Instruction file imported from krzysztofautomatyk/plc-ladder-sim (.github/instructions/rust.instructions.md). Copyright stays with the author.

Rust / Tauri backend instructions

This is the safety-conscious core of a PLC simulator. Correctness and determinism matter more than cleverness.

Non-negotiable rules

• Never panic in the scan path. User-logic faults (divide-by-zero, overflow, out-of-range address) must return a typed ScanError → the engine sets FAULT/STOP. Use checked arithmetic (checked_add, checked_mul, …) and the bounds-checked PlcMemory accessors — never index slices directly with input data.
• #![forbid(unsafe_code)] is set in lib.rs/main.rs. Do not introduce unsafe.
• Locks are parking_lot. PlcMemory::snapshot() takes all four process-image locks together for a coherent image — keep it that way.
• Modbus stays localhost-bound and write-gated (allow_modbus_write). Don't widen the bind address or remove the write gate.
• The audit trail is an append-only SHA-256 chain restored + verified on startup. Don't break verify_chain; new audited actions call AuditTrail::record.

Conventions

• Errors use thiserror enums; commands return CommandResult<T> (ok/err).
• Serde enums are #[serde(tag = "type", rename_all = "snake_case")]. When adding a LadderElement variant, also add the arm in compiler::compile_elements and, if it is renderable, a TS type + registry entry + glyph on the frontend.
• The compiled program hash is derived from bytecode only — display-only fields (labels, comments) must not change it.

Definition of done (run inside src-tauri/)

cargo fmt --all -- --check
cargo clippy --all-targets --all-features -- -D warnings   # warnings are errors
cargo test --all-features

Any change to execution semantics needs a matching test in plc/engine.rs or plc/compiler.rs. Prefer a deterministic test (see the existing LCG-based fuzz tests) over randomness with a real RNG.