The decoder context remains allocated and laid out by C because optional members
change with the active build configuration. Previously, the C adapter also
owned the platform leaf: its DYNAMIC_BMI2 branch queried CPU features and
assigned the private bmi2 member directly. That kept the policy in C and made
the Rust boundary depend on the context layout.
Move the leaf to Rust and give it an opaque scalar address plus the active
configuration value. The C adapter now only extracts the optional address when
that member exists and forwards it, while DYNAMIC_BMI2=0 passes NULL. Rust
performs the dynamic decision, queries CPU support, and writes the scalar. The
existing 68-word DCtx view is unchanged. Focused unit coverage verifies that
disabled dynamic dispatch leaves the scalar untouched and enabled dispatch
publishes the CPU feature result.
Test Plan:
- `cc -fsyntax-only -Werror -DDYNAMIC_BMI2=0 -Ilib -Ilib/common -Ilib/compress -Ilib/decompress -Ilib/dict -Ilib/legacy lib/decompress/zstd_decompress.c` -- passed
- `cc -fsyntax-only -Werror -DDYNAMIC_BMI2=1 -Ilib -Ilib/common -Ilib/compress -Ilib/decompress -Ilib/dict -Ilib/legacy lib/decompress/zstd_decompress.c` -- passed
- `rustfmt --edition 2021 --check rust/src/zstd_decompress.rs` -- passed
- `git diff --check -- lib/decompress/zstd_decompress.c rust/src/zstd_decompress.rs` -- passed
ddidderr
c2bed1242c