Replace scattered enum transmutes with an index_enum macro

[kirillbobyrev]

Summary

Fifth and last in the sequence (after #325–#328). The transmute safety cleanup.

Square, File, and Rank each hand-wrote unsafe { mem::transmute } to turn a u8 back into the enum — ~18 scattered call sites, each an independent chance to introduce UB if a bound was ever wrong.

Changes

An index_enum! macro generates, for a contiguous #[repr(u8)] enum:

• ALL — the variants indexed by discriminant;
• from_index — the safe, checked conversion (None out of range);
• from_index_unchecked — one audited, debug_assert-guarded unsafe constructor wrapping the single remaining transmute, with a documented safety contract.

Cold and fallible sites — FEN/UCI parsing, all the TryFrom impls, Display/Debug, Square::next — now use the safe from_index / ALL and contain no unsafe at all. The few hot infallible conversions (Square::new/file/rank/flip_perspective, Bitboard::as_square, and the bitboard iterator) call from_index_unchecked with a // SAFETY note explaining why the index is in range by construction.

Net: 18 scattered transmutes → 1, inside the macro.

Performance

This is on the movegen hot path, so I benchmarked it. A first, fully-safe version (array indexing everywhere) regressed movegen ~3.4%; keeping the hot infallible conversions zero-cost via from_index_unchecked brings it back to 25.9 ms vs. the 27.4 ms baseline — no regression (within noise, if anything slightly faster).

Testing

• 143 tests pass; the full expensive perft suite (incl. the shakmaty differential test) passes in release — move generation is exactly correct.
• cargo bench movegen: no regression vs. baseline.
• cargo clippy --all-targets --all-features zero warnings; cargo +nightly fmt --check clean.

Sequence complete

This finishes the five-PR plan you asked for: (1) unify Game + tablebase-in-search #325, (2) transposition table / tree reuse #326, (3) datagen self-play #327, (4) UCI completeness #328, and (5) this transmute cleanup.

🤖 Generated with Claude Code

https://claude.ai/code/session_01GVrKvkMe4xDmkRAksacBzk

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