Keep the C header's ZSTD_wildcopy signature as the enum-facing wrapper, but
move its over-copying implementation into rust/src/mem.rs. The wrapper passes
the enum's int representation to ZSTD_rust_wildcopy, where only the two valid
values are converted to ZstdOverlap. This leaves C's ZSTD_copy16/COPY16
helpers available to their direct compression callers and removes the now
unused COPY8 helper. ZSTD_copy16 is marked unused-safe for C translation units
that include the shared header without calling that direct helper.
The Rust leaf preserves the original first-copy behavior for zero and short
lengths, the source-before-destination 8-byte do-while path, the no-overlap
distance assertion, and the first-then-two-COPY16 loop. Its ABI contract does
not take ownership of caller buffers. ABI tests use 32-byte padded buffers and
exercise no-overlap plus offsets 8 and 15 across the boundary lengths, checking
both copied bytes and guard regions.
Test Plan:
- `cargo test mem::tests` -- passed (4 tests)
- `make lib-nomt` and `make lib-mt` -- passed
- `make -C tests test-zstream` -- passed
- `make -C tests test-fullbench` -- completed; its `-P0` run printed the
existing Scenario 17 diagnostic, but the target returned normally
- `cargo clippy`, `cargo clippy --benches`, `cargo clippy --tests`,
`cargo +nightly fmt`, then the same three clippy commands -- passed on the
final repeat
- `git diff --cached --check` -- passed
ddidderr
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