workspace demonstrates those interfaces through native, WebAssembly and C consumers. This is implementation evidence for the selected stack, not a comparative benchmark of programming languages.
Rust’s ownership and type system support the implementation’s memory-management and typed-operation requirements. They do not prove semantic correctness or eliminate unsafe behavior in dependencies. Bounded parsing, coverage-guided fuzzing and independent oracle comparisons remain necessary. The toolchain policy records the pinned compiler and minimum supported Rust version.
Alternatives and replacement criteria
No equivalent NUIF core has been implemented in C++, Zig, Go or TypeScript for a controlled comparison. Claims that these languages are categorically unsuitable are therefore unwarranted. A replacement proposal requires evidence for the actual workloads: hostile-input handling, canonical serialization, operation replay, native/browser embedding and maintenance cost.
Target-native adapter code remains appropriate where a host exposes its API through another language. The binding decision separates those host interfaces from the document model and does not make Rust a conformance requirement.
Dependency boundaries
The dependency register records each direct library’s role, alternatives and rationale. Taffy supplies layout evaluation; Harfrust and Skrifa supply shaping and font interpretation; Masonry and AccessKit supply the native shell and its semantic test surface. Source adapters use Tree-sitter where retained syntax is required. Each dependency remains behind NUIF-owned types or an explicitly declared host boundary.
The editor fork review records the retained patches and a CPU-rendered egui alternative. A small alternative-backend probe does not establish replacement parity for the editor. Migration requires the same canvas, semantic-action, text, rendering and host integration acceptance criteria as the current implementation.