build(rust): link migrated code into native libraries

Build feature-matched Rust archives for the native Make library path. Flatten
their members into libzstd.a so static consumers resolve C-to-Rust and
Rust-to-C references in one archive, and whole-archive link shared outputs so
every migrated public ABI export remains available.

Rust compression and decompression features now mirror the C partial-library
switches. This prevents compression-only artifacts from retaining DDict
references to decompression-only C helpers.

Add a C archive smoke test that round-trips data and exercises the DDict
lifecycle through lib/libzstd.a, rather than direct test-object links.

Test Plan:
- cargo fmt, strict Clippy, cargo test --all-targets, release build
- Rust checks with compression-only, decompression-only, and no features
- full, partial, forced-HUF, and 32-bit static/shared Make library probes
- test-rust-lib-smoke, fuzzer, zstreamtest, and invalidDictionaries

Refs: rust/README.md

rust/Cargo.toml

@@ -7,7 +7,9 @@ edition = "2021"
 crate-type = ["staticlib"]
 
 [features]
-default = []
+default = ["compression", "decompression"]
+compression = []
+decompression = []
 huf-force-decompress-x1 = []
 huf-force-decompress-x2 = []
 

rust/README.md

@@ -46,9 +46,12 @@ source file whose implementation has moved to Rust remains in the original
 makefile source list as a small shim so header configuration and platform
 preprocessor behavior stay available during the transition.
 
-The test and program makefiles select an archive directory for the active C
-configuration: default, forced HUF X1/X2, and the matching Rust target for
-32-bit C binaries. When the HUF mode changes, they also rebuild cached C
+The library, test, and program makefiles select an archive directory for the
+active C configuration: enabled compression/decompression modules, default or
+forced HUF X1/X2, and the matching Rust target for 32-bit C binaries. The
+native static archive flattens Rust object members rather than nesting a Rust
+archive, while the native shared library retains all migrated Rust exports.
+When the HUF mode changes, the test and program paths also rebuild cached C
 outputs before linking. This prevents original C tests from using a stale or
 configuration-incompatible implementation.
 
@@ -69,6 +72,7 @@ the narrow target for the component being migrated. For example:
 ```sh
 make -C tests fuzzer
 ./tests/fuzzer -i1 --no-big-tests
+make -C tests test-rust-lib-smoke
 ```
 
 Broader `tests/Makefile` targets remain the authoritative integration gates as

rust/src/lib.rs

@@ -7,14 +7,19 @@ pub mod cpu;
 pub mod debug;
 pub mod entropy_common;
 pub mod errors;
+#[cfg(feature = "compression")]
 pub mod fse_compress;
 pub mod fse_decompress;
+#[cfg(feature = "compression")]
 pub mod hist;
+#[cfg(feature = "decompression")]
 pub mod huf_decompress;
 pub mod mem;
 pub mod pool;
 pub mod threading;
 pub mod xxhash;
 pub mod zstd_common;
+#[cfg(feature = "decompression")]
 pub mod zstd_ddict;
+#[cfg(feature = "compression")]
 pub mod zstd_presplit;